We propose a characterization of measurement apparatuses based on their premeasurement reliability and accessibility. Our notion of premeasurement reliability parameterizes the possibility of getting unexpected wrong results when extracting information about a measured system from the apparatus in a given time window, and the one of accessibility describes the energy cost required to make the apparatus interact with the measured system. The characterization is then obtained by relating an apparatus's premeasurement reliability and accessibility to the time dependence of the overlap of its pointer states. As an example, we study a one-to-all qubit interaction in which the all act as a measurement apparatus for the one. This model shows that using randomly selected couplings results in accessible but unpredictable measurement apparatuses. Conversely, apparatuses with uniform coupling exhibit higher reliability but are energetically more costly.

We investigate the consequences of introducing nonuniform initial conditions in the dynamics of spin ensembles characterized by all-to-all interactions. Specifically, our study involves the preparation of a set of semiclassical spin ensembles with varying orientations. Through this setup, we explore the influence of such nonuniform initial states on the disruption of permutational invariance. Comparing this approach to the traditional scenario of initializing with spins uniformly aligned, we find that the dynamics of the spin ensemble now spans a more expansive effective Hilbert space. This enlargement arises due to the inclusion of off-diagonal coherences between distinct total angular momentum subspaces, an aspect typically absent in conventional treatments of all-to-all spin dynamics. Conceptually, the dynamic evolution can be understood as a composite of multiple homogeneous subensembles navigating through constrained subspaces. Notably, observables that are sensitive to the nonuniformity of initial conditions exhibit discernible signatures of these off-diagonal coherences. We adopt this fresh perspective to reexamine the relaxation phenomena exhibited by the Dicke model, as well as a prototypical example of a boundary time crystal. We show that ensembles initialized with inhomogeneous initial conditions can show distinctive behaviors when contrasted with canonical instances of collective dynamics. These behaviors encompass the emergence of gapless excitations, the manifestation of limit cycles featuring dressed frequencies due to superradiance, instances of frequency locking or beating synchronizations, and even the introduction of “extra” dimensions within the dynamics. In closing, we provide a brief overview of the potential implications of our findings in the context of modern cavity quantum electrodynamics (QED) platforms.

We propose a scheme for two-qubit gates between a flying photon and an atom in a cavity. The atom-photon gate setup consists of a cavity and a Mach-Zehnder interferometer with doubly degenerate ground- and excited-state energy levels mediating the atom-light interaction. We provide an error analysis of the gate and model important errors, including spatial mode mismatch between the photon and the cavity, spontaneous emission, cavity losses, detunings, and random fluctuations of the cavity parameters and frequencies. Error analysis shows that the gate protocol is more robust against experimental errors compared to previous atom-photon gates and achieves higher fidelity.

We study a one-dimensional two-component Fermi gas in a harmonic trapping potential using finite-temperature lattice quantum Monte Carlo methods. We are able to compute observables in the canonical ensemble via an efficient projective approach. Results for density profiles, correlations, as well as energy-related observables are presented for systems with up to 80 particles and various temperatures. Our simulations reproduce known numerical results and compare well against available experimental data close to the ground state, while at higher temperature they are benchmarked against the exact solution of the two-particle system. This provides an indication that a standard lattice discretization is sufficient to capture the physics of the trapped system. In the special case of a spin-imbalanced gas, we find no sign problem in the studied parameter ranges, allowing access without the need of specialized methods. This includes simulations close to the ground state and at large population imbalance, where we present results for density correlations, indicating pairing at finite total momentum.

The present work aims at providing a systematic analysis of the current density versus momentum characteristics for a fermionic superfluid throughout the BCS-BEC crossover, even in the fully homogeneous case. At low temperatures, where pairing fluctuations are not strong enough to invalidate a quasiparticle approach, a sharp threshold for the inception of a back-flow current is found, which sets the onset of dissipation and identifies the critical momentum according to Landau. This momentum is seen to smoothly evolve from the BCS to the BEC regimes, whereby a single expression for the single-particle current density that includes pairing fluctuations enables us to incorporate on equal footing two quite distinct dissipative mechanisms, namely, pair breaking and phonon excitations in the two sides of the BCS-BEC crossover, respectively. At finite temperature, where thermal fluctuations broaden the excitation spectrum and make the dissipative (kinetic and thermal) mechanisms intertwined with each other, an alternative criterion due to Bardeen is instead employed to signal the loss of superfluid behavior. In this way, detailed comparison with available experimental data in linear and annular geometries is significantly improved with respect to previous approaches, thereby demonstrating the crucial role played by quantum fluctuations in renormalizing the single-particle excitation spectrum.

We consider the evolution of two contact-interacting harmonically trapped particles following an arbitrary quench in interaction strength. We focus on the change in system energy, the work, associated with the quench. When quenching from any nonzero interaction strength to zero interaction strength we observe that the work done and particle separation diverge. In particular, the divergent behavior arises always and exclusively when quenching $\mathit{to}$ the noninteracting regime. We demonstrate that the source of the divergence is its instantaneous nature. This validates and builds upon previous work that found divergent behavior arises when quenching from the strongly interacting limit to the noninteracting limit in both the two- and three-body cases.

We investigate the dynamics of a spatial discrete soliton and the radiation families emitted by it inside a parity-time ($\mathcal{PT}$)-symmetric waveguide array with alternate gain-loss channels. A strong spatial soliton that evolves inside the waveguide array due to the balance between discrete diffraction and Kerr nonlinearity excites linear waves in the form of diffractive radiation when launched with an angle. The $\mathcal{PT}$-symmetric nature of the waveguide leads to additional radiations in Fourier space that have not been explored. In our paper, we mainly focus on the origin of these radiations and try to understand how to control them. Under strong $\mathcal{PT}$ symmetry, a discrete soliton launched normally to the waveguide array produces strong side lobes, which can lead to a population of the field at $\pm \pi /2$ in momentum space. In addition, a strong soliton with initial phase gradient radiates a unique $\mathcal{PT}$-symmetry-assisted linear wave. We establish a phase-matching condition to locate such radiation in momentum space. The periodic arrangement of the gain-loss channel also leads to radiations due to reflection and backscattering, which is prominent for a weak soliton. A linear Hamiltonian analysis for such a waveguide array is provided to identify the $\mathcal{PT}$-phase-transition regime and to optimize the parameters for stable discrete soliton dynamics. We thoroughly investigate the origin of all the radiations that emerge in the $\mathcal{PT}$-symmetric waveguide array and put forward the background theory, which is in good agreement with the full numerical results.

Quantum batteries are energy storage devices that satisfy quantum-mechanical principles. How to obtain analytical solutions for quantum battery systems and achieve a full charging is a crucial element of the quantum battery. Here, we investigate the Rosen-Zener quantum battery with $N$ two-level systems, which includes atomic interactions and external driving field. The analytical solutions of the stored energy, changing power, energy quantum fluctuations, and von Neumann entropy (diagonal entropy) are derived by employing the gauge transformation. We demonstrate that a full charging process can be achieved when the external driving field strength and scanning period conforms to a quantitative relationship. The local maximum value of the final stored energy corresponds to the local minimum values of the final energy fluctuations and diagonal entropy. Moreover, we find that the atomic interaction induces the quantum phase transition and the maximum stored energy of the quantum battery reaches the maximum value near the quantum phase transition point. Our result provides an insightful theoretical scheme to realize the efficient quantum battery.

Many protocols and tasks in quantum information science rely inherently on the fundamental notion of contextuality to provide advantages over their classical counterparts, and contextuality represents one of the main differences between quantum and classical physics. In this work we present a witness for preparation contextuality inspired by optimal two-state discrimination. The main idea is based on finding the accessible averaged success and error probabilities in both classical and quantum models. We can then construct a noncontextuality inequality and associated witness which we find to be robust against depolarising noise and loss in the form of inconclusive events.

We consider a toy model of noise channels, given by a random mixture of unitary operations, for state transfer problems with continuous variables. Assuming that the path between the transmitter node and the receiver node can be intervened, we propose a noise decoupling protocol to manipulate the noise channels generated by linear and quadratic polynomials of creation and annihilation operators, to achieve an identity channel, hence the term noise decoupling. For random constant noise, the target state can be recovered while for the general noise profile, the decoupling can be done when the interventions are fast compared to the noise. We show that the state at the transmitter can be written as a convolution of the target state and a filter function characterizing the noise and the manipulation scheme. We also briefly discuss that a similar analysis can be extended to the case of higher-order polynomial generators. Finally, we demonstrate the protocols by numerical calculations.

Quantum cooling has demonstrated its potential in quantum computing, which can reduce the number of control channels needed for external signals. Recent progress also supports the possibility of maintaining quantum coherence in large-scale systems. The limitations of classical algorithms trapped in the local minima of cost functions could be overcome using this scheme. According to this, we propose a hybrid quantum-classical algorithm for finding the global minima. Our approach utilizes quantum coherent cooling to facilitate coordinative tunneling through energy barriers if the classical algorithm gets stuck. The encoded Hamiltonian system represents the cost function, and a quantum coherent bath in the ground state serves as a heat sink to absorb energy from the system. Our proposed scheme can be implemented in the circuit quantum electrodynamics system using a quantum cavity. The provided numerical evidence demonstrates the quantum advantage in solving spin-glass problems.

Making and using polaritonic states (i.e., hybrid electron-photon states) for chemical applications has recently become one of the most prominent and active fields that connects the communities of chemistry and quantum optics. Modeling of such polaritonic phenomena using ab initio approaches calls for new methodologies, leading to the reinvention of many commonly used electronic structure methods, such as Hartree-Fock, density functional, and coupled cluster theories. In this work, we explore the formally exact diffusion quantum Monte Carlo approach to obtain numerical solutions to the polaritonic ground state during the dissociation of the ${\mathrm{H}}_2$ molecular system. We examine various electron-nuclear-photon properties throughout the dissociation, such as changes to the minimum of the cavity Born-Oppenheimer surface, the localization of the electronic wave function, and the average mode occupation. Finally, we directly compare our results to that obtained with state-of-the-art, yet approximate, polaritonic coupled cluster approaches.

We present a study of two-photon electron capture by H-like uranium ions. The energy of the incident electron was chosen to be in the region with the most significant contribution of the dielectronic recombination. We studied the photon emission spectrum. In particular, we calculated the differential over the photon energy cross section including the main resonance groups corresponding to the cascade transitions, and the low-energy photon region, where the infrared divergence required special processing. The calculations were performed within the framework of QED theory. The importance of generalized Breit interaction was discussed. We investigated the roles of the dielectronic recombination and the radiative recombination. We investigated the resonance approximation and the single-photon approximation, which are commonly used to describe radiation spectra.

We have performed systematic measurements of the resistive cooling behavior of bunches of highly charged ions in a Penning trap after injection from an external source. In particular, we have been able to measure the exponential cooling rate of the axial center-of-mass motion and experimentally show its linear dependence on the ion number within the bunch, as expected from theory. The common center-of-mass energy of the ions is reduced by more than four orders of magnitude on the time scale of milliseconds while the single-ion cooling occurs on the scale of seconds, representing a highly effective way to remove the dominating part of the ion kinetic energy.

The twisted light modes used in modern atomic physics experiments can be contaminated by small admixtures of plane-wave radiation. Although these admixtures hardly reveal themselves in the beam-intensity profile, they may seriously affect the outcome of high-precision spectroscopy measurements. In the present study we propose a method for diagnosing such a plane-wave contamination which is based on the analysis of the magnetic sublevel population of atoms or ions interacting with the “twisted $+$ plane-wave” radiation. In order to theoretically investigate the sublevel populations, we solve the Liouville–von Neumann equation for the time evolution of the atomic density matrix. The proposed method is illustrated for the electric dipole $5s{}^{2}S_{1/2}\text{−}5p{}^{2}P_{3/2}$ transition in Rb induced by (linearly, radially, or azimuthally polarized) vortex light with just a small contamination. We find that even tiny admixtures of plane-wave radiation can lead to remarkable variations in the populations of the ground-state magnetic sublevels. This opens up new opportunities for diagnostics of twisted light in atomic spectroscopy experiments.

Matter-wave interferometry with nanoparticles will enable the development of quantum sensors capable of probing ultraweak fields with unprecedented applications for fundamental physics. The high sensitivity of such devices, however. makes them susceptible to several noise and decoherence sources and, as such, can operate only when sufficient isolation from the environment is achieved. It is thus imperative to model and characterize the interaction of nanoparticles with the environment and to estimate its deleterious effects. This paper will aim to study the decoherence of the matter-wave interferometer due to dipole-dipole interactions, which is one of the unavoidable channels for decoherence, even for a neutral micro-crystal. We will use the scattering model for decoherence, characterized by the differential cross section, in order to obtain simple expressions for the decoherence rate due to dipole-dipole interactions in the short- and long-wavelength limits that can be readily applied to estimate the available coherence time. We will conclude by applying the formulas we obtained to estimate the dipole-dipole decoherence rate for the quantum gravity-induced entanglement of masses protocol and discuss if the effects should be mitigated.

We develop the moment method for Bose-Einstein condensates at finite temperatures that enable us to study collective sound modes from the hydrodynamic to the collisionless regime. In particular, we investigate collective excitations in a weakly interacting dilute Bose gas by applying the moment method to the Zaremba-Nikuni-Griffin equation, which is the coupled equation of the Boltzmann equation with the generalized Gross-Pitaevskii equation. Utilizing the moment method, collective excitations in the crossover regime between the hydrodynamic and collisionless regimes are investigated in detail. In the crossover regime, the second sound mode loses the weight of the density response function because of the significant coupling with incoherent modes, whereas the first sound shows a distinct but broad peak structure. We compare the result obtained by the moment method with that of the Landau two-fluid equations and show that the collective mode predicted by the Landau two-fluid equations well coincides with the result from the moment method even far from the hydrodynamic regime, whereas clear distinction also emerges in the relatively higher momentum regime.

We investigate, both numerically and analytically, the time evolution of a particle in an initial plane-wave state as it is subject to elastic scattering in a two-dimensional disordered system with Rashba spin-orbit coupling (SOC). In the analytic calculation, we treat the SOC nonperturbatively and the disorder perturbatively using the diffuson and the cooperon. We calculate the time dependence of coherent backscattering as a function of the strength of the SOC. We identify weak and strong SOC regimes and give the relevant time and energy scales in each case. By studying the time dependence of the anisotropy of the disorder-averaged momentum distribution, we identify the spin-relaxation time. We find a crossover from D'yakonov-Perel' spin relaxation for weak SOC to Elliot-Yafet-like behavior for strong SOC.