**Delayed 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

licensed under CC BY 4.0

**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 ** – F. Mascherpa, A. Smirne, S. F. Huelga and M. B. Plenio, *Physical Review Letters, 118, 100401* (2017) | ArXiv

**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 Dimensions ** – H.-K. Lau, R. Pooser, G. Siopsis and C. Weedbrook, *Physical Review Letters, 118, 080501* (2017) | ArXiv

**Probing 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** – A. Le Boité, M.-J. Hwang and M. B. Plenio, *Physical Review A, 95, 023829* (2017) | ArXiv

**Relations 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 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** – 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.