Accurate characterization of molecular alignment distribution is critical for correctly understanding results in strong-field experiments involving molecules. In this study, we theoretically propose a method to retrieve the alignment distribution of transiently aligned molecules from their terahertz spectra using a genetic algorithm based on the photocurrent model. The feasibility of our retrieval approach relies on the observation, verified by both experiments and simulations, that molecular terahertz radiation yields are strongly modulated by the alignment degree, and accordingly, alignment distribution of molecules. We demonstrate that the precise determination of alignment distribution of molecules can be achieved across a wide range of terahertz frequencies and driving laser intensities, if terahertz radiation is measured at different pump-probe angles. Notably, our retrieval procedure is straightforward to implement as it only requires knowledge about ionization probabilities obtained from simulations.

It is proposed to use rectangular Raman pulses for the technique of sequential large momentum transfer. It is shown that the small parameters that make it possible to use this technology for precision atom interferometry can be 40–200 times smaller than in the case of the Bragg regime. It is predicted that in the case of a nonequidistant timing of auxiliary pulses, one can observe oscillations in time of the interference picture with a period inversely proportional to the recoil frequency. Such an observation would be confirmation that Mach-Zehnder atom interference is a phenomenon caused by the quantization of the atomic center-of-mass motion. This effect is calculated for any shape of pulses. One can observe it in the Bragg regime as well. It is proposed to use noncontinuous composite Raman pulses as auxiliary beam splitters so that the effective Rabi frequency remains unchanged for the entire process. The gravity phase of an atomic interferometer is calculated for any shape, duration, and timing of Raman pulses, including the Bragg regime. The phase corrections caused by the finite pulses' durations are also calculated for the rectangular Raman pulse shape.

Non-Hermitian systems with complex-valued energy spectra and exceptional points show unconventional dynamics. Here we investigate the quantum interference of a self-interacting two-level system in which the coupling between the levels is nonreciprocal. We propose the multiple-passage Landau-Zener-Stückelberg-Majorana scheme to realize the quantum interferometry of such a non-Hermitian system, that is, the energy bias between two levels is periodically modulated, which may result in constructive or destructive interference. In the absence of nonlinear self-interaction, it exhibits obvious interference fringes in the weak nonreciprocal regime, whereas the interference is completely suppressed in the strong nonreciprocal regime due to the parity-time-symmetry breaking. The explicit expressions for the occupation probabilities are obtained within an effective rotating-wave approximation, which is consistent with the numerical results. In the presence of nonlinear interaction, the system shows a rich variety of dynamics and interference fringes. We further obtain phase diagrams for large ranges of nonreciprocity and nonlinear interaction parameters to explicitly demonstrate quantum interference within the nonlinear non-Hermitian system. The present work unlocks quantum interference characteristics from a nonreciprocal two-level system and provides a theoretical perspective for the manipulation of quantum states in non-Hermitian systems.

Quantum frequency up-conversion is a cutting-edge technique that leverages the interaction between photons and quantum systems to shift the frequency of single photons from a lower frequency to a higher frequency. If a photon before up-conversion was part of an entangled pair, it becomes crucial to comprehend the time-frequency entanglement after up-conversion. In this study, we present a theoretical analysis of the transformation of the time-dependent second-order quantum correlations in photon pairs and find the preservation of such correlations under fairly general conditions. Furthermore, we analyze Hong-Ou-Mandel interference between the signal and frequency-converted idler photons to gain insight into the indistinguishability of the two photons. The visibility of two-photon interference is sensitive to the magnitude of frequency conversion and improves as the frequency separation between two photons decreases.

We investigate the emergence of rogue quantum amplitudes in discrete-time quantum walks (DTQWs) influenced by phase disorder. Our study reveals the statistics of occupation probability amplitudes in space and time, uncovering optimal disorder regimes that favor rogue wave events. Through numerical simulations, we demonstrate that the probability of rogue waves increases with quantum coins close to the Pauli-$Z$ choice, regardless of the disorder degree. Conversely, for coins near the Pauli-$X$ choice, rogue events are scarce, except under weak disorder. A monotonic threshold is observed between rare and high-probability rogue wave regimes, depending on the quantum coin. We provide a comprehensive analysis of the coin-disorder interplay in regard to rogue wave events. Our findings shed light on the possible control of extreme quantum amplitudes through quantum coins in disordered DTQWs.

A formalism for quantum many-body systems is proposed through a semiclassical treatment in phase space, allowing us to establish stochastic thermodynamics incorporating quantum statistics. Specifically, we utilize a stochastic Fokker-Planck equation as the dynamics at the mesoscopic level. Here, the noise term characterizing the fluctuation of the flux density accounts for the finite-$N$ effects arising from random collisions between the system and the reservoir. Accordingly, the stationary solution is a quasiequilibrium state in a canonical system. We define stochastic thermodynamic quantities based on the trajectories of the phase-space distribution. The conservation law of energy, the $H$ theorem, and fluctuation theorems are therefore obtained. Our work sets an alternative formalism of quantum stochastic thermodynamics that is independent of the two-point measurement scheme. The numerous projective measurements of quantum systems are replaced by the sampling of the phase-space distribution, offering hope for experimental verifications in the future.

This study presents a quantum strategy for simultaneous estimation of two physical quantities using different entanglement resources. We explore the utilization of positively or negatively time-correlated photons. The proposed method enables the detection of the central position and relative velocity of multibody systems, as well as precise measurement of the size and velocity of moving objects. Comparative analysis with other strategies reveals the superior quantum advantage of our approach, particularly when appropriate entanglement sources with a high degree of entanglement are employed. These findings contribute to advancing our understanding of quantum strategies for accurate measurements.

A recent theoretical proposal for teleamplification requires preparation of Fock states, programmable interferometers, and photon-number resolving detectors to herald the teleamplification of an input state. These enable teleportation and heralded noiseless linear amplification of a photonic state up to an arbitrarily large energy cutoff. We report on adapting this proposal for the Borealis boson-sampling device and demonstrating teleamplification of squeezed-vacuum states with variable amplification factors. The results match the theoretical predictions and exhibit features of amplification in the teleported mode, with fidelities from 50 to 93%. This demonstration motivates the continued development of photonic quantum computing hardware for noiseless linear amplification's applications across quantum communication, sensing, and error correction.

Since the realization of quantum systems described by non-Hermitian Hamiltonians with parity-time ($\mathcal{PT}$) symmetry, interest in non-Hermitian, quantum many-body models has steadily grown. Most studies to date map to traditional quantum spin models with a non-Hermiticity that arises from making the model parameters complex or purely imaginary. Here we present a set of models with non-Hermiticity generated by splitting a Hermitian term into two Jordan-normal form parts, and the perturbations are confined to one or two sites. We present exact diagonalization results for a finite $\mathcal{PT}$ threshold in such models and provide an analytical approach for understanding the numerical results. Surprisingly, with non-Hermitian potentials confined to two or even a single site, the $\mathcal{PT}$ threshold seems insensitive to the size of the quantum spin chain. Our results provide a pathway to experimentally feasible non-Hermitian quantum spin chains where the confluence of many-body effects and non-Hermiticity effects can be observed.

Bell inequality is a vital tool to detect the nonlocal correlations, but the construction of it for multipartite systems is still a complicated problem. In this work, inspired via a decomposition of $(n+1)$-partite Bell inequalities into $n$-partite ones, we present a generalized iterative formula to construct nontrivial $(n+1)$-partite ones from the $n$-partite ones. Our iterative formulas recover the well-known Mermin-Ardehali-Belinskĭ-Klyshko (MABK) and other families in the literature as special cases. Moreover, a family of “dual-use” Bell inequalities is proposed, in the sense that for the generalized Greenberger-Horne-Zeilinger states these inequalities lead to the same quantum violation as the MABK family and, at the same time, the inequalities are able to detect the nonlocality in the entire entangled region. Furthermore, we present a generalization of the I3322 inequality to any $n$-partite case which is still tight, and of the 46 Śliwa's inequalities to the four-partite tight ones, by applying our iteration method to each inequality and its equivalence class.

Quantum entanglement and classical topology are two distinct phenomena that are difficult to be connected together. Here we discover that an open bosonic quadratic chain exhibits topology-induced entanglement effect. When the system is in the topological phase, the edge modes can be entangled in the steady state, while no entanglement appears in the trivial phase. This finding is verified through the covariance approach based on the quantum master equations, which provide exact numerical results without truncation process. We also obtain concise approximate analytical results through the quantum Langevin equations, which perfectly agree with the exact numerical results. We show the stationary entanglement originates from the matching between the near-zero eigenenergies of the topological edge states and the system-environment coupling (denoted by the dissipation rate). Our work reveals that the stationary entanglement can be a quantum signature of the topological phase in bosonic systems and, inversely, the topological quadratic systems can be powerful platforms to generate robust entanglement.

Generally, the three-body recombination (TBR) of the ultracold helium atoms in the zero-collision-energy limit could be described by the original zero-range (Efimov) universal function [E. Braaten, D. Kang, and L. Platter, Phys. Rev. A 75, 052714 (2007)] using the scattering length $a$ of the He-He interaction and a three-body parameter $a_{*}$ as input, where $a_{*}$ is associated with the positive scattering length at which an interference minimum in the TBR rate occurs at the threshold. This universal property has been reinvestigated by considering various post-Born-Oppenheimer (post-BO) effects on the relevant scattering process. It is found that a linear correction to $1/a_{*}$ versus $1/a$ should be introduced to give a better description of these considered TBR cases because of the finite-range effects. Particularly, such a correction may be ascribed not only to the general scattering properties associated with the Efimov channel at the short hyperradial distance, but also to the tunneling probability in the lowest incident channel. In addition, a successful fit of the modified universal function that includes correctly the finite-range effects [E. Garrido, M. Gattobigio, and A. Kievsky, Phys. Rev. A 88, 032701 (2013)] or the zero-range universal function with a linearly variational $1/a_{*}$ versus $1/a$ to numerically accurate data also indicates a clear manifestation of the van der Waals universality. Finally, the post-BO effects on the TBR process for the partial wave with the total angular momentum $J>0$ are also discussed.

We demonstrate a method for generating persistent currents in Bose-Einstein condensates by using a Gaussian process learner to experimentally control the stirring of the superfluid. The learner optimizes four different outcomes of the stirring process: (O.I) targeting and (O.II) maximization of the persistent current winding number and (O.III) targeting and (O.IV) maximization with time constraints. The learner optimizations are determined based on the achieved winding number and the number of spurious vortices introduced by stirring. We find that the learner is successful in optimizing the stirring protocols, although the optimal stirring profiles vary significantly depending strongly on the choice of cost function and scenario. These results suggest that stirring is robust and persistent currents can be reliably generated through a variety of stirring approaches.

An unusual metallic phase is argued to develop in the one-dimensional ionic Hubbard model, at half-filling and zero magnetization, at intermediate electron-electron repulsion $U$ when second-neighbors hopping is allowed and tuned close to a topological Lifshitz transition (connected with a change of the Fermi surface in the noninteracting system). The metallic state lies between a band insulator phase at low repulsion and a correlated (Mott-like) insulator phase at high repulsion. In approaching the latter, the model supports short-range antiferromagnetic order and spontaneous dimerization of both bond charge and nearest-neighbor antiferromagnetic correlations. A combination of mean-field and effective-field theory (bosonization) provides an analytical understanding of the physical processes underlying the argued phase transitions. The ground and low-energy excited states of finite-length chains are explored by density-matrix renormalization-group (DMRG) calculations, providing numerical evidence for the intermediate gapless phase. Such finite systems are attainable by cold atoms in optical lattices for a wide range of the parameter $U$.

We propose a feasible experimental scheme to improve the few-photon optomechanical effects, including photon blockade and mechanical-Schrödinger-cat-state generation, as well as photon-phonon entanglement in a tripartite microwave-optomechanical circuit. The system under consideration is formed by a single-Cooper-pair transistor, a microwave $LC$ resonator, and a micromechanical resonator. Our scheme is based on an additional higher-order (generalized) nonlinear cross-Kerr type of coupling, linearly dependent on photon number while quadratically dependent on mechanical phonon number, which can be realized via adjusting the gate charge of the Cooper-pair transistor. We show, both analytically and numerically, that the presence of both cross-Kerr and generalized cross-Kerr nonlinearities not only may give rise to the enhancement of one- and two-photon blockades as well as photon-induced tunneling but can also provide more controllability over them. Furthermore, it is shown that in the regime of zero optomechanical coupling, with the aid of generalized cross-Kerr nonlinearity, one can generate multicomponent mechanical superposition states which exhibit robustness against system dissipations. We also study the steady-state entanglement between the microwave and mechanical modes, the results of which signify the role of generalized cross-Kerr nonlinearity in enhancing the entanglement in the regime of large red detuning. The proposed generalized cross-Kerr optomechanical system can find potential applications in microwave quantum sensing, quantum telecommunication, and quantum information protocols.

We propose a scheme to create nonreciprocal entanglement in a spinning cavity magnomechanical system. The setup comprises a whispering gallery mode resonator with two counterpropagating modes, simultaneously coupled to yttrium iron garnet. By spinning the resonator, the counterpropagating modes undergo an opposite Fizeau drag caused by the optical Sagnac effect. We show that in a spinning resonator, the cavity-magnon-phonon modes get entangled along a chosen direction while remaining fully uncorrelated in the other. The degree of nonreciprocity is determined by the drive detuning and the spinning velocity of the resonator. Our work provides a different route to achieve chiral control of quantum entanglement involving magnon-photon-phonon modes.

Quantum optical sensors are ubiquitous in various fields of research, from biological or medical sensors to large-scale experiments searching for dark matter or gravitational waves. Gravitational-wave detectors have been very successful in implementing cavities and quantum squeezed light for enhancing sensitivity to signals from black hole or neutron star mergers. However, the sensitivity to weak forces is limited by the available energy and the optical decoherence in the system. Here, we derive the fundamental sensitivity limit of cavity- and squeezed-light-enhanced interferometers with optical loss. This limit is attained by the optimal use of an additional internal squeeze operation, which allows the mitigation of readout loss. We demonstrate the application of internal squeezing to various scenarios and confirm that it indeed allows one to reach the best sensitivity in cavity- and squeezed-light-enhanced linear force sensors. Our work establishes the groundwork for the future development of optimal sensors in real-world scenarios where, up until now, the application of squeezed light was curtailed by various sources of decoherence.

According to the general theory of relativity, time can flow at different rates depending on the configuration of massive objects, affecting the temporal order of events. Combined with quantum theory, this gravitational effect can result in events with an indefinite temporal order when a massive object is prepared in a suitable quantum state. This was argued to lead to a theory-independent test of the nonclassical order of events through the violation of Bell-type inequalities for temporal order. Here we show that the theory independence of this protocol is problematic: one of the auxiliary assumptions in the above approach turns out to be essential, while it is explicitly theory dependent. To illustrate this problem, we construct a complete scenario where accelerating particles interacting with optical cavities result in a violation of temporal Bell inequalities. Due to the equivalence principle, the same problem arises when one considers the gravitational case, and thus theory-dependent additional assumptions are needed behind Bell inequalities for temporal order to interpret a violation of the final inequalities as a signature of indefinite temporal order.

Overcoming the influence of noise and imperfections in quantum devices is one of the main challenges for viable quantum applications. In this article, we present different protocols, which we denote as “superposed quantum error mitigation,” that enhance the fidelity of single gates or entire computations by performing them in coherent superposition. Our results demonstrate that via our methods, significant noise suppression can be achieved for most kinds of decoherence and standard experimental parameter regimes. Our protocols can be either deterministic, such that the outcome is never postselected, or probabilistic, in which case the resulting state must be discarded unless a well-specified condition is met. By using sufficiently many resources and working under broad assumptions, our methods can yield the desired output state with unit fidelity. Finally, we analyze our approach for gate-based, measurement-based, and interferometric-based models, demonstrating the applicability in all cases and investigating the fundamental mechanisms they rely upon.

We discover a class of spacetime symmetries unique to time-periodic systems, which we term “mixing symmetry” due to its combination of space and time coordinates in the symmetry transformation. We systematically enumerate the symmetry groups and classify the corresponding Floquet-Bloch band theories by utilizing the winding number of quasienergy. Moreover, we provide a comprehensive scheme for the experimental realization of these symmetries. The particle propagator exhibits an intriguing pattern that remains invariant even under transformations mixing space and time coordinates. We anticipate that this distinct feature can be observed in current cold atom experiments.