The efficiency of solar energy harvesting systems is largely determined by their ability to transfer excitations from the antenna to the energy trapping center before recombination. Dark state protection achieved by coherent coupling between subunits in the antenna structure has been shown to significantly reduce radiative recombination and enhance the efficiency of energy trapping. Since the dark states cannot be populated by optical transitions from the ground, they are usually accessed through phononic relaxation from the bright state. In this study we explore a novel way of connecting the dark states and the bright states by optical transitions. In a ring-like chromophore system inspired by the natural photosynthetic antenna, the single-excitation bright state can be connected to the lowest energy single-excitation dark state optically through certain double-excitation states. We call such double-excitation states the ferry states and show that they are the result of accidental degeneracy between two categories of the double-excitation states. We then mathematically prove that the ferry states are only available when N the number of subunits on the ring satisfies N=4n+2 (n being an integer). Numerical calculations confirm the effect of having such ferry states by producing a significant energy transfer power spike at N=6 as compared to smaller N's when phononic relaxation is turned off. The proposed mathematical theory for the ferry states is not restricted to the particular numerical model and system but opens the possibility of new applications in any coherent optical system that adopts a ring structured chromophore arrangement.

Remarkable progress can be observed in recent years in the controlled emission, guiding and detection of coherent, free electrons. Those methods were applied in matter wave interferometers leading to high phase sensitivities and novel sensor technologies for dephasing influences such as mechanical vibrations or electromagnetic frequencies. However, the previous devices have been large laboratory setups. For future sensor applications or tests of the coherence properties of an electron source, small, portable interferometers are required. Here, we demonstrate a compact biprism electron interferometer that can be used for mobile applications. The design was optimized for small dimensions by beam path simulations. The interferometer has a length between the tip and the superposition plane before magnification of only 47 mm and provides electron interference pattern with a contrast up to 42.7 %. The detection of two dephasing frequencies at 50 and 150 Hz was demonstrated applying second order correlation and Fourier analysis of the interference data.

We present a feasible protocol using traveling wave field to experimentally observe negative response, i.e., to obtain a decrease in the output field intensity when the input field intensity is increased. Our protocol uses one beam splitter and two mirrors to direct the traveling wave field into a lossy cavity in which there is a three-level atom in a lambda configuration. In our scheme, the input field impinges on a beam splitter and, while the transmitted part is used to drive the cavity mode, the reflected part is used as the control field to obtain negative response of the output field. We show that the greater cooperativity of the atom-cavity system, the more pronounced the negative response. The system we are proposing can be used to protect devices sensitive to intense fields, since the intensity of the output field, which should be directed to the device to be protected, is diminished when the intensity of the input field increases.

We quantify the usefulness of a bipartite quantum state in the ancilla-assisted channel discrimination of arbitrary quantum channels, formally defining a worst-case-scenario channel discrimination power for bipartite quantum states. We show that such a quantifier is deeply connected with the operator Schmidt decomposition of the state. We compute the channel discrimination power exactly for pure states, and provide upper and lower bounds for general mixed states. We show that highly entangled states can outperform any state that passes the realignment criterion for separability. Furthermore, while also unentangled states can be used in ancilla-assisted channel discrimination, we show that the channel discrimination power of a state is bounded by its quantum discord.

Trapped-ion quantum platforms are subject to 'anomalous' heating due to interactions with electric-field noise sources of nature not yet completely known. There is ample experimental evidence that this noise originates at the surfaces of the trap electrodes, and models assuming fluctuating point-like dipoles are consistent with observations, but the exact microscopic mechanisms behind anomalous heating remain undetermined. Here we show that the normal-mode heating rates of a two-ion system can unveil new information about the underlying noise sources. This is because dipole fluctuators on the electrodes' surfaces necessarily result in a geometric transition where the heating rates of the normal modes equate. Experimentally-accessible characteristics of this crossover can determine uniquely the mean orientation of the dipoles. The spatial extent of correlations be- tween dipole fluctuators, i.e. the finite sizes of fluctuating patches, can also be determined with two ions. This information can be used to test the validity of candidate microscopic models, which predict correlation lengths spanning several orders of magnitude. Finally, we propose an experiment to measure these effects with currently-available traps and techniques.

We consider the problem of determining the energy distribution of quantum states that satisfy exponential decay of correlation and product states, with respect to a quantum local hamiltonian on a spin lattice. For a quantum state on a $D$-dimensional lattice that has correlation length $\sigma$ and has average energy $e$ with respect to a given local hamiltonian (with $n$ local terms, each of which has norm at most $1$), we show that the overlap of this state with eigenspace of energy $f$ is at most $exp(-((e-f)^2\sigma)^{\frac{1}{D+1}}/n^{\frac{1}{D+1}}D\sigma)$. This bound holds whenever $|e-f|>2^{D}\sqrt{n\sigma}$. Thus, on a one dimensional lattice, the tail of the energy distribution decays exponentially with the energy.

For product states, we improve above result to obtain a Gaussian decay in energy, even for quantum spin systems without an underlying lattice structure. Given a product state on a collection of spins which has average energy $e$ with respect to a local hamiltonian (with $n$ local terms and each local term overlapping with at most $m$ other local terms), we show that the overlap of this state with eigenspace of energy $f$ is at most $exp(-(e-f)^2/nm^2)$. This bound holds whenever $|e-f|>m\sqrt{n}$.

Cabello-Severini-Winter and Abramsky-Hardy (building on the framework of Abramsky-Brandenburger) both provide classes of Bell and contextuality inequalities for very general experimental scenarios using vastly different mathematical techniques. We review both approaches, carefully detail the links between them, and give simple, graph-theoretic methods for finding inequality-free proofs of nonlocality and contextuality and for finding states exhibiting strong nonlocality and/or contextuality. Finally, we apply these methods to concrete examples in stabilizer quantum mechanics relevant to understanding contextuality as a resource in quantum computation.

Concurrence, introduced by Hill and Wootters [Phys. Rev. Lett. 78, 5022 (1997)], provides an important measure of entanglement for a general pair of qubits that is faithful: strictly positive for entangled states and vanishing for all separable states. Such a measure captures the entire content of entanglement, providing necessary and sufficient conditions for separability. We present an extension of concurrence to multiparticle pure states in arbitrary dimensions by a new framework using the Lagrange's identity and wedge product representation of separability conditions, which coincides with the "I-concurrence" of Rungta et al. [Phys. Rev. A 64, 042315 (2001)] who proposed by extending Wootters's spin-flip operator to a so-called universal inverter superoperator. Our framework exposes an inherent geometry of entanglement, and may be useful for the further extensions to mixed and continuous variable states.

Microscopic imaging of local magnetic fields provides a window into the organizing principles of complex and technologically relevant condensed matter materials. However, a wide variety of intriguing strongly correlated and topologically nontrivial materials exhibit poorly understood phenomena outside the detection capability of state-of-the-art high-sensitivity, high-resolution scanning probe magnetometers. We introduce a quantum-noise-limited scanning probe magnetometer that can operate from room to cryogenic temperatures with unprecedented DC-field sensitivity and micron-scale resolution. The Scanning Quantum Cryogenic Atom Microscope (SQCRAMscope) employs a magnetically levitated atomic Bose-Einstein condensate (BEC), thereby providing immunity to conductive and blackbody radiative heating. It has a field sensitivity of 1.4 nT per resolution-limited point ($\sim$2 $\mu$m), or 6 nT/$\sqrt{\text{Hz}}$ per point at its duty cycle. Compared to point-by-point sensors, the long length of the BEC provides a naturally parallel measurement, allowing one to measure nearly one-hundred points with an effective field sensitivity of 600 pT$/\sqrt{\text{Hz}}$ for each point during the same time as a point-by-point scanner would measure these points sequentially. Moreover, it has a noise floor of 300 pT and provides nearly two orders of magnitude improvement in magnetic flux sensitivity (down to $10^{-6}$ $\Phi_0/\sqrt{\text{Hz}}$) over previous atomic probe magnetometers capable of scanning near samples. These capabilities are, for the first time, carefully benchmarked by imaging magnetic fields arising from microfabricated wire patterns, in a system where samples may be scanned, cryogenically cooled, and easily exchanged. The SQCRAMscope will provide charge transport images at temperatures from room to 4 K in unconventional superconductors and topologically nontrivial materials.

By using the technique of supersymmetric quantum mechanics, we study a quasi exactly solvable extension of the N-particle rational Calogero model with harmonic confining interaction. Such quasi exactly solvable many particle system, whose effective potential in the radial direction yields a supersymmetric partner of the radial harmonic oscillator, is constructed by including new long-range interactions to the rational Calogero model. An infinite number of bound state energy levels are obtained for this system under certain conditions. We also calculate the corresponding bound state wave functions in terms of the recently discovered exceptional orthogonal Laguerre polynomials.

When connecting a voltage-biased Josephson junction in series to several microwave cavities, a Cooper-pair current across the junction gives rise to a continuous emission of strongly correlated photons into the cavity modes. Tuning the bias voltage to the resonance where a single Cooper pair provides the energy to create an additional photon in each of the cavities, we demonstrate the entangling nature of these creation processes by simple witnesses in terms of experimentally accessible observables. To characterize the entanglement properties of the such created quantum states of light to the fullest possible extent, we then proceed to more elaborate entanglement criteria based on the knowledge of the full density matrix and provide a detailed study of bi- and multipartite entanglement. In particular, we illustrate how due to the relatively simple design of these circuits changes of experimental parameters allow one to access a wide variety of entangled states differing, e.g., in the number of entangled parties or the dimension of state space. Such devices, besides their promising potential to act as a highly versatile source of entangled quantum microwaves, may thus represent an excellent natural testbed for classification and quantification schemes developed in quantum information theory.

We numerically study the evolution of a small turbulent region of quantised vorticity in superfluid helium, a regime which can be realised in the laboratory. We show that the turbulence achieves a fluctuating steady-state in terms of dynamics (energy), geometry (length, writhing) and topology (linking). After defining the knot spectrum, we show that, at any instant, the turbulence consists of many unknots and few large loops of great geometrical and topological complexity.

It was shown [New J. Phys. 17, 103037 (2015)] that large and robust entanglement between two different mechanical resonators could be achieved, either dynamically or in the steady state, in an optomechanical system in which the two mechanical resonators are coupled to a single cavity mode driven by a suitably chosen two-tone field. An important limitation of the scheme is that the cavity decay rate must be much smaller than the two mechanical frequencies and their difference. Here we show that the entanglement can be remarkably enhanced, and the validity of the scheme can be largely extended, by adding a coherent feedback loop that effectively reduces the cavity decay rate.

We study the role played by noise on the QW introduced in [1], a 1D model that is inspired by a two particle interacting QW. The noise is introduced by a random change in the value of the phase during the evolution, from a constant probability distribution within a given interval. The consequences of introducing such kind of noise depend on both the center value and the width of that interval: a wider interval manifests as a higher level of noise. For some range of parameters, one obtains a quasi-localized state, with a diffusive speed that can be controlled by varying the parameters of the noise. The existence of this (approximately) localized state for such kind of time-dependent noise is, to the best of our knowledge, totally new, since localization (i.e., Anderson localization) is linked in the literature to a spatial random noise.

We describe a quantum state transfer protocol, where a quantum state of photons stored in a first cavity can be faithfully transferred to a second distant cavity via an infinite 1D waveguide, while being immune to arbitrary noise (e.g. thermal noise) injected into the waveguide. We extend the model and protocol to a cavity QED setup, where atomic ensembles, or single atoms representing quantum memory, are coupled to a cavity mode. We present a detailed study of sensitivity to imperfections, and develop a quantum error correction protocol to account for random losses (or additions) of photons in the waveguide. Our numerical analysis is enabled by Matrix Product State techniques to simulate the complete quantum circuit, which we generalize to include thermal input fields. Our discussion applies both to photonic and phononic quantum networks.

Potential Energy Surface for the ($\mathrm{\bar{p}} - \mathrm{He}^{2+} - \mathrm{He}$) system is calculated in the framework of the restricted (singlet spin state) Hartree-Fock method with subsequent account of the electronic correlations within the second order perturbation method (MP2). The geometry of heavy particles is described in variables of distance $r$ from nucleus $a$ to antiproton, distance $R$ from the center of mass of the $(\bar{p} - a)$ pair to $\mathrm{He}$ atom containing nucleus $b$, and an angle $\theta$ between $\mathbf{r}$ and $\mathbf{R}$. The potential $V(R,r,\cos\theta)$ of the interaction between $\mathrm{He}$ atom and a $\bar{p} - a$ subsystem involves a total energy of two electrons in the field of three heavy particles and Coulomb interactions of the $b$ nucleus with antiproton and the $a$ nucleus. The expansion of this potential in terms of Legendre polynomials $\mathrm{P}_k(\cos\theta)$ is obtained. Matrices of the multipole terms $V^k(r,R)$ ($k=0,1,2$) are obtained in the basis of antiprotonic helium ion states. The results are compared with the model potential that was used earlier in the calculations of collisional Stark transitions of $(\mathrm{\bar{p}He}^{2+})$ ion.

To reconstruct thermodynamics based on the microscopic laws is one of the most important unfulfilled goals of statistical physics. Here, we show that the first law and the second law for adiabatic processes are derived from an assumption that "probability distributions of energy in Gibbs states satisfy large deviation", which is widely accepted as a property of thermodynamic equilibrium states. We define an adiabatic transformation as a randomized energy-preserving unitary transformations on the many-body systems and the work storage. As the second law, we show that an adiabatic transformation from a set of Gibbs states to another set of Gibbs states is possible if and only if the regularized von Neumann entropy becomes large. As the first law, we show that the energy loss of the thermodynamic systems during the adiabatic transformation is stored in the work storage as "work," in the following meaning; (i) the energy of the work storage takes certain values macroscopically, in the initial state and the final state. (ii) the entropy of the work storage in the final state is macroscopically equal to the entropy of the initial state. As corollaries, our results give the principle of maximam work and the first law for the isothermal processes.

Author(s): Andrzej Raczyński, Jarosław Zaremba, and Sylwia Zielińska-Raczyńska

The propagation, storage, and release of two quantized light fields in an optically dressed atomic medium in the tripod configuration are studied. Using the formalism of spinor dark-state polaritons, the probabilities are obtained of finding outgoing photons of two possible localizations and two pos…

[Phys. Rev. A 95, 033836] Published Tue Mar 28, 2017

Author(s): Ariel Bendersky, Gabriel Senno, Gonzalo de la Torre, Santiago Figueira, and Antonio Acín

Quantum mechanics postulates random outcomes. However, a model making the same output predictions but in a deterministic manner would be, in principle, experimentally indistinguishable from quantum theory. In this work we consider such models in the context of nonlocality on a device-independent sce…

[Phys. Rev. Lett. 118, 130401] Published Tue Mar 28, 2017

Author(s): T. R. Tan, Y. Wan, S. Erickson, P. Bierhorst, D. Kienzler, S. Glancy, E. Knill, D. Leibfried, and D. J. Wineland

We report correlation measurements on two Be9+ ions that violate a chained Bell inequality obeyed by any local-realistic theory. The correlations can be modeled as derived from a mixture of a local-realistic probabilistic distribution and a distribution that violates the inequality. A statistical fr…

[Phys. Rev. Lett. 118, 130403] Published Tue Mar 28, 2017