We are proud that our group has two articles in the first issue ever of the new journal ‘Quantum Science and Technology’:
Coherent 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.
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.
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.
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.
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.
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.
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.