The Aharonov-Bohm (AB) phase is usually associated with a line integral of the electromagnetic vector potential generated by an external current source, such as a solenoid. According to this interpretation, the AB phase of a nonclosed path cannot be observed, as the integral depends on the gauge choice of the vector potential. Recent attempts to explain the AB effect through the interaction between a charged particle and an external current, mediated by the exchange of quantum photons, have assumed that the AB phase shift is proportional to the change in interaction energy between the charged particle and the external current source. As a result, these attempts argue that the AB phase change along a path does not depend on the gauge choice, and that the AB phase shift for a nonclosed path is in principle measurable. In this paper, we critically examine this claim and demonstrate that the phase obtained through this approach is actually gauge dependent and not an observable for a nonclosed path. We also provide a brief critical discussion of the proposed experiment for observing the AB phase shift of a nonclosed path.

We demonstrate that nonequilibrium steady states of the dissipative Rabi model show dramatic spikes in transport rates over narrow parameter ranges. Similar results are found for the Holstein and Dicke models. This is found to be due to avoided energy level crossings in the corresponding closed systems, and correlates with spikes in the entanglement entropy of key eigenstates, a signature of strong mixing and resonance among system degrees of freedom. Further, contrasting the Rabi model with the Jaynes-Cummings model reveals this behavior as being related to quantum integrability.

In this paper we propose a truncation method for determining the trace distance between two Gaussian states in fermionic systems. For two fermionic Gaussian states, characterized by their correlation matrices, we consider the von Neumann entropies and dissimilarities between their correlation matrices and truncate the correlation matrices to facilitate trace distance calculations. Our method exhibits notable efficacy in two distinct scenarios. In the first scenario, the states have small von Neumann entropies, indicating finite or logarithmic-law entropy, while their correlation matrices display near-commuting behavior, characterized by a finite or gradual nonlinear increase in the trace norm of the correlation matrix commutator relative to the system size. The second scenario encompasses situations where the two states are nearly orthogonal, with a maximal canonical value difference approaching 2. To evaluate the performance of our method, we apply it to various compelling examples. Notably, we successfully compute the subsystem trace distances between low-lying eigenstates of Ising and $XX$ spin chains, even for significantly large subsystem sizes. This is in stark contrast to existing literature, where subsystem trace distances are limited to subsystems of approximately ten sites. With our truncation method, we extend the analysis to subsystems comprising several hundred sites, thus expanding the scope of research in this field.

We consider the buildup and decay of two-spin entanglement through phase interactions in a finite environment of surrounding spins, as realized in quantum computing platforms based on arrays of atoms, molecules, or nitrogen vacancy centers. The non-Markovian dephasing caused by the spin environment through Ising-type phase interactions can be solved exactly and compared to an effective Markovian treatment based on collision models. In a first case study on a dynamic lattice of randomly hopping spins, we find that non-Markovianity boosts the dephasing rate caused by nearest-neighbor interactions with the surroundings, degrading the maximum achievable entanglement. However, we also demonstrate that additional three-body interactions can mitigate this degradation, and that randomly timed reset operations performed on the two-spin system can help sustain a finite average amount of steady-state entanglement. In a second case study based on a model nuclear magnetic resonance system, we elucidate the role of bath correlations at finite temperature on non-Markovian dephasing. They speed up the dephasing at low temperatures while slowing it down at high temperatures, compared to an uncorrelated bath, which is related to the number of thermally accessible spin configurations with and without interactions.

Vigorous optimization of quantum gates has led to bipotent quantum architectures, where the optimized gates are available for some qubits but not for others. However, such gate-level improvements limit the application of user-side pulse-level optimizations, which have proven effective for quantum circuits with a high level of regularity, such as the ansatz circuit of the quantum approximate optimization algorithm (QAOA). In this paper, we investigate the trade-off between hardware-level and algorithm-level improvements on bipotent quantum architectures. Our results for various QAOA instances on two quantum computers offered by IBM indicate that the benefits of pulse-level optimizations currently outweigh the improvements due to vigorously optimized monolithic gates. Furthermore, our data indicate that the fidelity of circuit primitives is not always the best indicator for the overall algorithm performance; their gate type and schedule duration should also be taken into account. This effect is particularly pronounced for QAOA on dense portfolio optimization problems, since their transpilation requires many swap gates, for which efficient pulse-level optimization exists. Our findings provide practical guidance on optimal qubit selection on bipotent quantum architectures and suggest the need for improvements of those architectures, ultimately making pulse-level optimization available for all gate types.

We consider ionized hydrogenlike atoms accelerated by an external electric field to detect Unruh radiation. By applying quantum field theory in the Rindler space-time, we show that the first-quantized description for hydrogenlike atoms cannot always be adopted. This is due to the frame-dependent definition of particles as positive and negative frequency field modes. We show how to suppress such a frame-dependent effect by constraining the atomic ionization and the electric field. We identify the physical regimes with nonvanishing atomic excitation probability due to the Unruh electromagnetic background. We recognize the observational limits for the Unruh effect via first-quantized atomic detectors, which appear to be compatible with current technology. Notably, the nonrelativistic energy spectrum of the atom cannot induce coupling with the thermal radiation, even when special relativistic and general relativistic corrections are considered. On the contrary, the coupling with the Unruh radiation arises because of relativistic hyperfine splitting and the Zeeman effect.

Until recently, theoretical and experimental studies of photoelectron angular distributions (PADs) including nondipole effects in atomic photoionization have been performed mainly for the conventional plane-wave radiation. One can expect, however, that the nondipole contributions to the angular- and polarization-resolved photoionization properties are enhanced if an atomic target is exposed to twisted light. The purpose of the present study is to develop a theory for PADs for the case of twisted light, especially for many-electron atoms. The theoretical analysis is performed for the experimentally relevant case of macroscopic atomic targets, i.e., when the cross-sectional area of the target is larger than the characteristic transversal size of the twisted beam. For such a scenario, we derive expressions for the angular distribution of the emitted photoelectrons under the influence of twisted Bessel beams. As an illustrative example, we consider helium photoionization in the region of the lowest dipole $2s2p\left[{{}^{1}P}_1\right]$ and quadrupole $2p^2\left[{{}^{1}D}_2\right]$ autoionization resonances. A noticeable variation of the PAD caused by changing the parameters of the twisted light is predicted.

Multifarious topological crystals are extensively explored because of their unique advantages in manipulating wave transport and immunity against local perturbations. Most of them are studied, however, in feature-fixed systems and therefore, commonly have a challenge in application that requires tunability, such as quantum switches, tunable quantum routers, and quantum storage and reading. Here, we theoretically suggest a reconfigurable valley topological quantum electrodynamics (QED) platform scheme based on the honeycomb lattice of Jaynes-Cummings emitters, which can be implemented by cavity- or circuit-QED cells. Based on such a reconfigurable platform, this work first designs a tunable topological quantum router and then demonstrates how to achieve topological quantum storage and reading in theory. They both signify the remarkable potential of the suggested reconfigurable topological QED platform for qubit operation.

We investigate nonreciprocal photon correlations in coupled microring resonators with directional quantum squeezing (quantum parametric amplification). We show that the degeneracy of two whispering-gallery modes (WGMs) in a microring resonator are broken by applying parametric amplification to one of the WGMs, which makes tunable chiral photon-qubit coupling and chiral photon hopping between two resonators feasible. Selecting optimal quantum squeezing strength, we predict the appearance of nonreciprocal unconventional photon blockade resulting from the interferences among different photon transition-dissipative paths both analytically and numerically. Moreover, flexible conversion between photon bunching and antibunching can be easily realized in a wide range of parameter regimes by modulating the quantum squeezing field. This work provides an alternative way for tunable nonreciprocal photon coherence manipulations in chiral quantum science and technologies.

We study the time-dependent quantum Fisher information (QFI) in an open quantum system satisfying the Gorini-Kossakowski-Sudarshan-Lindblad master equation. Most of our work concerns a simple three-level model for a single probe coupled to a sink, although some results for multiple probes are also given. We also study the dynamics of the system from an effective non-Hermitian dynamics standpoint and use it to understand the scaling of the QFI when multiple probes are used. A focus of our work is how the QFI is maximized at certain times, suggesting that the best precision in parameter estimation can be achieved by focusing on these times. The propagation of errors analysis allows us to confirm and better understand this idea. We also propose an optimum-time-based parameter estimation procedure involving relatively low-resource-consuming measurements followed by higher-resource-consuming measurements and demonstrate it in simulation.

We study the nucleation and dynamics of vortices in rotating lattice potentials where weakly linked condensates are formed with each condensate exhibiting an almost axial symmetry. Due to such a symmetry, the on-site phases acquire a linear dependence on the coordinates as a result of the rotation, which allows us to predict the position of vortices along the low-density paths that separate the sites. We first show that, for a system of atoms loaded in a four-site square lattice potential, subject to a constant rotation frequency, the analytical expression that we obtain for the positions of vortices of the stationary arrays accurately reproduces the full three-dimensional Gross-Pitaevskii results. We then study the time-dependent vortex nucleation process when a linear ramp of the rotation frequency is applied to a lattice with 16 sites. We develop a formula for the number of nucleated vortices which turns out to have a linear dependence on the rotation frequency with a smaller slope than that of the standard estimate which is valid in the absence of the lattice. From time-dependent Gross-Pitaevskii simulations we further find that the on-site populations remain almost constant during the time evolution instead of spreading outwards, as expected from the action of the centrifugal force. Therefore, the time-dependent phase difference between neighboring sites acquires a running behavior typical of a self-trapping regime. We finally show that, in accordance with our predictions, this fast phase-difference evolution provokes a rapid vortex motion inside the lattice. Our analytical expressions may be useful for describing other vortex processes in systems with the same on-site axial symmetry.

Recently created self-bound quantum droplets of binary Bose mixtures open intriguing possibilities for the study of impurity physics. We show that the properties of impurities embedded in quasi-one-dimensional droplets are determined by the interplay between back-action and quantum fluctuations. Due to such back-action, repulsive impurities may form a metastable quasi-bound state inside the droplet. In contrast, attractive impurities remain bound to the droplet, leading to the hybridization of droplet and impurity excitations, as well as to peculiar scattering resonances. Interestingly, impurity trapping may result solely from the effect of quantum fluctuations. These results may readily be probed experimentally by doping the currently available droplets of binary mixtures.

Photonic time crystals (PTCs) provide a completely new platform exhibiting light wave amplification owing to periodically varying electromagnetic properties. The need to control this amplification is becoming increasingly important, especially with the emergence of metasurface-based practical realization of PTCs. This paper investigates the introduction of isolated temporal defect in PTCs to establish an increased degree of control over the amplification. We find that in the presence of the defect, the transmittance and reflectance become close to unity for a specific value of momentum $k_d$ within the band gaps accompanied by a significant impact on the amount of amplification. We show the impact of the temporal defect on the exponential growth of the intensity with PTC periods. The effect primarily depends on the Floquet frequency of the PTC, which becomes real at $k_d$ giving rise to four pulses instead of two as an outcome of gap propagation. We further demonstrate that by manipulating the temporal and dielectric properties of the defect, the defect state in the momentum can be tuned to serve the design interest for specialty applications.

Two-dimensional periodic arrays of regions containing metallic nanoislands with random sizes and shapes, known as templated nanoisland arrays, can exhibit plasmonic phase-correlated dipole domains. These domains emerge at specific wavelengths where the lattice-induced plasmon dipoles of the nanoislands are coherently coupled together, forming collective plasmonic states that extend spatially over each region with scales of up to one micron S. M. Sadeghi and R. R. Gutha, Phys. Rev. Appl. 15, 034018 (2021). In this paper, we demonstrate that the far-field in-plane scattering of these arrays can be utilized to access the coherent signatures of the plasmonic phase-correlated dipole domains. Our results reveal that this process is particularly pronounced when the coherent coupling of these domains, facilitated by their hybridization with the Rayleigh Anomaly, gives rise to surface lattice resonances. These resonances effectively enhance the far-field in-plane scattering of templated nanoisland arrays, providing a platform to investigate the influence of plasmonic structures on the lattice-induced plasmon dipoles responsible for the emergence of such domains. They also allow us to explore the coherent networks formed through the coupling of these domains and with their neighboring domains. The findings of this study offer an opportunity to examine the effects of micron-scale coherently extended plasmon states and their potential applications in the long-range coherent transport of energy and information.

Multimode cavity quantum electrodynamics describes, for example, the coupling between an atom and a multimode electromagnetic resonator. The gauge choice is important for practical calculations in truncated Hilbert spaces, because the exact gauge invariance is recovered only in the whole space. An optimal gauge can be defined as the one predicting the most accurate observables for the same number of atomic levels and modes. Different metrics quantifying the gauge performance can be introduced depending on the observable of interest. In this paper we demonstrate that the optimal choice is generally mode dependent, i.e., a different gauge is needed for each cavity mode. While the choice of gauge becomes more important for increasing light-matter interaction, we also show that the optimal gauge does not correspond to the situation where the entanglement between light and matter is the smallest.

We have developed a theory of parametric photon generation in the waveguides coupled to arrays of quantum emitters with temporally modulated resonance frequencies. Such generation can be interpreted as a dynamical Casimir effect. We demonstrate numerically and analytically how the emission directionality and photon-photon correlations can be controlled by the phases of the modulation. The emission spectrum is shown to be strongly dependent on the anharmonicity of the emitter potential. Single- and double-excited state resonances have been identified in the emission spectrum.

A nuclear-spin-based RASER (radio-frequency amplification by stimulated emission of radiation) is an ideal experimental system to explore nonlinear interaction phenomena of nuclear spins coupled via virtual photons to a resonator. This is due to the RASER being stable for several hours, allowing for extended observation of these phenomena. Nonlinear phenomena in multimode RASERs range from mode oscillations in synchrony, frequency shifts, frequency combs, period doublings, and even chaos. These phenomena are observed in a parahydrogen-pumped two-compartment proton RASER. In two independently pumped compartments, the separation in frequency space between the two RASER modes is precisely controlled with a magnetic field gradient. By controlling the mode separation, we can select the type of nonlinear phenomena observed. A key finding is that the ranges of mode separation where chaos and synchrony occur are very close together. The experimental results are supported by numerical simulations, based on two-mode RASER equations.

Linear combinations of Bessel beams can be used to effectively trap light within cylindrical domains. Such hard traps can be used to produce states that exhibit stationary arrays of optical vortices from the perspective of a steadily rotating frame. These patterned singularities can be engineered to have singularities of the same or mixed charges and the requisite rotation rates are quantized even though the setting is purely linear. A hydrodynamic interpretation is that the vortices are at rest within a compressible, two-dimensional fluid of light.

We study modulational instability in a dispersion-managed optical fiber system where the sign of the group-velocity dispersion is changed at uniformly distributed random distances around a reference length. We find an instability gain of stochastic origin comparable to the conventional values found in a homogeneous anomalous dispersion fiber. We develop an accurate analytical technique based on transfer matrices to estimate the instability gain from the linearized nonlinear Schrödinger equation, which is also solved numerically. The comparison of numerical and analytical results confirms the validity of our approach. Modulational instability sidebands of purely stochastic origin appear and the competition between sidebands of periodic and stochastic origin is also discussed. These results may be of interest in tailoring and controlling modulational instability sidebands for telecommunications and parametric sources. Our method can also be applied to general linear stochastic differential equations with multiplicative noise, which broadly occur in Physics.