Quantum state transfer from an information-carrying qubit to a receiving qubit is ubiquitous for quantum information technology. In a closed quantum system, this task requires precisely timed control of coherent qubit-qubit interactions that are intrinsically reciprocal. Here, breaking reciprocity by tailoring dissipation in an open system, we show that it is possible to transfer a quantum state between stationary qubits autonomously without time-dependent control. We present the general requirements for this directional transfer process and show that the minimum system dimension for transferring one qubit of information is $3\times 2$ (between one physical qutrit and one physical qubit) plus one auxiliary reservoir. We propose realistic implementations in present-day superconducting circuit QED experiments and further propose schemes compatible with long-distance state transfer using impedance-matched dissipation engineering.

Topological properties of physical systems have attracted tremendous interest in condensed matter and atomic physics in the last decade. Recently, magnetic solid state compounds with and without magnetic order have become a focus. We provide evidence that ${\mathrm{BiCu}}_2{\mathrm{PO}}_6$ is a gapful, disordered quantum antiferromagnet with a non-trivial finite Zak phase, which characterizes one-dimensional systems. Our calculations show that in spite of the non-trivial topology no localized edge modes occur. This unexpected behavior is explained by a generic feature, namely the significance of direct and indirect gaps in topological systems.

We describe a framework for numerical calculation of time-periodically oscillating (breather) solutions to the discretized Lugiato-Lefever equation (LLE), as well as their linear stability as obtained from numerical Floquet analysis. Compared to earlier approaches, our work allows for the following conclusions: (i) The complete families of solutions are obtained also in regimes of instability; (ii) analysis of Floquet spectra and the corresponding eigenvectors show clearly the nature of the various Hopf and period-doubling bifurcations; (iii) properties of breather solutions to the continuous LLE are connected to corresponding oscillating solutions of the discrete LLE, which is of interest in its own right modeling coupled nonlinear cavities. In particular, we show that the oscillating discrete cavity solitons found in earlier work can be viewed as lattice versions of the continuous LLE breathers, as there is a smooth continuation in parameter space connecting them. Moreover, we confirm the existence of stable breathers at large detunings that was recently observed experimentally and describe their appearance from bifurcations.

Electrostatic cooling is known to occur in conductors and in porous electrodes in contact with aqueous electrolytes. Here we present evidence of electrostatic cooling at the junction of two electrolyte phases. These are, first, water containing salt and, second, an ion-exchange membrane, which is a water-filled porous layer containing a large concentration of fixed charges. When ionic current is directed through such a membrane in contact with aqueous phases on both sides, a temperature difference develops across the membrane which rapidly switches sign when the current direction is reversed. The temperature difference develops because one water-membrane junction cools down while the other heats up. Cooling takes place when the inner product of ionic current $\mathbf{I}$ and field strength $\mathbf{E}$ is a negative quantity, which is possible in the electrical double layers that form on the surface of the membrane. Theory reproduces the magnitude of the effect but overestimates the rate by which the temperature difference across the membrane adjusts itself to a reversal in current.

The spontaneous knotting of linear chains has been well studied, but little attention has been given to the self-entanglement of chains with more complex topologies. In this work, we perform experiments with granular chains that undergo tumbling motion to investigate the self-entanglement of circular chains, which lack the chain ends essential for forming knots. We study the entanglement probability and types of self-entanglements formed on linear and circular chains, using the well-studied self-entanglements on a linear chain to frame our understanding of self-entanglements on a circular chain. We describe a characterization method that views a self-entangled circular chain as a link of two components and use it to characterize the self-entanglements on circular chains with known topological descriptors from knot theory. Our experimental results show that an increase in circular chain length leads to an increase in entanglement probability and entanglement complexity until a plateau is reached, similar to the trends observed with linear chains. By examining the formation pathway of several self-entanglements, we infer a general mechanism for the self-entanglement of circular chains.

Atypically large fluctuations in macroscopic nonequilibrium systems continue to attract interest. Their probability can often be determined by the optimal fluctuation method (OFM). The OFM brings about a conditional variational problem, the solution of which describes the “optimal path” of the system which dominates the contribution of different stochastic paths to the desired statistics. The OFM proved efficient in evaluating the probabilities of rare events in a host of systems. However, theoretically predicted optimal paths were observed in stochastic simulations only in diffusive lattice gases, where the predicted optimal density patterns are either stationary, or travel with constant speed. Here we focus on the one-point height distribution of the paradigmatic Kardar-Parisi-Zhang interface. Here the optimal paths, corresponding to the distribution tails at short times, are intrinsically nonstationary and can be predicted analytically. Using the mapping to the directed polymer in a random potential at high temperature, we obtain “snapshots” of the optimal paths in Monte Carlo simulations which probe the tails with an importance sampling algorithm. For each tail we observe a very narrow “tube” of height profiles around a single optimal path which agrees with the analytical prediction. The agreement holds even at long times, supporting earlier assertions of the validity of the OFM in the tails well beyond the weak-noise limit.

We report the experimental observation of a cross-Kerr nonlinearity in a free-space medium based on resonantly excited, interacting Rydberg atoms and electromagnetically induced transparency. The nonlinearity is used to implement cross-phase modulation between two optical pulses. The nonlinear phase written onto the probe pulse is measured to be as large as 8 mrad per nanowatt of signal power, corresponding to a ${\chi }^{\left(3\right)}$ of ${10}^{-8}{\mathrm{m}}^2/{\mathrm{V}}^2$. Potential applications range from optical quantum information processing to quantum nondemolition measurement of photon number.

Rich quantum effects emerge when several quantum systems are indistinguishable from the point of view of the bath they interact with. In particular, delocalized excitations corresponding to coherent superposition of excited states (reminiscent of double slit experiments or beam splitters in interferometers) appear and change drastically the dynamics and steady state of the systems. Such phenomena, which are central mechanisms of superradiance, present interesting properties for thermodynamics and potentially other quantum technologies. Indeed, a recent paper [C. L. Latune, I. Sinayskiy, and F. Petruccione, Phys. Rev. A 99, 052105 (2019)] studies these properties in a pair of indistinguishable two-level systems and points out surprising effects of mitigation and amplification of the bath's action on the energy and entropy of the pair. Here, we generalize the study to ensembles of arbitrary number of spins of arbitrary size (i.e., dimension). We confirm that the previously uncovered mitigation and amplification effects remain, but also that they become more and more pronounced with growing number of spins and growing spin size. Moreover, we find that the free energy variation and the entropy production associated with the bath-driven dissipation are systematically reduced, formalising the idea of mitigation of the bath's action. Most remarkably, the combination of mitigation effects from two baths at different temperatures can result in amplifying their action. This is illustrated with cyclic thermal machines, and leads to large power enhancements. The reduction of irreversibility is also an interesting aspect since irreversibility is known to limit the performance of thermodynamic tasks. The above findings might also lead to interesting applications in collective work extraction, quantum battery charging, state protection, light harvesting devices, quantum biology, but also for the study of entropy production. Moreover, some experimental realisations and observations suggest that such effects are within reach.

Two-dimensional photon gases trapped in dye-filled microcavities can undergo thermalization and nearly ideal equilibrium Bose-Einstein condensation. However, they are inherently driven-dissipative systems that can exhibit an intricate interplay between the thermalizing influence of the environment given by the dye solution and the pump and loss processes driving the system out of equilibrium. We show that this interplay gives rise to a robust mechanism for two-mode emission, when the system is driven by an off-centered pump beam. Namely, after the system starts lasing in the dominantly pumped excited mode, in a second transition a photon condensate is formed in the ground mode, when the pump power is increased further. This effect is a consequence of the redistribution of excited dye molecules via the lasing mode in combination with thermalization. We propose to exploit this effect for engineering-controlled two-mode emission and demonstrate that by tailoring the transverse potential landscape for the photons, the threshold pump power can be tuned by orders of magnitude.

Water inside hygroscopic porous media such as plantlike systems can be found either freely penetrating in capillaries or absorbed into the solid phase (bound water). Here we demonstrate that the wetting properties (contact angle) of liquid along cell walls significantly depend on the amount of bound water absorbed: a change from poor to good wetting is observed when cell walls are saturated with bound water, which allows liquid displacement. We further show that this process is operative in hydrogels, suggesting that this might be a general property of porous hygroscopic systems. As a consequence, imbibition dynamics is controlled by water adsorption and diffusion in the walls, and even if the dynamics of capillary imbibition is strongly damped (by several orders of magnitude), water can freely climb over significant heights as long as sufficient water has been adsorbed into cell walls or in other hygroscopic walls. Under these conditions, the imbibition process in such systems is not described by the standard model but is analogous to the propagation of a front of solidification in a liquid. This process might contribute to the regulation of water absorption in unsaturated wood, allowing it to store available bound water in progressively higher depths, instead of leaving free water rapidly flow through it. Such a mechanism may be explored to design porous materials with tunable liquid adsorption timings for pharmaceutical or chemical engineering applications.

Spin inertia measurements are a novel experimental tool to study long-time spin relaxation processes in semiconductor nanostructures. We develop a theory of the spin inertia effect for resident electrons and holes localized in quantum dots. We consider the spin orientation by short optical pulses with arbitrary pulse area and detuning from the trion resonance. The interaction with an external longitudinal magnetic field and the hyperfine interaction with the nuclear spin bath is considered in both the ground and excited (trion) states of the quantum dots. We analyze how the spin inertia signal depends on the magnetic field (polarization recovery) and on the modulation frequency of the helicity of the pump pulses as well as on their power and detuning. In particular, we elaborate how approaching the saturation limit of the spin polarization influences the measurements. The quantitative description of spin inertia measurements will enable the determination of the parameters of spin dynamics such as the spin relaxation times in the ground and excited states and the parameters of the hyperfine interaction. Finally, we predict the emergence of resonant spin amplification due to the transverse components of the nuclear spin fluctuations, which manifests itself as oscillations of the spin polarization as a function of the longitudinal magnetic field.

Simulations of quantum transport in coherent conductors have evolved into mature techniques that are used in fields of physics ranging from electrical engineering to quantum nanoelectronics and material science. The most efficient general-purpose algorithms have a computational cost that scales as $L^{6,\cdots ,7}$ in three dimensions ($L$: length of the device), which on the one hand is a substantial improvement over older algorithms, but on the other hand still severely restricts the size of the simulation domain, limiting the usefulness of simulations through strong finite-size effects. Here we present a class of algorithms that, for certain systems, allows us to directly access the thermodynamic limit. Our approach, based on the Green's function formalism for discrete models, targets systems that are mostly invariant by translation, i.e., invariant by translation up to a finite number of orbitals and/or quasi-one-dimensional electrodes and/or the presence of edges or surfaces. Our approach is based on an automatic calculation of the poles and residues of series expansions of the Green's function in momentum space. This expansion allows us to integrate analytically in one momentum variable. We illustrate our algorithms with several applications: devices with graphene electrodes that consist of half an infinite sheet, Friedel oscillation calculations of infinite two-dimensional systems in the presence of an impurity, quantum spin Hall physics in the presence of an edge, and the surface of a Weyl semimetal in the presence of impurities and electrodes connected to the surface. In this last example, we study the conduction through the Fermi arcs of the topological material and its resilience to the presence of disorder. Our approach provides a practical route for simulating three-dimensional bulk systems or surfaces as well as other setups that have so far remained elusive.

Dark matter may induce apparent temporal variations in the physical “constants”, including the electromagnetic fine-structure constant and fermion masses. In particular, a coherently oscillating classical dark-matter field may induce apparent oscillations of physical constants in time, while the passage of macroscopic dark-matter objects (such as topological defects) may induce apparent transient variations in the physical constants. In this paper, we point out several new signatures of the aforementioned types of dark matter that can arise due to the geometric asymmetry created by the beam-splitter in a two-arm laser interferometer. These new signatures include dark-matter-induced time-varying size changes of a freely suspended beam-splitter and associated time-varying shifts of the main reflecting surface of the beam-splitter that splits and recombines the laser beam, as well as time-varying refractive-index changes in the freely suspended beam-splitter and time-varying size changes of freely suspended arm mirrors. We demonstrate that existing ground-based experiments already have sufficient sensitivity using existing data to probe extensive regions of the unconstrained parameter space in models involving oscillating scalar dark-matter fields and domain walls composed of scalar fields. In the case of oscillating dark-matter fields, Michelson interferometers—in particular, the GEO 600 detector—are especially sensitive. The sensitivity of Fabry-Perot-Michelson interferometers, including LIGO, VIRGO, and KAGRA, to oscillating dark-matter fields can be significantly increased by making the thicknesses of the freely suspended Fabry-Perot arm mirrors different in the two arms. Not-too-distantly separated laser interferometers can benefit from cross-correlation measurements in searches for effects of spatially coherent dark-matter fields. In addition to broadband searches for oscillating dark-matter fields, we also discuss how small-scale Michelson interferometers, such as the Fermilab holometer, could be used to perform resonant narrowband searches for oscillating dark-matter fields with enhanced sensitivity to dark matter. Finally, we discuss the possibility of using future space-based detectors, such as LISA, to search for dark matter via time-varying size changes of and time-varying forces exerted on freely floating test masses.

Strong multiple scattering of the probe in scanning transmission electron microscopy (STEM) means image simulations are usually required for quantitative interpretation and analysis of elemental maps produced by electron energy-loss spectroscopy (EELS). These simulations require a full quantum-mechanical treatment of multiple scattering of the electron beam, both before and after a core-level inelastic transition. Current algorithms scale quadratically and can take up to a week to calculate on desktop machines even for simple crystal unit cells and do not scale well to the nanoscale heterogeneous systems that are often of interest to materials science researchers. We introduce an algorithm with linear scaling that typically results in an order of magnitude reduction in computation time for these calculations without introducing additional error and discuss approximations that further improve computational scaling for larger-scale objects with modest penalties in calculation error. We demonstrate these speedups by calculating the atomic resolution STEM-EELS map using the $L$-edge transition of Fe, for a nanoparticle 80 Å in diameter, in 16 hours, a calculation that would have taken at least 80 days using a conventional multislice approach.

We study non-Markovian enhancement effects in the spontaneous emission of a collective excitation in a linear chain of up to 100 qubits coupled to a one-dimensional waveguide. We find that for a critical separation of qubits, the system exhibits super-superradiant (SSR) behavior leading to collective decay stronger than the usual Dicke superradiance. Here time-delayed coherent quantum feedback effects are at play on top of the usual Dicke superradiance effects. We find a linear scaling for the SSR decay rate with increasing qubit number $N$ such that ${\mathrm{\Gamma }}_{\mathrm{SSR}}\sim 2.277N{\gamma }_0$, where ${\gamma }_0$ is the single-emitter decay rate to a one-dimensional waveguide, as opposed to ${\mathrm{\Gamma }}_{\mathrm{Dicke}}\sim N{\gamma }_0$ for Dicke superradiance. The SSR decay rate can be tuned with qubit separation distance and may therefore have application for quantum technologies.

We show that any multiqudit entanglement witness leads to a nonseparability indicator for quantum optical fields, which involves intensity correlations. We get, e.g., necessary and sufficient conditions for intensity or intensity-rate correlations to reveal polarization entanglement. We also derive separability conditions for experiments involving multiport interferometers, now feasible with integrated optics. We show advantages of using intensity rates rather than intensities, e.g., a mapping of the Bell inequalities to ones for optical fields. The results have implication for studies of nonclassicality of “macroscopic” systems of undefined or uncontrollable number of “particles.”

We address the problem of electronic and nuclear spin resonance of an individual atom on a surface driven by a scanning tunneling microscope. Several mechanisms have been proposed so far, some of them based on the modulation of exchange and crystal field associated with a piezoelectric displacement of the adatom driven by the radio frequency (RF) tip electric field. Here we consider another mechanism, where the piezoelectric displacement modulates the $g$-factor anisotropy, leading both to electronic and nuclear spin flip transitions. We discuss thoroughly the cases of hydrogenated Ti ($S=1/2$) and Fe $(S=2)$ on MgO, relevant for recent experiments. We model the system using two approaches. First, an analytical model that includes crystal field, spin orbit coupling, and hyperfine interactions. Second, we carry out density-functional-based calculations. We find that the modulation of the anisotropy of the $g$ tensor due to the piezoelectric displacement of the atom is an additional mechanism for scanning tunneling microscopy (STM)-based single spin resonance that would be effective in $S=1/2$ adatoms with large spin orbit coupling. In the case of hydrogenated Ti on MgO, we predict a modulation spin resonance frequency driven by the DC electric field of the tip.

We investigate low-temperature electronic states of the series of organic conductors ${\beta }^{\prime \prime }-{\left[\text{bis(ethylenedithio)tetrathiafulvalene}\right]}_4\left[\left({\mathrm{H}}_3\mathrm{O}\right)M{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]G$, where $M$ and $G$ represent trivalent metal ions and guest organic molecules, respectively. Our structural analyses reveal that the replacement of $M$ and $G$ give rise to systematic change in the cell parameters, especially in the $b$-axis length, which has a positive correlation with the superconducting transition temperature $T_{\mathrm{c}}$. Analysis of temperature and magnetic field dependences of the electrical resistance including the Shubnikov–de Haas oscillations elucidates that the variation of charge disproportionation, the effective mass, and the number of itinerant carriers can be systematically explained by the change of the $b$-axis length. The changes of the transfer integrals induced by stretching/compressing the $b$ axis are confirmed by the band calculation. We discuss that electron correlations in quarter-filled electronic bands lead to charge disproportionation and the possibility of a novel pairing mechanism of superconductivity mediated by charge degrees of freedom.

One of the main difficulties of observing many-body localization in natural solid-state materials is creating strong enough disorder. A strong random local magnetic field is difficult to achieve in a solid state material. We propose exploiting large random magnetic anisotropy, either in magnitude or direction, which can be realized in organometallic quantum magnets. We present the phase diagram of an $S=1$ Heisenberg chain in terms of both a random magnetic anisotropy and a random magnetic field. The many-body localization phase emerges with sufficiently large anisotropy under very small random fields. We propose candidate materials of doped single-chain organometallic quantum magnets for realizing many-body localization, where either orientation disorder or substitution of metal ions can create large random magnetic anisotropy required in our prediction.

It is known that the squared modulus of the overlap (scalar product) of two energy eigenstates in a decaying quantum system is bounded from above by a function of the energy detuning and the individual decay rates. This is usually traced back to the positive definiteness of an appropriately defined decay operator. Here, we show that the weaker and more realistic condition of positive semidefiniteness is sufficient. We prove also that the bound becomes an equality for the case of single-channel decay. However, we show that the condition of positive semidefiniteness can be spoiled by quantum backflow. Hence, the overlap of quasibound quantum states subjected to outgoing-wave conditions can be larger than expected from the bound. A modified and less stringent bound, however, can be introduced. For electromagnetic systems, it turns out that a modification of the bound is not necessary due to the linear free-space dispersion relation. Finally, a geometric interpretation of the nonorthogonality bound is given which reveals that in this context the complex energy space can be seen as a surface of constant negative curvature.