Spin-Mechanical Scheme with Color Centers in Hexagonal Boron Nitride Membranes – M. Abdi, M.-J. Hwang, M. Aghtar, and M. B. Plenio, Physical Review Letters, 119, 233602 (2017) | ArXiv

Resource Theory of Superposition – T. Theurer, N. Killoran, D. Egloff, and M. B. Plenio, Physical Review Letters, 119, 230401 (2017) | ArXiv

Efficient tomography of a quantum many-body system – B. P. Lanyon, C. Maier, M. Holzäpfel, T. Baumgratz, C. Hempel, P. Jurcevic, I. Dhand, A. S. Buyskikh, A. J. Daley, M. Cramer, M. B. Plenio, R. Blatt and C. F. Roos, Nature Physics 13, 1158 (2017) | ArXiv


Quantum Redirection of Antenna Absorption to Photosynthetic Reaction Centers – F. Caycedo-Soler, C. A. Schroeder, C. Autenrieth, A. Pick, R. Ghosh, S. F. Huelga, and M. B. Plenio, J. Phys. Chem. Lett. 8, 6015 (2017) | ArXiv
The gist of it

Photosynthesis — the conversion of sunlight to chemical energy — is fundamental for supporting life on our planet. The early steps of photosynthesis involve the photo-excitation of reaction centres (RCs) and light-harvesting (LH) units. In this article, to the best of our knowledge, we present the first quantum mechanical effect of excitonic energy transfer, happening at physiological conditions, by illumination with thermal light. The historically overlooked excitonic delocalisation across RC and LH pigments in various photosynthetic organisms is shown to display a significant redistribution of absorption amplitudes, which benefits the absorption cross section of the optical bands associated with the RC of several species. Using stochastic optimisation, we are able to delineate clear guidelines and develop simple analytic expressions, in order to amplify the coherent redirection in artificial nano-structures.

Robust techniques for polarization and detection of nuclear spin ensembles – J. Scheuer, I. Schwartz, S. Müller, Q. Chen, I. Dhand, M. B. Plenio, B. Naydenov, and F. Jelezko, Physical Review B, 96, 174436 (2017) | ArXiv

Unambiguous nuclear spin detection using an engineered quantum sensing sequence – Z. Shu, Z. Zhang, Q. Cao, P. Yang, M. B. Plenio, C. Müller, J. Lang, N. Tomek, B. Naydenov, L. P. McGuinness, F. Jelezko, and J. Cai, Physical Review A, 96, 051402(R) (2017)

Colloquium_Quantum coherence as a resourceColloquium: Quantum coherence as a resource – A. Streltsov, G. Adesso, and M. B. Plenio, Reviews of Modern Physics, 89, 041003 (2017) | ArXiv
The gist of it

One of the central differences between classical and quantum physics is that individual quantum particles may exist in coherent superposition states while classically they cannot. This additional feature allows us to achieve physical effects that cannot be achieved in classical physics and therefore coherent superposition states represent a resource that we can make use of when our experimental capabilities are limited to classical devices. But how useful is a particular coherent superposition state, how costly is it to produce such states? This and many other natural questions can be posed and answered rigorously by formulating them as a mathematical theory of resources. This was proposed by our group at Ulm in 2014 and it has become a vibrant research area which the present work reviews.



JorgeArbitrary nuclear-spin gates in diamond mediated by a nitrogen-vacancy-center electron spin – J. Casanova, Z.-Y. Wang, and M. B. Plenio, Physical Review A, 96, 032314 (2017) | ArXiv
                                          The gist of it

We present a method to achieve general N-body entangling quantum gates between nuclear spins in solid-state platforms based on nitrogen vacancy centers in diamond or divacancies in silicon carbide. Our protocol is general and robust, and paves the way to the implementation of fermionic interactions which find applications in, for example, quantum chemistry in solid-state quantum registers.


Momentum coupling in non-Markovian quantum Brownian motionMomentum coupling in non-Markovian quantum Brownian motion – L. Ferialdi and A. Smirne, Physical Review A, 96, 012109 (2017) | ArXiv
The gist of it

Every quantum device unavoidably interacts with the surrounding environment. Generally, the resulting dynamics has to be described as non-Markovian, accounting for the memory effects due to the influence of the environment on the relevant system. A fundamental model of non-Markovian dynamics is provided by the so-called non-Markovian Brownian motion, where a system is bilinearly coupled via its position operator to a bath of harmonic oscillators. Such model is indeed at the same time physically meaningful and mathematically treatable in detail.

In this paper, we consider a generalization of the non-Markovian quantum Brownian motion, taking into account a coupling of the central harmonic oscillator both via its position and via its momentum operators with the bath. We derive the equations of motion for such a model, and we solve them for a generic Gaussian system state. We then investigate the resulting evolution of the first and second moments for different characterizations of the environment (spectral densities). In particular, we show that the coupling with the momentum enhances the dissipation experienced by the system, accelerating its relaxation to equilibrium and modifying the asymptotic state of the dynamics. We also quantify the memory effects experienced by the system, thus characterizing the non-Markovianity of its evolution.


PRXcoverAutonomous Quantum Clocks: Does Thermodynamics Limit Our Ability to Measure Time? – P. Erker, M. T. Mitchison, R. Silva, M. P. Woods, N. Brunner, and M. Huber, Physical Review X, 7, 031022 (2017) | ArXiv
See also the Physics viewpoint on the article.
The gist of it

Time is arguably one of the most prominent concepts in physics, yet it still holds a significant number of mysteries, particularly in the context of quantum physics. Quantum theory fails to provide a clear description of what time actually is and treats it simply as a classical external variable. It is often argued that this failure represents one of the obstacles to unifying quantum theory with general relativity. Here, we theoretically explore the ultimate limitations of measuring time based only on the laws of quantum physics. We introduce the concept of an autonomous quantum clock, which represents a minimal model of a quantum clock that is both complete and self-contained. It allows us to elucidate the fundamental limitations in the process of timekeeping without implicitly assuming unaccounted-for resources through external control. We assume that our out-of-thermal-equilibrium clock is powered by two thermal baths held at different temperatures. We show that the clock’s accuracy and resolution—its performance—are intimately related to the power the clock dissipates. In other words, measuring time results in an increase in entropy. This finding provides a quantitative basis for the intuitive connection between the second law of thermodynamics and the arrow of time.


Peter KnightJourneys from quantum optics to quantum technology – S. M. Barnett, A. Beige, A. Ekert, B. M. Garraway, C. H. Keitel, V. Kendon, M. Lein, G. J. Milburn, H. M. Moya-Cessa, M. Murao, J. K. Pachos, G. M. Palma, E. Paspalakis, S. J. D. Phoenix, B. Piraux o , M. B. Plenio, B. C. Sanders, J. Twamley, A. Vidiella-Barranco, M. S. Kim, Progress in Quantum Electronics, 54, 19 (2017) | ArXiv
                                          The gist of it

In this work some of the large number of PhD students and postdocs of Professor Sir Peter Knight FRS present short reminiscences of scientific life and the time they have spent in the Theoretical Quantum Optics group at Imperial College that Peter had founded. This presents a glimpse on the vast impact that Peter has had on the development of quantum optics and quantum technologies throughout his career.

Photo: Ralph Hodgson/Imperial College London

Bild1Stochastic unraveling of positive quantum dynamics – M. Caiaffa, A. Smirne, and A. Bassi, Physical Review A, 95, 062101 (2017) | ArXiv
The gist of it

The understanding of the dynamics of open quantum systems takes advantage of many different mathematical and conceptual tools, among which the stochastic unraveling is one of the most exploited. This method relies on a stochastic differential equation, which fixes trajectories in the set of the system’s pure states (the so-called quantum trajectories).

In this paper, we formulate a proper diffusive unraveling, which applies to positive, not necessarily completely positive dynamics. We prove in a constructive way how such an unraveling can be formulated for any positive semigroup dynamics, but also for a more general class of (divisible) open-system evolutions. In particular, the role of the rates and Lindblad operators in the standard unraveling is replaced by the eigenvalues and eigenvectors of a suitable rate operator. In this way, we provide a significant class of open-system dynamics with a useful tool to describe physical phenomena, which would be neglected within the usual, completely positive framework. The general approach is then applied to a prototypical model for the description of energy transfer in biomolecular networks.


Dissipatively Stabilized Quantum Sensor Based on Indirect Nuclear-Nuclear Interactions_biggerDissipatively Stabilized Quantum Sensor Based on Indirect Nuclear-Nuclear Interactions – Q. Chen, I. Schwarz, and M. B. Plenio, Physical Review Letters, 119010801 (2017) | ArXiv
                                          The gist of it

The Nitrogen vacancy (NV) center is attracting increasing attention due to two major applications: quantum register and sensing. However, there are several outstanding challenges caused by the relaxation and decoherence processes of the NV center electron spin as these limit quantum gate fidelities on nuclear registers as well as spectral resolution, selectivity and signal to noise ratio in sensing applications. Here, we address directly the challenges by using the NV center as a mediator to couple two nuclear spins, while eliminating the NV center and the effect of its decoherence and relaxation from the dynamics. The key idea is the use of the substantial second order coupling between the nuclear spins obtained through a strongly detuned NV centre which is periodically reinitialized by a dissipative process. Thus, high fidelity of selective quantum gates between the nuclear spins, as well as an improved sensing setup, are made possible even at ambient condition.


Steady-state preparation of long-lived nuclear spin singlet pairs at room temperatureSteady-state preparation of long-lived nuclear spin singlet pairs at room temperature – Q. Chen, I. Schwarz, and M. B. Plenio, Physical Review B, 95, 224105 (2017) | ArXiv
The gist of it

The preparation of nuclear spins in singlet states is attracting increasing attention due to their weak coupling to several environmental relaxation processes especially for closely spaced nuclei when their interaction with their environment is symmetric to a very good approximation and transitions between the singlet and triplet states are suppressed. However, the generation of a singlet state of nuclear spins in experiments by using unitary preparation is extremely difficult at room temperature. Here, we propose a dissipative scheme that achieves the preparation of pairs of nuclear spins in long-lived singlet states by a protocol that combines the interaction between the nuclei and a periodically reset electron spin of a nirogen-vacancy center with local radio-frequency control of the nuclear spins. The final state of this protocol is independent of the initial preparation of the nuclei, is robust to external field fluctuations, and can be operated at room temperature.


Protected ultrastrong coupling regime of the two-photon quantum Rabi model with trapped ionsProtected ultrastrong coupling regime of the two-photon quantum Rabi model with trapped ions – R. Puebla, M.-J. Hwang, J. Casanova, and M. B. Plenio,
Physical Review A, 95, 063844 (2017) | ArXiv
DOI: https://doi.org/10.1103/PhysRevA.95.063844
                                         The gist of it

The achievement of almost perfectly isolated quantum systems is nowadays possible thanks to the development of modern quantum technologies. However, imperfections or noises are inherent to any experimental platform. These imperfections, which stem from an uncontrolled interaction with the environment, lead to the loss of coherence and spoil a correct realization of a desired quantum isolated system. In this regard, there are a several schemes whose ultimate goal consists in shielding the quantum system from these imperfections, and in this manner quantum coherence can be preserved during longer times. This is of particular relevance when, in order to accomplish the quantum dynamics of a system, a long time is required, comparable or longer than the decoherence time.

In this work we propose a scheme based on a continuous dynamical decoupling scheme that allows us to realize a two-photon quantum Rabi model with a trapped ion, while at the same time, it largely reduces the impact of the magnetic dephasing noise. Indeed, this noise has been acknowledged as the main obstacle to achieve long-time coherent dynamics in ion-trap simulators.  Since the realization of the two-photon quantum Rabi model involves second-order sideband processes, the resulting dynamics becomes unavoidably slow in the ultrastrong coupling regime, and thus more exposed to noise. Here we show the suitability of the proposed scheme and discuss how dynamical decoupling methods take a dual role: they suppress the main source of decoherence, while they define the parameter regime of the simulated model.


Quantum – coherent dynamics in photosynthetic charge separation revealed by wavelet analysisQuantum – coherent dynamics in photosynthetic charge separation revealed by wavelet analysis – E. Romero, J. Prior, A. W. Chin, S. E. Morgan, V. I. Novoderezhkin, M. B. Plenio, and R. van Grondelle,
Nature Scientific Reports, 7, 2890
| ArXiv
 This work is licensed under a Creative Commons Attribution 4.0 International License.
The gist of it

Photosynthetic complexes need to absorb photons, transport the resulting excitons to the reaction center where they are then split into separated charges which, further down the line, drive the metabolism of the plants. All this happens under ambient conditions in complexes that are vibrating. In 2010 our group has proposed that the coupling between electronic and vibrational degrees of freedom plays an important role to facilitate some of these processes. Recent experiment using 2D-electronic spectroscopy have shown first evidence of this dynamics but he data were analysed in frequency space which tells you about resonances and coupling rates in the system but not about their temporal evolution. In order to get a better understanding, a paper from our group in 2013 proposed a wavelet analysis which allows us to observe time and frequency resolved information at the same time and hence make the dynamics of the photosynthetic complex visible. In the present work these tools have been put into action to analyse recent experimental results in the PSII reaction center and to provide further support of our hypothesis that vibronic effect support charge separation.


Submillihertz magnetic spectroscopy performed with a nanoscale quantum sensorSubmillihertz magnetic spectroscopy performed with a nanoscale quantum sensor – S. Schmitt, T. Gefen, F. M. Stürner, T. Unden, G. Wolff, C. Müller, J. Scheuer, B. Naydenov, M. Markham, S. Pezzagna, J. Meijer, I. Schwarz, M.B. Plenio, A. Retzker, L. P. McGuinness, and F. Jelezko,
Science, 356, 832 (2017)
Reprinted with permission from AAAS
DOI: 10.1126/science.aam5532
The gist of it

Precise timekeeping is critical to metrology, forming the basis by which standards of time, length and fundamental constants are determined. Stable clocks are particularly valuable in spectroscopy (e.g. nuclear magnetic resonance spectroscopy) as they define the ultimate frequency precision that can be reached. In quantum metrology, where the phase of a qubit is used to detect external fields, the clock stability is defined by the qubit coherence time, which determines the spectral linewidth and frequency precision. Here we demonstrate a quantum sensing protocol where the spectral precision goes beyond the sensor coherence time and is limited by the stability of a classical clock. Using this technique, we observe a precision in frequency estimation scaling in time 𝑇, as 𝑇^(−3⁄2) for classical oscillating fields instead of the typical 𝑇^(−1⁄2). The narrow linewidth magnetometer based on single spins in diamond is used to sense nanoscale magnetic fields with an intrinsic frequency resolution of 607µHz, 8 orders of magnitude narrower than the qubit coherence time.


Scheme for Detection of Single-Molecule Radical Pair Reaction Using Spin in Diamond

Scheme for Detection of Single-Molecule Radical Pair Reaction Using Spin in Diamond
– H. Liu, M. B. Plenio, and J. Cai, Physical Review Letters, 118, 200402 (2017) | ArXiv
The gist of it

It is well established that some birds, such as European Robbins, have a magnetic sense which supports their orientation during migration. What is not known however is the origin of this magnetic sense. There are two main theoretical models, one based on small magnetite particles that may reorient in an external magnetic field and the other based on the idea that upon photo excitation a certain type of molecules in the eye of a bird support a radical pair formed by two electrons which evolve under the joint action of the Zeeman interaction with the external magnetic field and the hyperfine interaction with the supporting molecule. Both are plausible and experiment needs to decide which is true. This is a very challenging task and in this work we propose the use of colour centers in diamond to try and observe the dynamics of the radical pair. The new twist here is that we are looking for the electric field rather than the magnetic field emanating from the radical pair. We show that in principle this should enable to observe some properties of the radical pair reaction such as the recombination rate.



Enhanced Resolution in Nanoscale NMR via Quantum Sensing with Pulses of Finite Duration1

Enhanced Resolution in Nanoscale NMR via Quantum Sensing with Pulses of Finite Duration
– J. E. Lang, J. Casanova, Z.-Y. Wang, M. B. Plenio, and T. S. Monteiro, Physical Review Applied, 7, 054009 (2017)
The gist of it

Nanoscale NMR and MRI employ microwave pulses to decouple the quantum sensor from sources of decoherence and to enhance the weak signals of single nuclear spins. In our work we show that the finite width of these pulses, typically thought of as an error source, can be exploited as a resource for increasing spectral resolution.



Regulating the Energy Flow in a Cyanobacterial Light-Harvesting Antenna ComplexRegulating the Energy Flow in a Cyanobacterial Light-Harvesting Antenna Complex – I. Eisenberg, F. Caycedo-Soler, D. Harris, S. Yochelis, S.F. Huelga, M.B. Plenio, N. Adir, N. Keren, and Y. Paltiel, J. Phys. Chem. B, 121, 1240-1247 (2017)
DOI: 10.1021/acs.jpcb.6b10590
Copyright {2017} American Chemical Society
The gist of it

Photosynthetic organisms are able to thrive in environments in which the light intensities are constantly changing, from extremely bright to weak or diffuse. This is made possible by the existence of molecular mechanisms that can switch between extremely efficient excitation energy transfer in low light conditions to drive light−chemical energy conversion and on the other hand means to quench energy in bright conditions. Here, we show that the cyanobacterial light-harvesting antenna complex may be able to regulate the flow of energy to switch reversibly from efficient energy conversion to photo-protective quenching via a structural change. Our experimental collaborators were able to isolate specific light-harvesting proteins and to measure their optical properties both in solution and in a desiccated state. The result indicate, supported by theoretical modeling, that the energy band structures are changed, generating a switch between the two modes of operation, exciton transfer and quenching. This flexibility can contribute greatly to the large dynamic range of cyanobacterial light-harvesting systems.



Fokker-Planck formalism approach to Kibble-Zurek scaling laws and nonequilibrium dynamics

Fokker-Planck formalism approach to Kibble-Zurek scaling laws and nonequilibrium dynamics – R. Puebla, R. Nigmatullin, T. E. Mehlstäubler, and M. B. Plenio, Physical Review B, 95, 134104 (2017) | ArXiv
The gist of it

A system undergoing a symmetry-breaking second-order phase transition is characterized by the emergence of singular behavior in distinct quantities at a critical value of an external parameter. However, while equilibrium or static properties of these phase transitions are well understood, features of the dynamics when traversing a phase transition at a finite rate are less clear and are object of current research. This includes the study of defect formation, how equilibrium singularities leave their imprint on the resulting nonequilibrium dynamics or how such dynamics are affected by finite size systems, among other aspects. In this context, the celebrated Kibble-Zurek mechanism merits special mention as it successfully predicts the scaling behavior of the formed defects as a function of the rate at which the second-order phase transition is traversed.

In this work we explore the aforementioned scenario in two different models, namely, a one-dimensional Ginzburg-Landau model and a linear to zigzag phase transition in an ion Coulomb crystal. We analyze the nonequilibrium dynamics resulting when traversing at finite rate the second-order phase transition by means of a Fokker-Planck formalism. This formalism allows us to obtain the probabilistic state of the system in a deterministic manner, and therefore, it aims to solve the nonequilibrium dynamics problem at the ensemble rather than at the individual realization level, as is the case in the Langevin approach. Furthermore, we show that the nonequilibrium results are well reproduced when nonlinear terms in both models are neglected, as for example the Kibble-Zurek scaling laws that dictate the dependence of spatial correlations on the quench rate. The developed framework is computationally efficient and enables the prediction of finite-size scaling functions. Additionally, it might be useful to investigate scaling laws of other important quantities in stochastic thermodynamics such as entropy production and work done.



Delayed entanglement echo for individual control of a large number of nuclear spinsDelayed entanglement echo for individual control of a large number of nuclear spins – Z.-Y. Wang, J. Casanova and M. B. Plenio, Nature Communications, 8, 14660 (2017) | ArXiv
This work is licensed under a Creative Commons Attribution 4.0 International License.
The gist of it

Quantum technologies require reliable quantum control on individual elements of microscopic quantum systems. But for potential massive quantum resources such as hundreds of long-lived nuclear spins near the electron of single nitrogen-vacancy (NV) centers in diamond, individual detection and manipulation are challenging.  In this work, we solve the difficulties and problems ahead by using a delayed entanglement operation. With our protocol one can detect, address and control nuclear spins around an electron spin unambiguously and individually in a broad frequency band. Hybrid quantum systems can be naturally incorporated to our scheme for improved performance. Our work would allow large-scale quantum information processing and quantum simulation on nuclear spin qubits, as well as atomic-scale imaging for biomolecules. There are also applications of our method in traditional fields of nuclear magnetic resonance (NMR) and electron-nuclear double resonance (ENDOR), for example, in analysis of chemical shifts and materials.


Open Systems with Error Bounds Spin-Boson Model with Spectral Density Variations

Open Systems with Error Bounds: Spin-Boson Model with Spectral Density Variations – F. Mascherpa, A. Smirne, S. F. Huelga and M. B. Plenio, Physical Review Letters, 118, 100401 (2017) | ArXiv
DOI: https://doi.org/10.1103/PhysRevLett.118.100401
The gist of it

Open quantum systems in harmonic environments are often modeled theoretically and simulated numerically in terms of the spectral density of the bath, which details how strongly the central system couples to each of the environmental modes as a function of the mode frequency. The spectral density is not usually known exactly; in this paper, we investigate the sensitivity of operator expectation values to variations and errors in it, and derive upper bounds on the error affecting the predictions as a function of the spectral deviation considered.


Quantum Machine Learning over Infinite DimensionsQuantum Machine Learning over Infinite Dimensions – H.-K. Lau, R. Pooser, G. Siopsis and C. Weedbrook

Physical Review Letters, 118, 080501 (2017) | ArXiv
DOI: https://doi.org/10.1103/PhysRevLett.118.080501
The gist of it

Machine learning is a data-manipulation techniques that has been increasingly important in e.g. finance and national security.  Recently, it is discovered that quantum computer can reduce the resource requirement of machine learning.  Nevertheless, current quantum machine learning algorithms store information as qubits, which can be implemented on only discrete-variable type of quantum systems.  In this work, we generalize quantum machine learning algorithm to continuous-variable type of quantum system, which could be found in various physical platforms and could store more information in each degree of freedom.  Specifically, we developed a continuous-variable version of exponential-swap operation.  We showed how exponential-swap can be applied to various machine learning tasks, which include Matrix inversion, Principal component analysis, and Vector distance computation.  We also discussed potential optical implementation of the operations.

This work has recently attracted some public attention. For a popular science report, please visit https://phys.org/news/2017-03-physicists-quantum-machine-infinite-dimensions.html


Probing the Dynamics of a Superradiant Quantum Phase Transition with a Single Trapped IonProbing the Dynamics of a Superradiant Quantum Phase Transition with a Single Trapped Ion – R. Puebla, M.-J. Hwang, J. Casanova and M. B. Plenio,
Physical Review Letters, 118, 073001 (2017) | ArXiv

The gist of it

Phase transitions typically take place in the limit of infinitely many particles, the so-called thermodynamic limit. A recent theoretical finding has shown, however, that a quantum phase transition can occur even in finite-component systems of coupled bosons and spins in the limit of extremely large coupling strength and large detuning (see Phys. Rev. Lett. 115, 180404 (2015)). In this work, we demonstrate for the first time that such an extreme parameter regime can be indeed achieved using a single trapped ion with currently available technology and that it is possible to probe the universal properties of the nonequilibrium dynamics of the phase transition. Our work demonstrates that the trapped ion system can serve as an ideal platform to explore the physics of a phase transition both in and out of equilibrium without the daunting task of scaling up the number of ions.

Metastability in the driven-dissipative Rabi model


Metastability in the driven-dissipative Rabi model – A. Le Boité, M.-J. Hwang and M. B. Plenio,
Physical Review A, 95, 023829 (2017) | ArXiv
The gist of it

Experiments in cavity quantum electrodynamics (cavity QED), involving a strong interaction between an atom and a cavity photonic mode, have proved to be very powerful for testing our understanding of the quantum world. In this context, a common way to probe the quantum nature of the interaction between light and matter is to drive the system with a classical light field (such as a laser), and record the statistics of the photons emitted from the cavity. The number of photons in a coherent light field like a laser follows a Poissonian distribution. A sub-Poissonian statistics, which shows less fluctuations in the number of photons, is however a genuine quantum effect and an important evidence of effective photon-photon interactions induced by the atom-cavity coupling.

From a theoretical point of view, the interaction between a two-level atom and a single cavity mode is well captured by the quantum Rabi model. A driven-dissipative version of this model, including the external driving field and the inevitable leakage of photons out of the cavity, is thus well suited to describe the cavity QED experiments mentioned above. Until now, most of the studies have focused on the steady-state of the system, reached when the external driving exactly compensate the different losses mechanisms. In this paper, we go beyond the study of steady-state properties and explore the transient dynamics of the driven-dissipative Rabi model.
In particular, we show that, as the atom-cavity coupling strength becomes larger than the cavity frequency, a new time scale emerges. This time scale, much larger than the natural relaxation time of the atom and the cavity, leads to long-lived metastable states susceptible to being observed experimentally. By systematically investigating the set of possible metastable states, we find that their properties can differ drastically from those of the steady state and relate these properties to the energy spectrum of the Rabi Hamiltonian.



Relations between dissipated work in non-equilibrium process andRelations between dissipated work in non-equilibrium process and the family of Rényi divergences – B.-B. Wei and M. B. Plenio,
New Journal of Physics, 19, 0023002 (2017) | ArXiv
licensed under CC BY 3.0
The gist of it

While the statistical mechanics of thermodynamical system in equilibrium is well understood and taught at undergraduate level, a similar theory for systems that are far off equilibrium has not been established yet. What is, for example, the work done when a system is pushed far away from equilibrium? Even such a seemingly, simple question that has a straightforward answer in equilibrium statistical mechanics is difficult to assess far away from equilibrium. This is a question that we consider in our work. More specifically, we link the dissipated work done on a system driven arbitrarily far from equilibrium to a concept from quantum information science, namely the family of Rényi divergences between initial and final state along the forward and reversed dynamics.


Universal continuous-variable quantum computation without coolingUniversal continuous-variable quantum computation without cooling – H.-K. Lau and M. B. Plenio, Physical Review A, 95, 022303 (2017) | ArXiv
The gist of it

When we do quantum computation, the computer is usually initiated in a known state, i.e., pure state.  If the quantum computer is composed of harmonic oscillators, i.e., continuous-variable quantum computer, it is usually initialised by ground-state cooling, which is unfortunately not an easy process for many quantum systems.  In this work, we show that, surprisingly, quantum computation is possible even if we do not completely know the quantum state, i.e., mixed state, so ground-state cooling is not necessary.  We explicitly propose a two-qumode parity encoding that each qubit is represented by two mixed-state harmonic oscillators, and outline how the quantum logic gates can be implemented.  We show that in some situation, our scheme allows quantum computation at which ground-state cooling is challenging.  Our scheme can also tolerate a wider range of error, and reduce the fundamental initialisation energy, than any pure-state scheme.


Signatures of spatially correlated noise and non-secular effects in two-dimensional electronic spectroscopy

Signatures of spatially correlated noise and non-secular effects in two-dimensional electronic spectroscopy – J. Lim, D. J. Ing, J. Rosskopf, J. Jeske, J. H. Cole, S. F. Huelga and M. B. Plenio,
J. Chem. Phys. 146, 024109 (2017)|ArXiv
The gist of it

Several competing theoretical models have been proposed to explain long-lived quantum coherences in photosynthetic complexes observed by using two-dimensional (2D) electronic spectroscopy. These models consider different vibrational structures, such as correlated fluctuations in dephasing noise and disorder induced by delocalised phonon modes coupled to several pigments, and vibronic features in phonon spectral densities induced by underdamped vibrational modes. Vibronic models have been tested both experimentally and theoretically for many biological and artificial systems, as shown in Nature Comm. 6, 7755 (2015) by our group. However, correlated fluctuation models have received little attention in the context of 2DES simulations, even though recent 2D experiments suggested the presence of correlated fluctuations in some biological and engineered molecular systems. In this work, we theoretically investigate how correlations in the noise affect 2D optical responses with the aim to identify the signatures of correlated fluctuations in 2D electronic spectra.





decoherence-enhanced-performance-of-quantum-walks-applied-to-graph-isomorphism-testingDecoherence-enhanced performance of quantum walks applied to graph isomorphism testing – M. Bruderer and M.B. Plenio,
Phys. Rev. A 94, 062317 (2016)|ArXiv
The gist of it

Are two graphs – the mathematical term for a network of links and nodes – the same, even if they look different at first glance? This supposedly simple problem has been intriguing researchers for decades and crops up in numerous applications. While many ways for distinguishing graphs have been suggested we took an unusual approach:

Imagine there is a particle moving on the graph, that is to say the particle can hop between nodes whenever they are linked. Now one would expect that the motion of the particle depends on the overall shape of the graph. In our paper we show that indeed if the particle is observed for a long time, even if only between two nodes, it is possible to identify the underlying graph. To our surprise we found that this approach works best neither for classical particles nor for particles following the laws of quantum mechanics, but somewhere on the border between the classical and quantum world. This contradicts the established paradigm that quantum is always better and it seems that a good mix between quantum and classical is the best choice for solving some mathematical problems.


tracking-the-coherent-generation-of-polaron-pairs-in-conjugated-polymersTracking the coherent generation of polaron pairs in conjugated polymers – A. De Sio, F. Troiani, J. Rehault, E. Sommer, J. Lim, S.F. Huelga, M.B. Plenio, M. Maiuri, G. Cerullo, E. Molinari, and Ch. Lienau,
Nature Comm.7, 13742 (2016)|ArXiv
licensed under 
CC BY 4.0
The gist of it

Organic solar cells have attracted considerable interest in recent years due to their potential as efficient and low-cost solar energy converters. For various conjugated polymers in solar cell applications, experimental studies have reported ultrafast charge separation at the interfaces between polymers and fullerenes on a sub-100 fs timescale. Theoretical studies based on time-dependent density functional theory suggested that strong electronic and vibronic couplings of organic photovoltaics may support ultrafast charge separation dynamics. However, the functional relevance of the coherent electronic and vibronic couplings under noise and disorder and their experimental verification remain open questions. In this work, we investigate the polaron pair formation in a reference conjugated polymer for solar cell applications by means of two-dimensional electronic spectroscopy. Here the excitons created by light absorption dissociate into polaron pairs that have been considered as the precursors of free charges. We show that experimentally observed polaron pair formation on a sub-100 fs timescale is governed by coherent electronic and vibronic couplings even in the presence of a high noise level at room temperature and a large energetic disorder of the conjugated polymer. We show that a coherent, vibronic model can reproduce the main features of experimental finding, including vibronic-level structures observed in experiments, shown in the figure, which cannot be accounted by an incoherent model where charge separation is governed by a classical rate equation. Based on model parameters estimated from experimental data, we show that non-equilibrium vibrational motions of the C=C stretch mode of the conjugated polymer underpin ultrafast polaron pair formation and that this mechanism is robust against noise and disorder due to the strong electronic coupling and large Huang-Rhys factors of the present system. Our results suggest the possibility that vibronic coupling, which was shown to play a central role in the excitonic dynamics of natural and artificial light-harvesting systems, also plays an important role in the charge separation dynamics in the organic photovoltaics.



A note on coherence power of N-dimensional unitary operatorsA note on coherence power of N-dimensional unitary operators – M. Garcia-Diaz, D. Egloff, and M.B. Plenio,
Quant. Inf. Comp.16, 1282 (2016)|ArXiv
The gist of it

At the heart of quantum mechanics lies the principle that states can be in a linear superposition even while only being observable in discrete quanta. Recently the notion of how much superposed a state is relative to a given basis has been formalized and named coherence theory. This theory hence allows to compare different states in their degree of superposition. In a similar spirit one might now wonder on how much superposing (and hence not classical) a map is and therefore ask how much coherence it can maximally produce; in other words, one might ask what its coherence power is. In this notes we discuss basic properties of the coherence power and show tools that are useful for calculating the coherence power of maps, explicitly giving a new proof for the two-dimensional unitary case and analysing some instructive examples.



a-robust-scheme-for-the-implementation-of-the-quantum-rabi-model-in-trapped-ionsA robust scheme for the implementation of the quantum Rabi model in trapped ions – R. Puebla, J. Casanova, and M.B. Plenio,
New J. Phys. 18, 113039 (2016)|ArXiv
licensed under 
CC BY 3.0

(Here is the link to the video abstract)
The gist of it

The maintenance of quantum coherence is a crucial ingredient to exploit the advantages of quantum settings with respect to their classical counterparts. However, experimental imperfections or, simply, the unavoidable presence of noises severely restrict the quantum coherence to a short duration. Hence, a long-time preservation of the quantum coherence is highly desired although its realization constitutes a formidable task. A number of techniques have been proposed to handle different noise scenarios to maintain quantum coherence during a prolonged time. Among them, we focus on the continuous dynamical decoupling technique, which consists in applying continuous drivings that provide a dressed basis in which the noise effects are largely suppressed, if the noise has vanishingly small frequency components close to the frequency splitting of the new dressed basis. Interestingly, this method allows for a concatenated scheme, i.e., introducing additional continuous drivings to handle different sources of noise, as demonstrated in New J. Phys. 14 113023 (2012) for an electron spin of nitrogen vacancy centers in diamond.

More specifically, in our work we analyze the applicability of the continuous dynamical decoupling technique, in its concatenated configuration, to trapped-ion settings. In particular, we illustrate how to realize a noise-resilient and tunable quantum Rabi model with a trapped-ion. The paradigmatic quantum Rabi model describes the interaction between a single two-level system and a bosonic field mode. Note that despite its apparent simplicity, this model shows a rich variety of physics depending on the parameter regime as the transition from Jaynes-Cummings model to deep strong coupling regime, the relativistic Dirac equation or the emergence of a second-order quantum phase transition. We consider magnetic dephasing noise, which is the main limitation to the quantum coherence in trapped-ions, and fluctuating laser intensities as secondary noise source. Then, by means of detailed numerical simulations, we demonstrate the noise-resilient realization of the quantum Rabi model making use of the CCD scheme in different parameter regimes with respect to its unprotected realization.


pulse-phase-control-for-spectral-disambiguation-in-quantum-sensing-protocolsPulse-phase control for spectral disambiguation in quantum sensing protocols. – J. F. Haase, Z.-Y. Wang, J. Casanova, and M. B. Plenio
Phys. Rev. A 94, 032322 (2016)|ArXiv
The gist of it

Whenever a quantum device (e.g. the electron spin qubit of the NV center in diamond) is used as a sensor to measure parameters of interest, one needs to ensure that the collected data is interpreted correctly. In the case of remaining uncertainties, proper tools are required to eliminate them completely.

In this work, we address the emergence of spurious resonances in pulsed NMR sequences and develop a criterion to identify these fake signals. These resonances occur due to the specific design of the pulse sequences which itself are already meant to suppress errors, however the finite width of the pulses causes the accumulation of the undesired signals. The proposed criterion relays on the free choice of the initial phase relation of the equally weighted superposition of the sensing qubit’s energy eigenstates, which is generally used as the input state in sensing experiments. It turns out, that a variation of this phase relation causes an oscillating amplitude of spurious signals while real signals remain stable. Additionally, we find that the effect of a varying phase relation is equivalent to a variation in phase of the applied pulses, which qualifies the criterion for an easy implementation in existing experimental setups.

To gain a deep understanding of the criterion’s working principle, we motivate it with an approximated analytical calculation and verify its validity with an extensive numerical simulation.



noise-resilient-quantum-computing-with-a-nitrogen-vacancy-center-and-nuclear-spinsNoise-Resilient Quantum Computing with a Nitrogen-Vacancy Center and Nuclear Spins. – J. Casanova, Z.-Y. Wang, and M. B. Plenio
Phys. Rev. Lett. 117, 130502 (2016)|ArXiv
The gist of it

Solid-state platforms as diamond doped with nitrogen impurities present enormous advantages to hold a scalable quantum processor. These systems combine the presence of many stable quantum registers, i.e. nuclear spins, together with the possibility to coherently interact with them via a color impurity, e.g. and NV center implanted on the system, controlled with external radiation. In this work, we present a study of the effect in the hybrid system of an NV-center surrounded by nearby 13C atoms, each of them holding a stable qubit, of the minimal ingredients required for quantum computing. These are, robust sequences of microwave pulses applied on the NV-center, combined with the presence of radio frequency fields for both single qubit operations and internuclear decoupling. Our study finds an appropriate operating regime for the joined action of these tools, and reveals the presence of an experimentally friendly low-energy resonance branch for accurate quantum control of the nearby qubits.


fate-of-photon-blockade-in-the-deep-strong-coupling-regimeFate of photon blockade in the deep strong-coupling regime. – A. Le Boité, M.-J. Hwang, H. Nha and M.B. Plenio, Phys. Rev. A 94, 033827 (2016)|ArXiv
The gist of it

Experiments in cavity quantum electrodynamics (cavity QED), involving a strong interaction between an atom and a cavity photonic mode, have proved very powerful both for testing fundamental laws of quantum physics and implementing quantum information protocols.  One important milestone was to reach an atom-cavity interaction larger than any dissipation rate. In this so-called “strong-coupling regime”, the coupling of light to matter induces effective photon-photon interactions which give rise to interesting nonlinear effects. One of the most famous is the photon blockade effect, where the presence of a single photon inside the cavity is sufficient to inhibit the absorption of another photon. A consequence of the photon blockade is that photons have a tendency to come out of the cavity one by one, which is a purely quantum effect.

The effective photon-photon interaction in a cavity QED system is well captured, in the strong coupling regime, by the Jaynes-Cummings (JC) model. Its energy spectrum has a nonlinear ladder structure responsible for the photon blockade.  Furthermore, the JC model predicts that the nonlinearity of the energy spectrum increases with the coupling strength and that the photon-blockade effect gets more pronounced. We show in this paper that this is no longer true when the coupling strength is increased even further and becomes comparable or larger than the cavity frequency. In this regime, called the “ultrastrong-coupling regime”, the rotating-wave approximation on which the JC model is based is no longer valid and a description of the system based on the Rabi model is necessary.
Within this theoretical framework,  we show  that, as the atom-cavity coupling strength increases, the system undergoes multiple transitions in the photon statistics. In particular, a first  breakdown of the photon blockade is shown to be the consequence of a parity shift in the energy spectrum. A subsequent revival of the photon blockade and the emergence of the quasi-coherent statistics, for even larger coupling rates, are attributed to an interplay between the nonlinearity in the energy spectrum and the transition rates between the eigenstates of the Rabi Hamiltonian.


quantum-phase-transition-in-the-finite-jaynes-cummings-lattice-systemsQuantum phase transition in the finite Jaynes-Cummings lattice systems. – M.-J. Hwang and M.B. Plenio, Phys. Rev. Lett. 117, 123602 (2016)|ArXiv
The gist of it

The Jaynes-Cummings (JC) model is one of the most important model in the field of quantum optics, which describes the interaction between a quantized light and an atom. Ever since the first introduction of the model early in 60s, this exactly solvable model has played a key role in understanding many beautiful quantum optical phenomena observed in the laboratory. This paper reveals yet another fascinating aspect of the JC model that has remained hidden in more than half a century of extensive studies. We demonstrated that the exact solution of the JC model exhibits all the hallmarks of the second-order quantum phase transition (QPT) with a broken continuous symmetry.

In an earlier work done at our institute Phys. Rev. Lett. 115, 180404 (2015), we have demonstrated that the Rabi model may undergo a QPT in a particular limit of large detuning and large coupling strength. This was the first example where the system far from the standard thermodynamic limit of infinite particles exhibits a QPT. Surprisingly, the QPT of the JC model, which has a different symmetry and different microscopic interaction from the Rabi model, occurs in the same limit. Furthermore, in this paper, we also demonstrate that the lattice extension of the JC model, the JC lattice model, also undergoes a QPT in the same limit even when the number of lattice sites is finite. Therefore the current paper together with the earlier work on the Rabi model demonstrates that the finite-system QPT is a generic phenomena of quantum mechanics not limited to a particular symmetry, microscopic interaction, and spatial dimension, suggests that they will be present in a broad range of physical systems, and establishes a general principle of realizing the finite-system QPT.

universal-quantum-computing-with-arbitrary-continuous-variable-encodingUniversal Quantum Computing with Arbitrary Continuous-Variable Encoding. – H.-K. Lau and M.B. Plenio, Phys. Rev. Lett. 117, 100501 (2016)|ArXiv
The gist of it

When we build a quantum computer, the first thing we need to specify is the quantum states that represent the quantum bit (qubit) of information, 0 and 1.  If the quantum system have energy eigenstates with specific properties, such as the electronic states of trapped ions and diamond NV centres, the qubit states can be two of the energy eigenstates, and we can control them one-by-one.  On the other hand, there are quantum systems that behave like harmonic oscillators, e.g. atom ensembles and mechanical oscillators.  These systems have infinitely many energy eigenstates, but it is difficult to single out two of them as a qubit.  In the literature, quite a number of “encodings” have been proposed to represent a qubit by specially-chosen superpositions of multiple eigenstates.  For examples, the qubit states could be vacuum and single photon states, or two coherent states with different amplitude, or two Schrodinger’s Cat states.  Due to the difference of physical behaviours, each encoding is advantageous in some specific parts of quantum computation.  However the difference of encoding also diversifies the architecture of quantum computers, because the quantum computer logic gates have to be implemented by the interaction that depends on the encoding.

In our work, we ask if exists a unified set of interaction that perform the logic gates for every encoding.  Surprisingly, the answer is yes.  Our idea is to represent the qubit by the parity of any two encoding states.  We show that all quantum logic gates can be preformed by the newly found exponential-swap interaction.  Because the interaction is the same, our method unifies the architecture of harmonic oscillator quantum computer.  Our scheme also brings additional benefits that different computational parts could employ different encodings, and the quantum information can be protected from a typical class of noise: collective noise

energy-based-scheme-for-reconstruction-of-piecewise-constant-signals-observed-in-the-movement-of-molecular-machinesAn Energy Based Scheme for Reconstruction of Piecewise Constant Signals observed in the Movement of Molecular Machines. – J. Rosskopf, K. Paul-Yuan, M.B. Plenio and J. Michaelis
Phys. Rev. E 94, 022421 (2016)|ArXiv
The gist of it

Analyzing the physical and chemical properties of single DNA based molecular machines such as polymerases and helicases requires to track stepping motion on the length scale of base pairs. Although high resolution instruments have been developed that are capable of reaching that limit, individual steps are oftentimes hidden by experimental noise which complicates data processing. Here, we present an effective two-step algorithm which detects steps in a high bandwidth signal by minimizing an energy based model (Energy based step-finder, EBS).


First, an efficient convex denoising scheme is applied which allows compression to tuples of amplitudes and plateau lengths. Second, a combinatorial clustering algorithm formulated on a graph is used to assign steps to the tuple data while accounting for prior information. Performance of the algorithm was tested on poissonian stepping data simulated based on published kinetics data of RNA Polymerase II (Pol II). Comparison to existing step-finding methods shows that EBS is superior in speed while providing competitive step detection results especially in challenging situations. Moreover, the capability to detect backtracked intervals in experimental data of Pol II as well as to detect stepping behavior of the Phi29 DNA packaging motor is demonstrated.


Excited-state quantum phase transition in the Rabi model
Excited-state quantum phase transition in the Rabi model
. – R. Puebla, M.-J. Hwang and M.B. Plenio
Phys. Rev. A 94, 023835 (2016) |ArXiv
The gist of it

A two-level system coupled to a single harmonic oscillator, which is known as Rabi model, can undergo a quantum phase transition. This phase transition takes place when the ratio between the two-level transition frequency is much larger than the oscillator frequency and in the regime of a very strong coupling strength, as shown in M.-J. Hwang et al, Phys. Rev. Lett. 115, 180404 (2015). Here, we show that the quantum phase transition accompanies critical behavior in higher energy excited states, known as excited-state quantum phase transition. The main hallmark of excited-state quantum phase transitions consists in a singular density of states at a critical excitation energy rather than at a critical value of an external control parameter. With help of a semiclassical approach, we find analytically the density of states, which diverges logarithmically as the excitation energy approaches to its critical value. Additionally, relevant observables, such as boson number or two-level polarization, display singularities as a consequence of the diverging density of states at the critical excitation energy.

This work shows that excited-state quantum phase transitions can be realized in a simple quantum system, as it is a single two-level system interacting with a single bosonic field, without resorting to the thermodynamic limit, i.e. the traditional limit of infinitely many system components.



Picture1Coherent control of quantum systems as a resource theory. – J.M. Matera, D. Egloff, N. Killoran, and M.B. Plenio
Quantum Sci. Technol. 1, 01LT01 (2016)|ArXiv
The gist of it

While controlling a quantum system is a standard task nowadays, we are still far away from developing quantum computers, and one might wonder what is the difference between the two. Qualitatively the difference is that for quantum computing one needs to control quantum systems in a quantum way, using quantum systems instead of directly using the large apparata or (classical) electromagnetical fields that often are enough to control a quantum system directly. In this letter we make this idea precise by building a theory which allows us to quantify the usefulness of controlling a quantum system through a quantum system instead of using a classical one.

Realising a quantum absorption refrigerator with an atom-cavity system
. – M. Mitchison, M. Huber, J. Prior, M.P. Woods and M.B. Plenio
Quantum Sci. Technol. 1, 015001 (2016)|ArXiv
licensed under CC BY 3.0
The gist of it

Cooling of atomic motion is an essential precursor for many interesting experiments and technologies, such as quantum computing and simulation using trapped atoms and ions. In most cases, this cooling is performed using lasers to create a kind of light-induced friction force which slows the atoms down. This process is often rather wasteful, because lasers use up a huge amount of energy relative to the tiny size of the atoms we want to cool. Here, we propose to solve this problem using a quantum absorption refrigerator: a machine that is powered only by readily available thermal energy, such as sunlight, as it flows through the device. We describe how to build such a refrigerator, and predict that sunlight could actually be used to cool an atom to nearly absolute zero temperature. The refrigerator works by trapping the sunlight between two mirrors, in such a way that every single photon makes a significant contribution to the friction force slowing the atom down. Similar schemes could eventually be important for reducing the energy cost of cooling in future quantum technologies.

Laser cooling of a high-temperature oscillator by a three-level system
Laser cooling of a high-temperature oscillator by a three-level system
– H.-K. Lau and M.B. Plenio,
Phys. Rev. B. 94, 054305 (2016)|ArXiv
The gist of it

To observe the quantum effect, we usually need to cool an object to nearly the ground state.  If the object is interacting with a two-level system that is dissipative, then we can apply laser cooling.  Although different kinds of object, ranging from singly trapped atom to visibly large mechanical oscillator, have been successfully cooled by laser cooling, it is believed to be efficient only under demanding conditions.  One such condition is that the background temperature is sufficiently low that satisfies the so-called Lamb-Dicke regime.  Surprisingly, it was recently found that laser cooling is sometimes possible even when the background temperature is higher than the Lamb-Dicke regime.  In our work, we study this phenomenon more closely.  We start from developing a technique to separate the classical part from the quantum part of the dynamics of the object, so the system is easier to study.  Then we extend previous analysis to three-level systems, which are more easily found in the real world.  We confirm previous observation of high temperature laser cooling, and provide a modified criterion to judge when laser cooling is efficient.  Surprisingly, we also find some situations that laser cooling can cool the object only for a limited time, after that the object would be heat up significantly

High-sensitivity magnetometer using a single atom

Ultrasensitive magnetometer using a single atom
. – I. Baumgart, J.M. Cai, A. Retzker, M.B. Plenio and Ch. Wunderlich,
Phys. Rev. Lett. 116, 240801 (2016)|ArXiv
The gist of it

The development of highly sensitive methods for the detection of minute fields lies at the heart of quantum metrology and has, over the history of science, led to many discoveries. This motivates the continuous drive towards the development of ever more sensitive metrology methods. Of particular interest in this context are atoms and ions that are trapped in ultra-high vacuum as they can be isolated to a remarkable degree from environmental influences. Still noise will impose limitations and needs to be addressed.

We demonstrate a novel method for sensing magnetic fields and demonstrate that it can achieve the best sensitivity ever realized for a single trapped atomic particle and it can do so over a broad range of frequencies. State-of-the-art magnetometers reach their best sensitivity in a limited frequency-band or do not work at all (for all practical purposes) outside a certain frequency range. The type of magnetometer introduced here could be used to detect fields from direct-current to the gigahertz regime – an unprecedented range of frequencies – using an atom confined to a nanometer-sized region in space. Moreover, the magnetometer is essentially immune against magnetic disturbances and reaches a sensitivity close to the standard quantum limit.


Quantum metrology enhanced by repetitive quantum error correction
Quantum metrology enhanced by repetitive quantum error correction. – Th. Unden, P. Balasubramanian, D. Louzon, Y. Vinkler, M.B. Plenio M. Markham, D. Twitchen, I. Lovchinsky, A.O. Lovchinsky, M.D. Lukin, A. Retzker, B. Naydenov, L. McGuinness and F. Jelezko,
Phys. Rev. Lett. 116, 230502 (2016)|ArXiv
The gist of it

In sensing and metrology noise is a fundamental problem because it randomises the sensor and therefore limits its precision. Quantum information science has found different ways to reduce the impact of noise and one of these is quantum error correction. Typically, quantum error correction requires a significant overhead in qubits, one needs at least three qubits to encode and protect a single qubit against phase noise. But this overhead can be reduced when combining stable qubits for encoding with error prone qubits for sensing. This has been achieved experimentally in our recent work where a sensing electron spin is combined with a encoding nuclear spin and repeated error correction has been achieved in combination with sensing.

Diamond Quantum Devices in BiologyDiamond Quantum Devices in Biology. – Y. Wu, F. Jelezko, M.B. Plenio and T. Weil
Angewandte Chemie – International Edition Minireview 55, 6586 – 6598 (2016)
The gist of it

The Institute of Theoretical Physics is Corresponding PI of the ERC Synergy Grant BioQ “Diamond Quantum Devices and Biology” which is looking in the development of quantum technologies for studying biology, the impact of quantum dynamics on biological function and the use of self-organised structures for creating quantum devices. This invited minireview is summarizing recent developments in the first and the third topic (for a review on the second topic see Contemp. Phys. 54, 181 (2013)).


Necessary and sufficient condition for quantum adiabatic evolution by unitary control fieldsNecessary and sufficient condition for quantum adiabatic evolution by unitary control fields. – Z.-Y. Wang, M.B. Plenio,
Phys. Rev. A 93, 052107 (2016)|ArXiv
The gist of it

This work derives the necessary and sufficient condition for quantum adiabatic evolution, that is, when a system remains in an eigenstate of a Hamiltonian even if this Hamiltonian changes over time. It settles a problem that had been identified in the conditions for adiabatic quantum evolution that had been formulated over the last 50 years or so. With the finding that the widely used quantum adiabatic quantitative condition is not always valid, various new necessary or sufficient adiabatic conditions were proposed for replacing the traditional one. However, none of these conditions has been successfully shown to be both necessary and sufficient. Our work settles the problem by providing a simple condition with proofs for both its necessity and sufficiency, by using a gauge invariant formalism to extract all the nonadiabatic transitions. Counterintuitively, the condition reveals that quantum adiabatic evolution allows rapid changes and/or arbitrary numbers of energy crossings in the system Hamiltonian. New ways to achieve quantum adiabatic evolution by pulse sequences or fast varying fields are demonstrated.



summary_PRB_93_174104 (003)Positioning nuclear spins in interacting clusters for quantum technologies and bioimaging. – Z.-Y. Wang, J. F. Haase, J. Casanova, and M. B. Plenio,
Phys. Rev. B 93, 174104 (2016)|ArXiv
The gist of it

In this paper we present a method to detect, locate, and control individual nuclei in interacting clusters by using a single nitrogen-vacancy (NV) center and external control provided by microwave and radio-frequency radiation. The method overcomes the inability of previous schemes to address individually spins that are interacting. It also does not suffer from serious technical drawbacks related to the reorientation of the external alignment field, which affects the NV readout and initialization and is time consuming in current laboratory setups. Detailed numerical simulations demonstrate the precision that our method can achieve by resolving 3D structures of nuclear spins in complex ensembles with Angstrom fidelity. With this information at hand, different applications that include quantum information processing using clusters of carbon-13 nuclei in diamond or identification of the stereoisomeric forms of molecules are available. Note that the former paves the way to exploit the exceptional properties of nuclear spins as solid-state quantum registers while, the latter, corresponds to a question that could previously be answered only when billions of these molecules were available.

Sensing in the presence of observed environments

Sensing in the presence of observed environments.
– M.B. Plenio and S.F. Huelga,
Phys. Rev. A 93, 032123 (2016)|ArXiv
The gist of it

Sensing and metrology in the presence of environmental noise represents a considerable challenge because the noise limits the observation time and hence the achievable precision. In this work we explore the idea what may happen when we have limited access to the environment, e.g. we can measure whether the sensor has spontaneously emitted a photon. This information alone may improve the achievable precision quite considerably and the present work shows this at the hand of some examples.


Recurrent Delocalization and Quasiequilibration of Photons in Coupled Systems in Circuit Quantum Electrodynamics
Recurrent Delocalization and Quasiequilibration of Photons in Coupled Systems in Circuit Quantum Electrodynamics.
– Myung-Joong Hwang, M. S. Kim, and Mahn-Soo Choi,
Phys. Rev. Lett. 116, 153601 (2016)|ArXiv
The gist of it

Charged particles can interact with each other, or feel the presence of other charged particles, through the Coulomb interaction. While such an interaction mechanism does not naturally exist for photons, it is well-known that a single atom placed inside of a cavity can mediate an interaction among photons inside of the cavity. The magnitude of this induced photon-photon interaction is often thought to monotonically increase as a function of the atom-photon interaction strength. Contrary to this belief, we show in our work that beyond a certain threshold value, the stronger atom-photon interaction starts to reduce the induced photon-photon interaction. This rather counter-intuitive property of the induced photon-photon interaction is discussed in the context of the photon population dynamics in a coupled cavity array where there occurs a double dynamical transition from a delocalization to localization, back to delocalization, of the photon population. Moreover, it is found that the second delocalization dynamics shows a quasi-equilibration despite of being a closed, finite quantum system.


Ultimate precision limits for noisy frequency estimationUltimate precision limits for noisy frequency estimation. – A. Smirne, J. Kołodynski, S.F. Huelga and R. Demkowicz-Dobrzanski,
Phys. Rev. Lett. 116, 120801 (2016)|ArXiv
The gist of it

How precisely can we estimate the value of an unknown parameter? In classical experiments involving N sensing particles, i.e. N probes, the best estimation strategies lead to an error (as measured by the variance), which scales at most as 1/N, according to the central limit theorem. On the other hand, the use of entangled states can yield a further factor 1/N of improvement, which shows in a paradigmatic way that quantum features can be exploited to get a significant advantage compared to any classical strategy. Such a quantum advantage is nevertheless jeopardized by the interaction of the probing system with the surrounding environment. Previous results showed that the quantum and classical strategies become completely equivalent in the presence of random fluctuations of the parameter to be estimated, due to the influence of a fast-decaying environment.

In this work, we show how the advantage provided by using entangled states can be (partially) re-established, if one deals with a more general and more realistic type of system-environment interactions. Classical strategies can be outperformed if the probes are measured on time-scales short enough, in order to access the universal dynamical regime of open systems, where the survival probabilities decay less than linearly with time. In particular, we derive a lower bound to the estimation error, which holds for a wide and well-defined type of dynamics and we show its attainability, as well as pointing out the crucial dynamical features, which discriminate between classical or super-classical limits to the parameter estimation.


la boite


Power laws in the dynamic hysteresis of quantum nonlinear photonic resonators. – W. Casteels, F. Storme, A. Le Boité, and C. Ciuti,
Phys. Rev. A 93, 033824 (2016)|ArXiv
The gist of it

In a wide class of systems, sweeping back and forth the driving amplitude of a nonlinear resonator produces a hysteresis cycle, which for electromagnetic resonators is referred to as optical bistability. For weak nonlinearities, it can be described by a semiclassical approach neglecting quantum fluctuations. It is known that quantum fluctuations induce switching between two classically stable branches. The steady-state is then unique and consists of a statistical mixture of the two branches. in other words, in the quantum regime (for large nonlinearities), there is no static hysteresis.
We show in this work that there is nonetheless a dynamic hysteresis in the quantum regime. By sweeping the driving amplitude in a finite time we show that the area of the hysteresis cycle exhibits a rich temporal power-law behavior, qualitatively different from semiclassical predictions. We connect this behavior to a nonadiabatic response of the system and establish a link with the Kibble-Zurek mechanism for quenched phase transitions.



Practical Entanglement Estimation for Spin-System Quantum Simulators.png
Practical Entanglement Estimation for Spin-System Quantum Simulators.
– O. Marty, M. Cramer and M.B. Plenio,
Phys. Rev. Lett. 116, 105301 (2016)|ArXiv
The gist of it

Strong entanglement, a quintessential feature of quantum mechanics, is typically a signature of a successfully implemented quantum simulator. Indeed, the aim of these experiments is the preparation and control of systems which show genuine quantum features. For example, recent experiments with trapped ions were designed to implement critical systems – a scenario which is known to become hard to simulate numerically with increasing number of spins due to the presence of large quantum correlations. In order to determine the entanglement in an experiment, one has to take into account that measurements to extract the required information are not only limited by the specifications of the different experimental platforms but, more generally, also by the large dimension of the Hilbert space. A familiar choice to overcome these issues are entanglement witnesses. In our work we determine entanglement witnesses that are quantitative in the sense that they determine lower bounds to the logarithmic negativity, a widely used entanglement measure, that can be obtained to reveal the behavior of entanglement across quantum phase transitions experimentally. In particular, we describe how these witnesses can be measured, e.g., using a short sequence of gate operations and we study the robustness of these measurements in the presence of noise.


The electronic spin of nitrogen-vacancy (NV) centers in diamond is extremely sensitive to local magnetic fieldsSensing of single nuclear spins in random thermal motion with proximate nitrogen-vacancy centers. – M. Bruderer, P. Fernández-Acebal, R. Aurich and M. B. Plenio,
Phys. Rev. B 93, 115116 (2016)|ArXiv
The gist of it

The electronic spin of nitrogen-vacancy (NV) centers in diamond is extremely sensitive to local magnetic fields, which makes them a valuable tool for sensing nuclear and electronic spins in their vicinity. While the detected spin targets are static in most applications, the situation is entirely different for ambient thermal conditions as encountered, for example, in biological environments. In this case, the spin-carrying molecules and thus the spins undergo random diffusive motion. In our work we simulate the motion of diffusive spins for specific molecules and derive effective models for the interaction between NV centers and spin targets. We find in particular that spins undergoing rapid diffusion in a restricted domain behave as “averaged” spins, which can still be detected under realistic experimental conditions.


Universality in the Dynamics of Second-Order Phase TransitionsUniversality in the Dynamics of Second-Order Phase Transitions. – G. Nikoghosyan, R. Nigmatullin and M.B. Plenio,  Phys. Rev. Lett. 116, 080601 (2016)|ArXiv

DOI:https://doi.org/10.1103/PhysRevLett.116.080601 The gist of it

Symmetry breaking phase transitions and their dynamics are at the heart of a broad range of physical phenomena ranging from the physics of the early universe to solid state physics. Of particular interest are dynamical properties when traversing such a symmetry-breaking second-order phase transition at a finite rate as then spatially separated parts of the system may chose symmetry-broken phases independently and where such choices are incompatible, topological defects form whose number dependence on the quench rate is given by simple power laws.

Typically these powerlaws are derived by physical arguments which works well in simple cases but can lead to wrong conclusions in complex situations involving for example spatial inhomogeneities. We propose a general approach for the derivation of such scaling laws that is based on the analytical transformation of the associated equations of motion to a universal form rather than employing plausible physical arguments. We demonstrate the power of this approach by deriving the scaling of the number of topological defects in both homogeneous and nonhomogeneous settings.



Resonance-inclined optical nuclear polarization of liquids in diamond structures.
– Q. Chen, I. Schwarz, F. Jelezko, A. Retzker and M.B. Plenio,
Phys. Rev. B 93, 060408(R) (2016)|ArXiv
The gist of it

Nuclear spin hyperpolarization (DNP) is a key emerging method for increasing the sensitivity of nuclear magnetic resonance (NMR). Using DNP, a wide range of novel applications in biomedical sciences is made possible, such as metabolic MR imaging or the characterization of molecular chemical compositions. The prevalent methods for achieving DNP in solutions are typically most effective in the regime of small interaction correlation times between the electron and nuclear spins, limiting the size of accessible molecules. To solve this limitation, we design a mechanism for DNP in the liquid phase that is applicable for large interaction correlation times (e.g. slow-moving molecules). We combine this scheme with optically polarized nitrogen-vacancy (NV) center spins in diamonds which provides near perfect electron polarization source at room temperature. Considering the model in a flow cell containing nanodiamonds immobilized in a hydrogel, numerical illustration shows flowing water molecules can be polarized over 1000-fold, in sufficient volumes for detection by current NMR scanners.


Converting non-classicality into entanglement

Converting non-classicality into entanglement. – N. Killoran, F.E.S. Steinhoff and M.B. Plenio,
Phys. Rev. Lett. 116, 080402 (2016)|ArXiv
The gist of it

There is a longstanding discussion about the question where exactly the border between the classical and the quantum world might lie and depends very much on which features of the world we define as non-classical in the first place. Entanglement is one of the few features that we tend to agree upon as being non-classical. Hence we decided to attack the question of non-classicality from the viewpoint of entanglement by examining which states can be interconverted into entangled states. The results that we obtain generalize and link convertibility properties from the resource theory of coherence, spin coherent states, and optical coherent states, while also revealing important connections between local and nonlocal  pictures of non-classicality.



Efficient simulation of non-Markovian system-environment interactionEfficient simulation of non-Markovian system-environment interaction. – R. Rosenbach, J. Cerrillo, S.F. Huelga, J.S. Cao and M. B. Plenio,
New J. Phys. 18, 023035 (2016)|ArXiv
licensed under CC BY 3.0
The gist of it

The numerical simulation of the interaction of electronic degrees of freedom with their environment is highly challenging when we want to know precise answers and when the environment has a complicated structure so that the evolution cannot be approximated by master equations. This is just the regime in which biological excitation energy transfer in photosynthetic complexes is taking place. In our group we had developed earlier the TEDOPA method while Javier Cerrillo, a former PhD student in the group, had developed another approach, the memory kernel method. During a visit at MIT and a return visit at Ulm we decided to merge the two methods to combine their strengths and the present paper is the result.

Dynamical error bounds for continuum discretization via Gauss quadrature rules – a Lieb-Robinson bound approach
Dynamical error bounds for continuum discretization via Gauss quadrature rules – a Lieb-Robinson bound approach.
– M.P. Woods and M.B. Plenio,
J. Math. Phys. 57, 022105 (2016)|ArXiv
The gist of it

The interaction of a quantum system with its environment is of fundamental importance for the development of quantum technologies. The spin-boson model in which a spin degree of freedom couples to a continuum of harmonic oscillators is one of the most fundamental models in this area. A continuum of harmonic oscillators is not easy to treat numerically and ideally one would like to reduce them to a finite number of harmonic oscillators, i.e. to discretize the harmonic environment. This will make us to commit errors that we need to bound. Our work uses a chain mapping that we had developed earlier together with Lieb-Robinson type methods which limit the speed at which perturbation travel through such chains to develop such error bounds.

mediumPhase-dependent exciton transport and non-equilibrium energy harvesting from thermal environments – S. Oviedo, J. Prior, A.W. Chin, R. Rosenbach, S.F. Huelga and M.B. Plenio,
Physical Review A 93, 020102(R) (2016)|ArXiv
The gist of it

When a physical system in an excited state is coupled to a cold environment, typically the direction of energy flow is from “hot” system to “cold” environment. This situation changes when the environment is non-Markovian, that is, when its spectral distribution is not flat and when the system exhibits quantum coherence. In this work we show that under these circumstance uphill energy transport is possible, i.e. for the system to absorb energy temporarily from the colder environment thus violating detailed balance.

Formation of helical ion chains
Formation of helical ion chains
– R. Nigmatullin, A. del Campo, G. De Chiara, G. Morigi, M.B. Plenio and A. Retzker,
Physical Review B 93, 014106 (2016)|ArXiv
The gist of it

Symmetry breaking phase transitions and their dynamics are at the heart of a broad range of physical phenomena ranging from the physics of the early universe to solid state physics. Of particular interest are dynamical properties when traversing such a symmetry-breaking second-order phase transition at a finite rate as then spatially separated parts of the system may chose symmetry-broken phases independently and where such choices are incompatible, defects form whose number dependence on the quench rate is given by simple power laws.

Here we study the dynamics of symmetry breaking phase transitions in the formation of helical ion strings that are being held in ion traps that are rotationally invariant around one trap axis. The structures that can occur can be visualised by taking a belt from your trouser and holding it front of you with both hands. Turning one end of the belt creates a helical structure with a winding number that depends on the number of turns. When you go through a phase transition in which the planar structure becomes unstable, we will end up with a finite winding number depending on how fast we traverse the phase transition. We derive analytical scaling laws and confirm them with extensive numerical simulations.



Optically_induced_dynamics_1Optically induced dynamics nuclear spin polarization in diamond – J. Scheuer, I. Schwarz, Q. Chen, D. Schulze-Sünninghausen, P. Carl, P. Höfer, A. Retzker, H. Sumiya, J. Isoya, B. Luy, M.B. Plenio, B. Naydenov, and F. Jelezko
 New Journal of Physics 18, 013040 (2016)|ArXiv
The gist of it

Magnetic resonance imaging (MRI) detects the tiny magnetic moment generated by the nuclear spin of hydrogen atoms in your body. Its sensitivity depends strongly on nuclear spin polarisation, i.e. the magnetisation of the body. Motivated by this observation, dynamical nuclear spin polarisation (DNP), the transfer of electron spin polarization to nuclear spins, has recently been applied to enhance MRI signal. Typically, large signal enhancements (hyperpolarization) require cryogenic temperatures (< 2 K) and bulky, specialized equipment for achieving first a high electron polarization. The Nitrogen-Vacancy defect (NV centre) in diamonds and diamond nanocrystals (nanodiamonds) provides a unique alternative for DNP as the NV centre electron spin can be optically polarized to over 90% polarization at room temperature by short laser pulses.
In our paper we demonstrate an efficient scheme that realises laser induced 13C nuclear spin hyperpolarization in a bulk diamond at room temperature and low ambient magnetic field. Importantly, our protocol is robust against an unknown alignment of the diamond crystal, making this a viable protocol for optical polarization of solutions of nanodiamond. This result gives rise to the possibility of using nanodiamonds, suitably treated to attach themselves to targets that we would like to detect, as MRI markers for molecular imaging applications.


Dynamical nuclear polarization using multi-colour control of color centers in diamondDynamical nuclear polarization using multi-colour control of color centers in diamond – P. Yang, M.B. Plenio and J.M. Cai
EPJ Quantum Technology 3, 1 (2016)
The gist of it

Using laser light it is possible to polarize the electron spin of a colour center in diamond even at room temperature. Resonant microwave radiation can then be used to transfer this polarization to surrounding nuclear spins. Repeating this procedure many times over can lead to very strongly (hyper)-polarized nano-diamonds that may be useful as a contrast agent in MRI. One of the challenges is the random orientation of nanodiamonds in a solution which makes it difficult to achieve resonance with microwaves. In this work we explore the use of microwave frequency combs, i.e. the use of many microwave frequencies to achieve better performance.


* All figures on this page are taken from the corresponding papers


Accelerated 2D magnetic resonance spectroscopy of single spins using matrix completionAccelerated 2D magnetic resonance spectroscopy of single spins using matrix completion-J. Scheuer, A. Stark, M. Kost, M. B. Plenio, B. Naydenov, and F. Jelezko
Scientific Reports 5, 17728 (2015) | ArXiv
licensed under CC BY 4.0
The gist of it

Two dimensional spectroscopy suffers from huge data acquisition times, that make it unattractive for a lot of applications. However, tailored optimization routines lead to a significant acceleration in this step. One of the most promising optimization ideas is the completion of redundant matrices from only a few of its entries. This so called matrix completion can be implemented in various algorithms. In this paper we demonstrate the application of the so called singular value thresholding algorithm to enable time efficient 2D magnetic resonance spectroscopy with Nitrogen Vacancy (NV) centers in diamond, speeding up the data acquisition by a factor of 5-10.


Optical hyperpolarization of 13C nuclear spins in nanodiamond ensemblesOptical hyperpolarization of 13C nuclear spins in nanodiamond ensembles -Q. Chen, I. Schwarz, F. Jelezko, A. Retzker, and M. B. Plenio
Physical Review B 92, 184420 (2015) | ArXiv
The gist of it

Magnetic resonance techniques enables a wide range of novel applications in chemistry, physics and biomedical sciences, such as powerful imaging tools which have revolutionized medicine. In the past decades, dynamical nuclear polarization (DNP) has been utilized to get orders of magnitude enhancements of nuclear magnetic resonance signals. However, current implementations of DNP require cryogenic temperatures and long times for achieving high polarization. Here we propose and analyze in detail protocols that can achieve rapid hyperpolarization of 13C nuclear spins in randomly oriented ensembles of nanodiamonds at room temperature. Our basic idea is transfer of the optical polarization of electron spins in nitrogen-vacancy centers to 13C nuclei. Severe challenges are posed by the random orientation of the nanodiamonds and their nitrogen-vacancy centers. We address these challenges by a combination of (1) an off-resonant microwave double resonance scheme in conjunction with (2) a realization of the integrated solid effect. Together with adiabatic rotations of the external magnetic fields or rotations of the nanodiamonds leads to over 10,000-fold enhancement in the 13C polarization. This high levels of hyperpolarization, together with the long nuclear-spin polarization lifetimes in nanodiamonds and the relatively high density of 13C nuclei, turn functionalized and hyperpolarized nanodiamonds into attractive MRI probes for molecular imaging both in vitro and in vivo.


Quantum Phase Transition and Universal Dynamics in the Rabi Model


Quantum Phase Transition and Universal Dynamics in the Rabi Model – M.-J. Hwang, R. Puebla, and M. B. Plenio
Phys. Rev. Lett. 115, 180404 (2015) | ArXiv
The gist of it

Quantum phase transition describes an abrupt change in ground state properties of a quantum system upon a change of a system parameter. The phase transition is generally held to occur in a system where the number of system components is infinite, realizing the so-called thermodynamic limit. In this paper we have challenged this notion by showing that even a single two-level atom coupled to a single harmonic oscillator can undergo a quantum phase transition. It is identified that an atomic transition frequency that is much larger than an oscillator frequency in the regime of a very strong coupling strength can play the same role of the traditional thermodynamic limit achieved by the many atoms. Moreover, we have also confirmed that its adiabatic dynamics is governed by a Kibble-Zurek mechanism, which is a common trait of a critical system undergoing a phase transition.



enhancing-light-harvesting-power-pngEnhancing light-harvesting power with non-classical vibrational interactions: A quantum heat engine picture – N. Killoran, S. F. Huelga and M. B. Plenio
J. Chem. Phys. 143, 155102 (2015)| ArXiv
The gist of it

Biological systems are very small and highly sophisticated thermodynamic engines that are operating in a regime in which quantum fluctuations are not completely negligible anymore.
In this work we are taking this viewpoint seriously and study an aspect of the energy conversion process from sunlight to chemical energy in the presence of exciton-vibrational coupling. To this end we design a prototypical quantum heat engine which defines carefully all quantities and then we proceed to demonstrate quantitatively the increase in power that is achievable due to the presence of exciton-vibrational coupling.




High-fidelity quantum simulation using hybrid dressed states
High-fidelity quantum simulation using hybrid dressed states
– J.M. Cai, I. Cohen, A. Retzker and M. B. Plenio
Physical Review Letters 115, 160504 (2015) | ArXiv
The gist of it

Trapped ions represent a promising technology for the implementation of quantum information processing. However, a wide variety of noise sources reduce the fidelity of quantum gates and require careful attention. Besides the improvement of the hardware, software solutions, that is improved protocols, represent important steps towards ion trap based quantum information processing. Each such strategy tends to have strengths and weaknesses and happy marriages of several techniques has the potential of achieving significantly improved performance. In this work we combine two approaches, continuous dynamical decoupling and decoherence free subspaces to combine their advantages. We show that this combination can be used to achieve a high fidelity quantum simulation.


Robust dynamical decoupling sequences for individual-nuclear-spin addressing
Robust dynamical decoupling sequences for individual-nuclear-spin addressing
– J. Casanova, Z.-Y. Wang, J.F. Haase, and M. B. Plenio
Physical Review A 92, 042304 (2015) | ArXiv
The gist of it

Nuclear spins are exceptional candidates to store and process quantum information. Our method generates highly selective filter functions ensuring individual addressing and control of nuclear spins in realistic scenarios. In the same way, when applied to an NV center system implanted close to the diamond surface, our protocol is of relevance for the precise identification and localization of nuclear spins in large molecules located on top of the diamond. Therefore, with our work we are paving the way to the control of robust large-scale quantum platforms and the performance of magnetic resonance imaging of individual molecules.



Optical signatures of quantum delocalization over extended domains in photosynthetic membranes is witnessed by polarized light excitationOptical signatures of quantum delocalization over extended domains in photosynthetic membranes is witnessed by polarized light excitation – C. Schroeder, F. Caycedo-Soler, S.F. Huelga, and M.B. Plenio, J. Phys. Chem.  A 119, 9043 – 9050 (2015)                                                 The gist of it

Electronic excitations in photosynthetic membranes occur when a biomolecule (pigment) is excited by light. Many pigments constitute these membranes, which due to their mutual interaction, are able to be excited collectively. Based on Quantum Mechanics, the collective behavior results in an electronic excitation delocalized over these interacting pigments. Historically, excitonic delocalization has been observed to occur between pigments that strongly interact. A clear border that states how strong the interaction must be in order to set a size for the excitons, is still under debate. In our paper we demonstrate theoretically that the excitonic delocalization in photosynthetic membranes can extend over greater domains than previously assumed. These excitons were thought to reside over single ring-like harvesting structures in purple bacteria. As we show here, these excitons rather involve several light-harvesting rings, in a phenomenon that can be verified unambiguously through polarized absorption spectra. This work opens a way to experimentally verify how optical observables are a key tool to progress in the understanding of the interplay between classical and quantum mechanics in the biologically relevant structures for the photosynthesis of purple bacteria.


Simulating Bosonic Bath with Error BarsSimulating Bosonic Bath with Error Bars – M. P. Woods, M. Cramer, and M. B. Plenio
Physical Review Letters 115, 130401 (2015) | ArXiv
The gist of it

The simulation of the dynamics of a quantum system with a structured environment is a topic of longstanding interest as it has a broad range of applications ranging from the simulation of quantum technologies to the study of quantum effects in biological systems. In 2010 our group has developed a new approach, TEDOPA,  to this problem (Prior, J., Chin, A. W., Huelga, S. F., & Plenio, M. B. (2010). Efficient simulation of strong system-environment interactions. Physical review letters, 105(5), 050404) combining the theory of orthogonal polynomials with time dependent renormalisation group methods. What was missing though were rigorous error bounds on the various approximations that are made in this approach when it is implement numerically. The present work achieves this goal and establishes TEDOPA as, to our knowledge, the first method with such bounds.

 Experimental Detection of Quantum Coherent Evolution through the Violation of Leggett-Garg-Type Inequalities – Z.-Q. Zhou, S. F. Huelga, C.-F. Li, and G.-C. Guo
Physical Review Letters 115, 113002 (2015) | ArXiv

Filter design for hybrid spin gates


Filter design for hybrid spin gates
A. Albrecht, and M. B. Plenio
Physical Review A 92, 022340 (2015) | ArXiv
The gist of it

Quantum systems are generally subjected to noise from their environment. In many practical cases this noise has a finite width frequency spectrum, or equivalently a finite correlation time. For such noise it is possible to make use of dynamical decoupling methods that had been invented over the last 5 decades in nuclear magnetic resonance to average out the action of the noise. Recently, these methods have received considerable attention with the advent of solid state implementations of quantum information processing and they are also finding applications in trapped ion technologies. In this work we have furthered the development of these methods by introducing alternating pulse sequences that lead to tunable filter functions. This is the first of several works in this direction to emerge from this group: Robust dynamical decoupling sequences for individual-nuclear-spin addressingPositioning nuclear spins in interacting clusters for quantum technologies and bioimaging.



Time reversal and charge conjugation in an embedding quantum simulator
Time reversal and charge conjugation in an embedding quantum simulator
– X. Zhang, Y. Shen, J. Zhang, J. Casanova, L. Lamata, E. Solano, M.-H. Yung, J.-N.Zhang, and K. Kim
Nature Communications 6, 7917 (2015) | ArXiv
licensed under CC BY 4.0
The gist of it

Quantum computers or quantum simulators are quantum devices that may soon outperform current classical computations for analyzing complex quantum phenomena. However, they are not yet able to perform some basic arithmetic calculations such as finding the complex conjugate. Operations involving the complex conjugate require an anti-unitary process, an impossible task if directly implemented in a quantum system restricted to unitary gates. In addition, such complex conjugate is deeply connected to the important concepts in quantum field theory like charge conjugation and time reversal. For the first time, we perform the complex conjugation and these symmetry operations in our trapped-ion quantum system through the use of enlarged spaces and the novel concept of embedding a quantum simulator. Our realization can be applied to numerous quantum calculations and simulations that require access to the implementation of unphysical operations.



 Efficiency of quantum controlled non-Markovian thermalization – V. Mukherjee, V. Giovannetti, R. Fazio, S. F. Huelga, T. Calarco, and S. Montangero
New Journal of Physics 17, 063031 (2015) | ArXiv
licensed under CC BY 3.0

Vibronic origin of long-lived coherence in an artificial molecular light harvesterVibronic origin of long-lived coherence in an artificial molecular light harvester – J. Lim, D. Paleček, F. Caycedo-Soler, C. N. Lincoln, J. Prior, H. von Berlepsch, S. F. Huelga, M. B. Plenio, D. Zigmantas, and J. Hauer                                                                                       Nature Communications 6, 7755 (2015) | ArXiv
                                            The gist of it

With the advent of two-dimensional electronic spectroscopy, long-lasting quantum coherences have been reported for several photosynthetic systems. To explain how these quantum coherences are sustained under noisy biological environments at ambient temperatures, our team proposed a vibronic coupling mechanism, published in Phys. Rev. Lett. 105, 050404 (2010) and Nature Physics 9, 113 (2013), where the intramolecular vibrational motion of light-absorbing pigments enhances coherent electronic processes. However, unambiguous test of the vibronic coupling mechanism has been a challenging task due to the complexity of biological systems and their two-dimensional electronic spectra. To test the theoretical hypothesis for biological systems unambiguously, our team together with colleagues from Vienna University of Technology (Austria), Lund University (Sweden), Charles University in Prague (Czech Republic), Universidad Politécnica de Cartagena (Spain) and Freie Universität Berlin (Germany) assembled and investigated an artificial molecular light-harvester, called J-aggregates of cyanine dyes. The relatively simple electronic and vibrational structure of the artificial light-harvester reduced the complexity of two-dimensional electronic spectra significantly and allowed to test the vibronic coupling mechanism quantitatively.

In this paper we demonstrated that the coherent interaction between intramolecular vibrations and electronic degrees of freedom induces long-lasting quantum coherences in J-aggregates, which provided quantitative agreement with our experimental observations. This implies that vibrations, which are typically considered as a source of noise destroying electronic coherences, can play an opposite role, namely enhancing coherent electronic motions, such that electronic excitations are transferred quantum-mechanically through a molecular system in a wave-like manner.

Irreversible processes without energy dissipation in an isolated Lipkin-Meshkov-Glick model
– Ricardo Puebla and Armando Relaño
Physical Review E 92, 012101 (2015) | ArXiv
The gist of it

Irreversibility stems from the inability of a system to recover or restore its initial state without help of an external action or investing energy into it, and it is characterized by an entropy production.  Moreover, it is also known that there is a strong connection between irreversibility and information gained or lost by the system, manifested in the physical consequences of the Szilard engine and expressed in the Landauer principle of information erasure. Therefore, a complete description of the entropy production requires the account of information flow.

In this work, we analyse how the (ir)reversible behavior is affected by extra information in a simple and isolated model. A symmetry-breaking initial state is continuously driven from a degenerate to a non-degenerate phase, performing a closed cycle. When the driving is performed infinitely slow, there is no transition among eigenstates and hence, the population of each eigenstate in the final state perfectly matches the ones of the initial state. Nevertheless, the initial information regarding the symmetry-breaking is unavoidably lost, provided the initial state is spread over many eigenstates. This constitutes a source of irreversibility which is not linked to energy dissipation, but to a loss of information. We provide a thermodynamic interpretation relying on equilibrium states, which sets a lower bound of the entropy production equals to the information loss, log2, which stems from the two choices of symmetry breaking present in the system.


Improved scaling of time-evolving block-decimation algorithm through reduced-rank randomized singular value decompositionImproved scaling of time-evolving block-decimation algorithm through reduced-rank randomized singular value decomposition – D. Tamascelli, R. Rosenbach and M.B. Plenio
Physical Review E 91, 063306 (2015) | ArXiv
The gist of it

The simulation of quantum system by classical computers is, in general, a complex task. The mere description of the state of a collection of a relatively small number, say n, of interacting d-level quantum systems, requires of the order of d^n complex numbers. This simple fact of life is also known as the “curse of dimensionality”. Most of these parameters are required to describe correlations, i.e. entanglement, in the state. When entanglement in a quantum system is limited, the relevant dynamics of the system can be restricted to a very small part of the state space where the description of the state requires a number of parameters that scales polynomially, and not exponentially, with the number of quantum systems. A class of algorithms based on the Matrix Product States representation of quantum states take advantage of this observation; DMRG and t-DMRG usually select the relevant subspace through a decimation technique based on the singular value decomposition (SVD). The time-evolving block-decimation is an MPS-based algorithm for the simulation of the evolution of a quantum system. The complexity of each time-evolution step is dominated by the SVD, which absorbs, depending on the size of the simulated system, up to 90% of the total simulation time.

In this paper, we show that, by applying a randomized version of the SVD routine, which we refer to as Reduced-rank Randomized SVD (RRSVD), the power law governing the computational complexity of TEBD is lowered by one degree, resulting in a considerable speed-up. We exemplify the potential gains in efficiency in some real world examples, taken from the simulation of open quantum systems via Time-Evolving Density Matrix using Orthogonal Polynomial algorithm (TEDOPA). We moreover show that for those systems RRSVD delivers results as accurate as state-of-the-art deterministic SVD routines.

The algorithm described in the paper has been implemented in different versions able to exploit either the Intel MKL library or the NVIDIA General Purpose – Graphics Processing Units (GP-GPU).


Resolving single molecule structures with Nitrogen-vacancy centers in diamond
Resolving single molecule structures with Nitrogen-vacancy centers in diamond
 – M. Kost, J.-M. Cai and M.B. Plenio
Scientific Reports 5, 11007 (2015)ArXiv
licensed under CC BY 4.0
The gist of it

The Nitrogen Vacancy (NV) center in diamond allows extremely sensitive magnetic field measurements under room temperature conditions. Implanted close to the surface, the enormous sensitivity allows for the resolution of level splittings induced by dipolar coupling in molecules attached to the surface. Exploiting correlated spectroscopy (COSY), we can read out these splittings to compute the spatial structure of the molecule and its constituents. In the strong coupling regime, the pure interaction between NV center and the molecule is sufficiently strong, to get a full contrast signal even without polarizing the nuclear spins in the molecule. We further demonstrate the speed up in data acquisition by matrix completion procedures.


Scalable Reconstruction of Unitary Processes and HamiltoniansScalable Reconstruction of Unitary Processes and Hamiltonians – M. Holzäpfel, T. Baumgratz, M. Cramer and M.B. Plenio
Physical Review A 91, 042129 (2015) | ArXiv
The gist of it

Determining the state of a quantum system is a non-trivial task due to the probabilistic nature of quantum measurements: The resulting data only allows for an estimation of the true state of the system. The same is true for characterizing a quantum process, i.e. how a complete set of states is changed e.g. by a unitary time evolution. To determine an arbitrary state of n qubits, we also have to measure a number of observables which grow exponentially with n. However, using basic linear algebra one can find efficient representations of many many-qubit states and it has been shown that such states often allow for characterization with a much smaller number of observables. In our simulation, we efficiently determine an unknown quantum process of a many-qubit system by adapting existing methods for state characterization. We use a known correspondence between quantum processes on n qubits and quantum states on 2n qubits, but it turns out that this enlarged system will not be necessary in an actual experiment.


Universal Set of Gate for Microwave Dressed-state Quantum Computing

Universal Set of Gate for Microwave Dressed-state Quantum Computing
– G. Mikelsons, I. Cohen, A. Retzker and M.B. Plenio
New Journal of Physics 17, 053032 (2015) | ArXiv
licensed under CC BY 3.0
The gist of it

Trapped ions represent one of the frontrunner technologies towards the realization of medium for large scale quantum computation. Currently the standard approach uses lasers to control and entangle the ions but some 15 years ago an alternative approach had been suggested which uses microwaves applied to ions in the presence of a magnetic field gradient to achieve spin-motion coupling and, building on that, entanglement between ions. The qubit states that were required for that are magnetically sensitive and therefore susceptible to magnetic field fluctuations. in Nature 476, 185 (2011) we proposed continuous dynamical decoupling as a means to overcome this problem and contributed to a first experimental realization. In the present work we have analysed our approach in considerable detail. We demonstrate that it is capable of delivering quantum gates of a fidelity that exceed fault tolerant thresholds and that therefore, large scale quantum computing is made possible in principle.


 Redfield equations for modeling light-harvesting complexes – J. Jeske, D. Ing, M. B. Plenio, S. F. Huelga and J. H. Cole
Journal of Physical Chemistry 142, 064104 (2015) | ArXiv

Two-Dimensional Spectroscopy for the Study of Ion Coulomb Crystals
Two-Dimensional Spectroscopy for the Study of Ion Coulomb Crystals
– A. Lemmer, C. Cormick, C. T. Schmiegelow, F. Schmidt-Kaler and M. B. Plenio
Physical Review Letters 114, 073001 (2015) | ArXiv
The gist of it

Trapped atomic ions have proven to be a very versatile tool for quantum information processing and the simulation of quantum dynamics. More recently, they have also become a testbed for statistical mechanics. For the study of statistical mechanical systems it is desirable to extend the toolbox of analysis techniques available with trapped ions.
In this work we show how the technique of two-dimensional (2D) spectroscopy that has been very successful in other fields, such as NMR and quantum biology, can be applied to trapped ions. It turns out that the method is particularly well-suited to study non-linear interactions in ion traps. We illustrate the usefulness of the technique with two concrete examples: one is the detection of the onset of a phase transition while the second is resonant non-linear exchange of energy between motional modes.


Nondestructive selective probing of phononic excitations in a cold Bose gas using impuritiesNondestructive selective probing of phononic excitations in a cold Bose gas using impurities – D. Hangleiter, M. T. Mitchison, T. H. Johnson, M. Bruderer, M. B. Plenio, and D. Jaksch
Physical Review A 91, 013611 (2015) | ArXiv
The gist of it

Ultracold quantum gases are an extremely clean realization of interacting quantum many-body systems and exhibit interesting properties such as superfluidity and quantum phase transitions. For studying these phenomena it is necessary to probe quantum gases with high accuracy while leaving their characteristic quantum features intact. In our work we suggest a scheme for measuring temperatures and excitations with a single impurity atom that is immersed in a Bose gas. The impurity is confined by a double-well potential and acts as an effective qubit coupled to phononic excitations of the quantum gas. We show how to use the detector to observe coherent density waves and to measure temperatures down to the nanokelvin regime. The scheme could be realized experimentally, including the possibility of using an array of multiple impurities to achieve greater precision.


Appearance of Gibbs states in quantum-state tomography – J. Rau
Physical Review A, 90, 062114 (2014) | ArXiv

Quantum channels and memory effects – F. Caruso, V. Giovannetti, C. Lupo and S. Mancini
Reviews of Modern Physics 86, 1203 (2014) | ArXiv

Dephasing-assisted transport in linear triple quantum dots – L. D. Contreras-Pulido, M. Bruderer, S. F. Huelga, and M. B. Plenio
New Journal of Physics 16, 113061 (2014) | ArXiv

 Experimental Reconstruction of Work Distribution and Study of Fluctuation Relations in a Closed Quantum System – Tiago B. Batalhão, Alexandre M. Souza, Laura Mazzola, Ruben Auccaise, Roberto S. Sarthour, Ivan S. Oliveira, John Goold, Gabriele De Chiara, Mauro Paternostro, and Roberto M. Serra
Physical Review Letters 113, 140601 (2014)ArXiv
*Editors’ Suggestion

 Quantifying Coherence – T. Baumgratz, M. Cramer, and M. B. Plenio
Physical Review Letters 113, 140401 (2014)ArXiv
*Editors’ Suggestion

 Testing quantum gravity by nanodiamond interferometry with nitrogen-vacancy centers – A. Albrecht, A. Retzker and M.B. Plenio
Physical Review A 90, 033834 (2014) | ArXiv

 Self-assembling hybrid diamond-biological quantum devices – A. Albrecht, A. Retzker, G. Koplovitz, F. Jelezko, S. Yochelis, Y. Nevo, O. Shoseyov, Y. Paltiel and M.B. Plenio
New Journal of Physics 16, 093002 (2014) | ArXiv

 Nuclear magnetic resonance spectroscopy with single spin sensitivity – A. C. Müller, X. Kong, J.-M. Cai, K. Melentijevic, A. Stacey, M. Markham, J. Isoya, S. Pezzagna, J. Meijer, J. Du, M.B. Plenio, B. Naydenov, L.P. McGuinness and F. Jelezko
Nature Communications 5, 4703 (2014)

 Quantum non-Markovianity: characterization, quantification and detection – A. Rivas, S.F. Huelga and M.B. Plenio
Reports on Progress in Physics 77, 094001 (2014) | ArXiv

All-optical magnetic resonance of high spectral resolution using a nitrogen-vacanvy spin in diamond – Z.-Y. Wang, J.M. Cai, A. Retzker and M.B. Plenio
New Journal of Physics 16, 083033 (2014) | ArXiv

 Quantum metrology subject to spatially correlated Markovian noise: restoring the Heisenberg limit – J. Jeske, J.Cole and S.F. Huelga
New Journal of Physics 16, 073039 (2014) | ArXiv

 Competition between memory-keeping and memory-erasing decoherence channels – T.J.G. Apollaro, S. Lorenzo, C. Di Franco, F. Plastina and M. Paternostro
Physical Review A 90, 012310 (2014) ArXiv

 Tuning heat transport in trapped-ion chains across a structural phase transition – A. Ruiz, D. Alonso, M.B. Plenio and A. del Campo
Physical Review B 89, 214305 (2014) ArXiv

 Hybrid sensor based on colour centres in diamond and piezoactive layers – J.M. Cai, F. Jelezko and M.B. Plenio
Nature Communications 5, 4065 (2014) | ArXiv

 Phonon-induced dynamic resonance energy transfer – J. Lim, M. Tame, K. Hyuk Yee, J.-S. Lee and J. Lee
New Journal of Physics 16, 053018 (2014) | ArXiv

Transport enhancement from incoherent coupling between one-dimensional quantum conductor – J. J. Mendoza-Arenas, M. T. Michison, S. R. Clark, J. Prior, D. Jaksch and M. B. Plenio
New Journal of Physics 16, 053016 (2014) | ArXiv

 Extracting Entanglement from Identical Particles – N. Killoran, M. Cramer, and M. B. Plenio
Physical Review Letters 112, 150501 (2014)ArXiv

 Realistic and verifiable coherent control of excitonic states in a light-harvesting complex – S. Hoyer, F. Caruso, S. Montangero, M. Sarovar, T. Calarco, M. B. Plenio und B. Whaley
New Journal of Physics 16, 045007 (2014) | ArXiv

 Inverse counting statistics for stochastic and open quantum systems: the characteristic polynomial approach – M. Bruderer, L. D. Contreras-Pulido, M. Thaller, L. Sironi, D. Obreschkow and M. B. Plenio
New Journal of Physics 16, 033030 (2014) ArXiv

 Quantifying entanglement with scattering experiments – O. Marty, M. Epping, H. Kampermann, D. Bruß, M. B. Plenio and M. Cramer
Physical Review B 89, 125117 (2014) ArXiv

 Mappings of open quantum systems onto chain representations and Markovian embeddings – M. P. Woods, R. Groux, A. W. Chin, S. F. Huelga and M. B. Plenio
Journal of Mathematical Physics 55, 032101 (2014) ArXiv


• Origin of long-lived oscillations in 2D-spectra of a Quantum Vibronic Model: Electronic versus Vibrational coherence – M.B. Plenio, J. Almeida and S.F. Huelga
Journal of Chemical Physics 
139, 235102 (2013)
 | ArXiv

• Time-frequency resolved ultrafast spectroscopy techniques using wavelet analysis – J. Prior, E. Castro, A.W. Chin, J. Almeida, S.F. Huelga and M.B. Plenio
Journal of Chemical Physics 
139, 224103 (2013)
 | ArXiv

A scalable maximum likelihood method for quantum state tomography – T. Baumgratz, A. Nüßeler, M. Cramer and M.B. Plenio
New Journal of Physics 
15125004 (2013)
 | ArXiv

Chemical Compass Model for Avian Magnetoreception as a Quantum Coherent Device – J.M. Cai and M.B. Plenio
Physical Review Letters
111, 230503 (2013)

Effective cutting of a quantum spin chain by bond impurities – T.J.G. Apollaro, F. Plastina, L. Banchi, A. Cuccoli, R. Vaia, P. Verrucchi, and M. Paternostro
Physical Review A
88, 052336 (2013)

Vibriations, Quanta and Biology – S.F. Huelga and M.B. Plenio
Contemporary Physics
54, 181 (2013)
| ArXiv

• Dynamics of topological defects in ion Coulomb crystals – H.L. Partner, T. Burgermeister, K. Pyka, J. Keller, T.E. Mehlstäubler, R. Nigmatullin, A. Retzker and M.B. Plenio
New Journal of Physics 
15103013 (2013)
 | ArXiv

• Entanglement Amplification in the Non-Perturbative Dynamics of Modular Quantum Systems – A. Bayat, S.M. Giampaolo, F. Illuminati and M.B. Plenio
Physical Review A
88, 022319 (2013)
| ArXiv

• Entanglement of bosons in optical lattices – M. Cramer, A. Bernard, N. Fabbri, L. Fallani, C. Fort, S. Rosi, F. Caruso, M. Inguscio and M.B. Plenio
Nature Communications
4, 2161 (2013)

• Detecting and polarizing nuclear spins with nuclear double resonance on a single electron spin – P. London, J. Scheuer, J.M. Cai, I. Schwarz, A. Retzker, M.B. Plenio, M. Katagiri, T. Teraji, S. Koizumi, J. Isoya, R. Fischer, L.P. McGuinness, B. Naydenov and F. Jelezko
Physical Review Letters
111, 067601 (2013)

• Topological defect formation and spontaneous symmetry breaking in ion Coulomb crystals – K. Pyka, J. Keller, H. Partner, R. Nigmatullin, T. Burgermeister, D.M. Meier, K. Kuhlmann, A. Retzker, M.B. Plenio, W.H. Zurek, A. del Campo, T.E. Mehlstäubler
Nature Communications
4, 2291 (2013)

• Observation of the Kibble-Zurek scaling law for defect formation in ion crystals – S. Ulm, J. Roßnagel, G. Jacob, C. Degünther, S.T. Dawkins, U.G. Poschinger, R. Nigmatullin, A. Retzker, M.B. Plenio, F. Schmidt-Kaler and K. Singer
Nature Communications
4, 2290 (2013)

• Coupling of nitrogen vacancy centers in nanodiamonds by means of
– A. Albrecht, A. Retzker, F. Jelezko and M.B. Plenio
New Journal of Physics 
15083014 (2013)
 | ArXiv

• Detection of a few metallo-protein molecules using color centers in nanodiamonds – A. Ermakova, G. Pramanik, J.M. Cai, G. Algara-Siller, U. Kaiser, T. Weil, Y.-K. Tzeng, H. C. Chang, L. P. McGuinness, M.B. Plenio, B. Naydenov, and F. Jelezko
Nano Letters
13, 3305 (2013)

• Driven geometric phase gates with trapped ions – A. Lemmer, A. Bermudez and M.B. Plenio
New Journal of Physics 
15083001 (2013)
 | ArXiv

• Matrix product state representation without explicit local Hilbert space truncation with applications to the sub-ohmic spin-boson model – M. Frenzel and M.B. Plenio
New Journal of Physics 
15073046 (2013)
 | ArXiv

• Controlling and Measuring Quantum Transport of Heat in Trapped-Ion Crystals – A. Bermudez, M. Bruderer, and M. B. Plenio
Physical Review Letters
111, 040601 (2013)
| ArXiv

• Preparation of the ground state of a spin chain by dissipation in a structured environment – C. Cormick, A. Bermudez, S.F. Huelga and M.B. Plenio
New Journal of Physics
15, 073027 (2013)
| ArXiv

Reconstructing density matrices efficiently – T. Baumgratz, D. Gross, M. Cramer and M.B. Plenio
Physical Review Letters 111, 020401 (2013) | ArXiv

Extreme alien light allows survival of terrestrial bacteria – N. Johnson, G. Zhao, F. Caycedo, P. Manrique, H. Qi, F. Rodriguez and L. Quiroga
Scientific Reports 3, 2198 (2013)

Probing quantum coherence in superconducting qubit arrays – J. Almeida, P.C. de Groot, S.F. Huelga, A.M. Liguori and M.B. Plenio
Journal of Physics B 46, 104002 (2013) | ArXiv

• Quantum diffusion with disorder, noise and interaction – C. D’Errico, M. Moratti, E. Lucioni, L. Tanzi, B. Deissler, M. Inguscio, G. Modugno M.B. Plenio and F. Caruso
New Journal of Physics
15, 045007 (2013)
| ArXiv

Dissipation assisted quantum information processing with trapped ions – A. Bermudez, T. Schätz, and M.B. Plenio
Physical Review Letters 110, 110502 (2013) | ArXiv

• Exploiting Structured Environments for Efficient Energy Transfer: The phonon antenna mechanism – M. del Rey, A.W. Chin, S.F. Huelga and M.B. Plenio
Journal of Physical Chemistry Letters 4
, 903-907 (2013)
| ArXiv

A large-scale quantum simulator on a diamond surface at room temperature – J.M. Cai, A. Retzker, F. Jelezko and M.B. Plenio
Nature Physics
9, 168 (2013)
| ArXiv

A Paradox in Bosonic Energy Calculations via Semidefinite Programming Relaxations – M. Navascués, A. Garcia-Sáez, A. Acin, S. Pironio and M. B. Plenio
New Journal of Physics 15, 023026 (2013) | ArXiv

• Quantum speed limits in open quantum systems – A. delCampo, I.L. Egusquiza, M.B. Plenio, and S.F. Huelga
Physical Review Letters 110, 050403 (2013) | ArXiv

Entanglement Replication in Driven Dissipative Many-Body systems – S. Zippilli, M. Paternostro, G. Adesso and F. Illuminati
Physical Review Letters 110, 040503 (2013) | ArXiv

Erratum: Entanglement Replication in Driven Dissipative Many-Body systems [Phys. Rev. Lett. 110, 040503 (2013)] – S. Zippilli, M. Paternostro, G. Adesso and F. Illuminati
Physical Review Letters 111, 169901 (2013)

• Diamond based single molecule magnetic resonance spectroscopy – J.M. Cai, F. Jelezko, M.B. Plenio and A. Retzker
New Journal of Physics 15, 013020 (2013) | ArXiv

• Vibrational structures and long-lived electronic coherences – A.W. Chin, J. Prior, R. Rosenbach, F. Caycedo-Soler, S.F. Huelga and M.B. Plenio
Nature Physics
9, 113 (2013)
| ArXiv

Quantum dynamics in photonics crystals – J. Prior, I. de Vega, A.W. Chin, S.F. Huelga and M.B. Plenio
Physical Review A 87, 013428 (2013) | ArXiv


• Precise experimental investigation of eigenmodes in a planar ion crystal – H. Kaufmann, S. Ulm, G. Jakob, U. Poschinger, H. Landa, A. Retzker, M.B. Plenio and F. Schmidt-Kaler
Physical Review Letters 109, 263003 (2012) | ArXiv

Quantum Metrology in Non-Markovian Environments – A. W. Chin, S. F. Huelga and M.B. Plenio
Physical Review Letters 109, 233601 (2012) | ArXiv

Linear optics simulation of quantum non-Markovian dynamics – A. Chiuri, C. Greganti, L. Mazzola, M. Paternostro and O. Mataloni
Scientific Reports 2, 968 (2012) | ArXiv

• Transferring entanglement to the steady state of flying qubits – Y. Guo, J. Li, T. Zhang, and M. Paternostro
Physical Review A 86, 052315 (2012) | ArXiv

• Recursive quantum detector tomography – L. Zhang, A. Datta, H.B. Coldenstrodt-Ronge, X.-M. Jin, J. Eisert, M.B. Plenio and I.A. Walmsley
New Journal of Physics 14, 115005 (2012) | ArXiv

Robust dynamical decoupling with concatenated continuous driving – J.-M. Cai, B. Naydenov, R. Pfeiffer, L. P. McGuinness, K. D. Jahnke, F. Jelezko, M. B. Plenio and A. Retzker
New Journal of Physics 14, 113023 (2012) | ArXiv

Entanglement control in hybrid optomechanical systems – B. Rogers, M. Paternostro, G.M. Palma and G. De Chiara
Physical Review A 86, 042323 (2012) | ArXiv

Emergent Thermodynamics in a Quenched Quantum Many-Body System – R. Dorner, J. Goold, C. Cormick, M. Paternostro and V. Vedral
Physical Review Letters 109, 160601 (2012) | ArXiv

Input-output Gaussian channels: theory and applications – T. Tufarelli, A. Retzker, M.B. Plenio and A. Serafini
New Journal of Physics 14, 093046 (2012) | ArXiv

• Quantum Magnetism of Spin-Ladder Compounds with Trapped-Ion Crystals – A. Bermudez, J. Almeida, K. Ott, H. Kaufmann, S. Ulm, F. Schmidt-Kaler, A. Retzker and M.B. Plenio
New Journal of Physics 14, 093042 (2012) | ArXiv

• Long-lived driven solid-state quantum memory – J.-M. Cai, F. Jelezko, N. Katz, A. Retzker and M.B. Plenio
New Journal of Physics 14, 093030 (2012) | ArXiv

Dipolar Bose-Einstein condensate of Stationary-Light Dark-state Polaritons – F.E. Zimmer, G. Nikoghosyan and M.B. Plenio
Physical Review A 86, 023854 (2012) | ArXiv

Computation of 2-D spectra assisted by compressed sampling – J. Almeida, J. Prior and M.B. Plenio
Journal of Physical Chemistry Letters 3, 2692-2696 (2012) | ArXiv

• Qubit-assisted thermometry of a quantum harmonic oscillator – M. Brunelli, S. Olivares, M. Paternostro, and M.G. A. Paris
Physical Review A 86, 012125 (2012) | ArXiv

Coherence and decoherence in biological systems: principles of noise-assisted transport and the origin of long-lived coherences – A.W. Chin, S.F. Huelga and M.B. Plenio
Phil. Trans. Act. Roy. Soc. 370, 3638 (2012) | ArXiv

Spin Peierls Quantum Phase Transitions in Coulomb Crystals – A. Bermudez and M.B. Plenio
Physical Review Letters 109, 010501 (2012) | ArXiv

 Mapping coherence in measurement via full quantum tomography of a hybrid optical detector – L. Zhang, H. Coldenstrodt-Ronge, A. Datta, G. Puentes, J.S. Lundeen, X.-M. Jin, B.J. Smith, M.B. Plenio, I.A. Walmsley
Nature Photonics 6, 364-368 (2012) | ArXiv

 Coherent open-loop optimal control of light-harvesting dynamics – F. Caruso, S. Montangero, T. Calarco, S.F. Huelga and M.B. Plenio
Physical Review A 85, 042331 (2012) | ArXiv

 Non-Markovianity-Assisted Steady State Entanglement – S.F. Huelga, Á. Rivas, and M.B. Plenio
Physical Review Letters 108, 160402 (2012) | ArXiv

 The nature of the low energy band of the Fenna-Matthews-Olson complex: vibronic signatures – F. Caycedo-Soler, A.W.  Chin, J. Almeida, S.F. Huelga and M.B. Plenio
Journal of Chemical Physics 136, 155102 (2012) | ArXiv

 Quantum limits for the magnetic sensitivity of a chemical compass – J. Cai, F. Caruso and M.B. Plenio
Physical Review A 85, 040304(R) (2012) | ArXiv

 Pulsed Laser Cooling for Cavity Optomechanical Resonators – J. Cerrillo, S. Machnes, M. Aspelmeyer, W. Wieczorek, M.B. Plenio and A. Retzker
Physical Review Letters 108, 153601 (2012) | ArXiv

 Robust trapped-ion quantum logic gates by continuous dynamical decoupling – A. Bermudez, P. O. Schmidt, M.B. Plenio and A. Retzker
Physical Review A 85, 040302 (2012) | ArXiv

 Probing biological light-harvesting phenomena by optical cavities – F. Caruso, S. K. Saikin, E. Solano, S.F. Huelga, A. Aspuru-Guzik and M.B. Plenio
Physical Review B 85, 125424 (2012) | ArXiv

 Generation of Mesoscopic Entangled States in a Cavity Coupled to an Atomic Ensemble – G. Nikoghosyan, M. J. Hartmann and M.B. Plenio
Physical Review Letters 108, 123603 (2012) | ArXiv

Compact Continuous-Variable Entanglement Distillation – A. Datta, L. Zhang, J. Nunn, N.K. Langford, A. Feito, M.B. Plenio and I.A. Walmsley
Physical Review Letters 108, 060502 (2012) | ArXiv

Lower bounds on ground state energies – T. Baumgratz and M.B. Plenio
New Journal of Physics
14, 023027 (2012)
 | ArXiv


Arrays of waveguide-coupled optical cavities that interact strongly with atoms – G. Lepert, M. Trupke, M.J. Hartmann, M.B. Plenio and E.A. Hinds
New Journal of Physics 13, 113002 (2011) | ArXiv

Frustrated Quantum Spin Models with Cold Coulomb Crystals – A. Bermudez, J. Almeida, F. Schmidt-Kaler, A. Retzker and M.B. Plenio
Physical Review Letters
107, 207209 (2011)
 | ArXiv

Generalized Polaron Ansatz for the Ground State of the Sub-Ohmic Spin-Boson Model: An Analytic Theory of the Localization Transition – A.W. Chin, J. Prior, S.F. Huelga and M.B. Plenio
Physical Review Letters
107, 160601 (2011)
| ArXiv

Electron-Mediated Nuclear-Spin Interactions Between Distant NV-Centers – A. Bermudez, F. Jelezko, M.B. Plenio and A. Retzker
Physical Review Letters
107, 150503 (2011)
| ArXiv
*Selected for a Viewpoint in Physics and as Editors’ suggestion

Quantum dynamics of bio-molecular systems in noisy environments – S.F. Huelga and M.B. Plenio
Procedia Chemistry
3, 248 – 257 (2011)

Renormalization Group algorithms with graph enhancement: reprise – R. Hübener, C. Kruszynska, L. Hartmann, W. Dür, M.B. Plenio and J. Eisert
Physical Review B
84, 125103 (2011)
| ArXiv

Heterogeneous Kibble-Zurek mechanism: vortex nucleation during Bose-Einstein condensation – A. del Campo, A. Retzker and M.B. Plenio
New Journal of Physics
13, 083022 (2011)
| ArXiv

Quantum Gates and Memory using Microwave Dressed States – N. Timoney, I. Baumgart, M. Johanning, A. F. Varon, M.B. Plenio, A. Retzker and Ch. Wunderlich
476, 185 (2011)
 | ArXiv

Experimental quantitative verification of entanglement in photonic cluster state experiments – H. Wunderlich, G. Vallone, P. Mataloni and M.B. Plenio
New Journal of Physics
13, 033033 (2011)
| ArXiv
*Chosen for IOP Select 2011

• Enhancement of laser cooling by the use of magnetic field gradients – A. Albrecht, A. Retzker, Ch. Wunderlich, and M.B. Plenio
New Journal of Physics
13, 033009 (2011)
| ArXiv

 • Atomic Fock states by gradual trap reduction: From sudden to adiabatic limits – D. Sokolovski, M. Pons, A. del Campo and J.G. Muga
Physical Review A 83,  013402 (2011) | ArXiv

 • Simulation of noise-assisted transport via optical cavity networks – F. Caruso, N. Spagnolo, C. Vitelli, F. Sciarrino, and M.B. Plenio
Physical Review A
83,  013811 (2011)
| ArXiv

• Fast frictionless dynamics as a toolbox for low-dimensional Bose-Einstein condensates – A. del Campo
Europhysics Letters
 96, 60005 (2011)
 | ArXiv

• Frictionless quantum quenches in ultracold gases: a quantum dynamical microscope – A. del Campo
Physical Review A
 84, 031606(R) (2011)
 | ArXiv

Comparing, Optimising and Benchmarking Quantum Control Algorithms in a Unifying Programming Framework – S. Machnes, U. Sander, S.J. Glaser, P. de Fouquieres, A. Gruslys, S. Schirmer, T. Schulte-Herbrueggen
Physical Review A
84, 022305 (2011)
| ArXiv

Interaction-dependent temperature effects in Bose-Fermi mixtures in optical lattices – M. Cramer
Physical Review Letters
106, 215302 (2011)
| ArXiv

Measuring entanglement in condensed matter systems – M. Cramer, M.B. Plenio, and H. Wunderlich
Physical Review Letters
106, 020401 (2011)
| ArXiv

Local hypothesis testing between a pure bipartite state and the white noise state – M. Owari, and M. Hayashi

Reconstructing quantum states efficiently – M. Cramer and M.B. Plenio

Quantum memory for entangled continuous-variable states – K. Jensen, W. Wasilewski, H. Krauter, T. Fernholz, B. M. Nielsen, M. Owari, M. B. Plenio, A. Serafini, M. M. Wolf and E. S. Polzik
Nature Physics
7, 13-16 (2011)
| ArXiv