On quantum gravity tests with composite particles, S. P. Kumar and M. B. Plenio, Nature Communications 11 (2020) 3900
DOI: https://doi.org/10.1038/s41467-020-17518-5

Temporal correlations of sunlight may assist photoprotection in bacterial photosynthesis, A. M. De Mendoza, F. Caycedo-Soler, S. F. Huelga, and M. B. Plenio, New Journal of Physics 22 (2020) 073042
DOI: https://doi.org/10.1088/1367-2630/ab99e0

The gist of it

Photosynthetic systems adapt and respond efficiently to fluctuations in their light environment.  As a result, large photosynthetic yields can be achieved in conditions of low light intensity, while photoprotection mechanisms are activated in conditions of elevated light intensity. In sharp contrast with these observations, current theoretical models predict bacterial cell death for physiologically high light intensities. To resolve this discrepancy, we consider the three stages of photosynthesis in natural conditions, namely light absorption, exciton transfer, and charge separation dynamics.  The thermal light absorption at the antenna complexes is characterized by photo-bunching, which in turn, is partially maintained until the reaction centers after transport. There, finite charge recombination times lead to energy dissipation because of the gaps between the bunches of excitations arrivals. The dissipated excitations never get to induce Quinol production and the acidity across the membrane can be adjusted to safe values for the cell, depending on the Spatio-temporal correlations of the absorbed light. Our approach allows us to identify a mechanism of bacterial photoprotection that relies on the sunlight photon-statistics. 

A Complex Comprising a Cyanine Dye Rotaxane and a Porphyrin Nanoring as a Model Light‐Harvesting System, J. Pruchyathamkorn, W. J. Kendrick, A. T. Frawley, A. Mattioni, F. Caycedo‐Soler, S. F. Huelga, M. B. Plenio, and H. L. Anderson, Angewandte Chemie, Int. Ed. 59 (2020) 1
DOI: https://doi.org/10.1002/anie.202006644

The gist of it

In photosynthetic organisms, right after a photon is absorbed, electronic energy is efficiently transferred from peripheral antenna complexes to the reaction center to be further processed. Studying the absorption and energy transfer mechanism in biological samples can be extremely challenging, due to the presence of highly complex protein structures that bind together the pigments in characteristic geometries. In this work, we present a unique artificial light-harvesting complex, specifically synthesized to reproduce the light-harvesting architecture of purple bacteria. Through a combination of experiments and theoretical modeling, we are able to characterize the picosecond energy transfer dynamics, revealing a mechanism analogous to the one that takes place in the original biological complex. This comparison represents an important step towards bio-mimetic artificial photosynthesis.

Experimental control of the degree of non-classicality via quantum coherence - A. Smirne, T. Nitsche, D. Egloff, S. Barkhofen, S. De, I. Dhand, C Silberhorn, S. F. Huelga and M. B. Plenio, Quantum Science and Technology, 5 (2020) 4LT01
DOI: doi.org/10.1088/2058-9565/aba03

The gist of it

Which predictions of quantum mechanics can and which cannot be reproduced by means of any plausible classical theory? This question is at the basis of upcoming quantum technologies including sensing, computation and communication, and it is central to determine if certain phenomena are genuinely quantum, for instance in biological or thermodynamical systems. Different strategies have been developed to assess the quantumness of physical systems without having to rely on the knowledge of the microscopic details of the system at hand, but rather directly evaluating the probability distributions of the measurement outcomes with respect to specific traits of classical statistics, such as locality, non-contextuality, and measurement non-invasiveness. The latter means that, at least in principle, one can access the value of an observable without altering the statistics associated with its sequential measurements at different times, a property typical of classical statistical theories, which is instead not generally valid in the quantum realm.

Here, we study, both theoretically and experimentally, to what extent non-classicality can be linked with quantum coherence. We show when the coherence of an observable is linearly related to the degree of violation of the Kolmogorov condition, which quantifies the deviation from any classical explanation of the multi-time statistics. Experimentally, we probe this connection in a time-multiplexed optical quantum walk, by comparing the standard statistics, where properties of the walker are measured only at the final step, with the statistic associated with a quantum walk undergoing measurements at an intermediate time [see the figure]. Achieving exquisite control of quantum coherence of the walker by varying the degree of coherent superposition effected by the coin, we show a concomitant variation in the degree of non-classicality of the walker statistics.

Motional Dynamical Decoupling for Interferometry with Macroscopic Particles – J. S. Pedernales, G. W. Morley, and M. B. Plenio, Physical Review Letters, 125, 023602 (2020)
DOI: https://doi.org/10.1103/PhysRevLett.125.023602

Universal Anti-Kibble-Zurek Scaling in Fully Connected Systems – R. Puebla, A. Smirne, S. F. Huelga, and M. B. Plenio, Physical Review Letters, 124, 230602 (2020)
DOI:https://doi.org/10.1103/PhysRevLett.124.230602

The gist of it

The Kibble-Zurek mechanism dictates a fundamental scaling relation between the equilibrium critical exponents of a phase transition, the quench rate with which the system is driven and the number of nonequilibrium excitations upon traversing the phase transition. In the quantum realm, these universal scaling laws are sensitive to the unavoidable system-environment interaction, i.e. to the open nature of the system dynamics. Such system-environment interaction will typically lead to an anti-Kibble-Zurek behavior: the slower the quench, the more excitations, thus implying a breakdown of the universal scaling relation dictated by the original Kibble-Zurek formulation. However, as we have shown in this work, such anti-Kibble-Zurek behavior can still acquire a universal form: The excitations can increase as a power law of the quench rate, whose exponent is again solely determined by the equilibrium critical exponents of the system’s phase transition. Moreover, all the phenomenology associated to the standard Kibble-Zurek scaling applies here as well, that is, finite-size effects and the non-trivial modification of the power-law exponent under nonlinear drivings towards the phase transition. This constitutes a new phenomenon in the context of nonequilibrium critical dynamics and open quantum systems.

Optimized auxiliary oscillators for the simulation of general open quantum systems – F. Mascherpa, A. Smirne, A. D. Somoza, P. Fernández-Acebal, S. Donadi, D. Tamascelli, S. F. Huelga, and M. B. Plenio, Physical Review A, 101, 052108 (2020)
DOI:https://doi.org/10.1103/PhysRevA.101.052108

Enhancing the Robustness of Dynamical Decoupling Sequences with Correlated Random Phases – Z. Wang, J. Casanova, and M. B. Plenio, Symmetry 2020, 12, 730 (2020)
DOI:https://doi.org/10.3390/sym12050730

The gist of it

This paper demonstrates that the performance of dynamical decoupling sequences can be improved by imposing correlated random phases to the basic pulse units. These correlated random phases cancel the leading order errors in the basic pulse units such as the static fluctuation in the Rabi frequency and frequency detuning of the control field. Furthermore, these correlated random phases suppress spurious responses, which is due to limited control power and environmental perturbation and can lead to false signal identification in quantum sensing, e.g., the misidentification of carbon nuclei for proton nuclei.

Efficient simulation of open quantum systems coupled to a fermionic bath – A. Nüßeler, I. Dhand, S. F. Huelga, and M. B. Plenio, Physical Review B, 101, 155134 (2020)
DOI:https://doi.org/10.1103/PhysRevB.101.155134

The gist of it

The problem of open quantum systems coupled to fermionic environments arises in a variety of fields ranging from quantum dots to quantum transport at zero and finite temperature. This work presents and analyzes a tensor-network based time integration scheme for this problem, which enables simulations of such systems with controllable and certified error. Furthermore, it is shown that performing simulations in the Heisenberg picture can lead to significant efficiency gains.

Limited-control metrology approaching the Heisenberg limit without entanglement preparation – B. Tratzmiller, Q. Chen, I. Schwartz, S. F. Huelga, and M. B. Plenio, Physical Review A, 101, 032347 (2020)
DOI:https://doi.org/10.1103/PhysRevA.101.032347

The gist of it

A basic question in quantum metrology is how precise a quantity (e.g. magnetic field strength) can be measured given a number of particles M and a total time T. The ultimate precision limit is the Heisenberg limit, which requires entanglement between the particles involved. In this work we present a scheme that approaches the Heisenberg limit without requiring the preparation of an entangled state by using nuclear spins which are read out by a nearby NV center. We compare the scheme to other approaches and discuss applications.

Hybrid Microwave-Radiation Patterns for High-Fidelity Quantum Gates with Trapped Ions – I. Arrazola, M.B. Plenio, E. Solano, and J. Casanova, Physical Review Applied, 13, 024068 (2020)
DOI: doi.org/10.1103/PhysRevApplied.13.024068

The gist of it

Quantum processors based on trapped ions are one of the leading candidates to build quantum simulators and computers, due to their high controllability and long coherence times. Although laser control has been extremely successful achieving high-fidelity quantum gates, the scaling up of these devices requires the precise control of several laser sources, which is a hard technological challenge. Alternatives to laser control that are easier to scale up involve microwave radiation and magnetic field gradients. In this work, we propose microwave radiation patterns that lead to high-fidelity two-qubit gates while being robust against errors in the magnetic or microwave fields. Our method combines continuous dynamical decoupling techniques with phase modulated drivings, phase-flips and refocusing pi pulses, leading to high-fidelity quantum gates in realistic experimental scenarios.

Quantum coherence and state conversion: theory and experiment – K.-D. Wu, T. Theurer, G.-Y. Xiang, C.-F. Li, G.-C. Guo, M. B. Plenio, and A. Streltsov, npj Quantum Information, 6, 22 (2020)
DOI: doi.org/10.1038/s41534-020-0250-z

The gist of it

The resource theory of coherence studies the operational value of superpositions in quantum technologies. A key question in this theory is which resources, i.e., quantum states showing coherence, can be obtained from a given initial state using only operations that are unable to create coherence, so called incoherent operations. This determines the relative value of states for quantum technologies: a state that can be obtained from another one via incoherent operations contains less coherence and is hence less valuable. Here, we solve this question completely for qubit states by determining the optimal single-shot probabilities for mixed-state conversions via stochastic incoherent operations. Extending the discussion to distributed scenarios, we introduce and address the task of assisted incoherent state conversion, where the process is enhanced by making use of correlations with a second party. Building on these results, we demonstrate experimentally that the optimal state-conversion probabilities can be achieved in a linear optics setup. This paves the way towards real world applications of coherence transformations in current quantum technologies.

Protecting Quantum Spin Coherence of Nanodiamonds in Living Cells – Q.-Y. Cao, P.-C. Yang, M.-S. Gong, M. Yu, A. Retzker, M.B. Plenio, C. Müller, N. Tomek, B. Naydenov, L.P. McGuinness, F. Jelezko, and J.-M. Cai, Physical Review Applied, 13, 024021 (2020)
DOI: doi.org/10.1103/PhysRevApplied.13.024021

The gist of it

Because of its excellent coherent and optical properties at room temperature, the nitrogen-vacancy (NV) center in diamond, especially when located in nanodiamonds, represents a promising tool for sensing applications in biological environments. However, in nanodiamonds the relatively short NV electron spin coherence times require microwave control to decouple the electron spin of the NV center from its noisy environment. In a biological environment this needs to be achieved with minimal microwave power in order to reduce possible heating effects. Building on earlier work from our group here we demonstrate energy-efficient protection of NV spin coherence in nanodiamonds using concatenated continuous dynamical decoupling. When this is applied to nanodiamonds in living cells, we are able to extend the spin coherence time by an order of magnitude to the T1 limit of 30 μs. Further analysis demonstrates concomitant improvements of sensing performance, which shows that our results provide an important step toward in vivo quantum sensing using NV centers in nanodiamond.