We show that the methods for quantification of system-environment entanglement that were recently developed for interactions that lead to pure decoherence of the system can be straightforwardly generalized to time-dependent Hamiltonians of the same type. This includes the if-and-only-if criteria of separability, as well as the entanglement measure applicable to qubit systems, and methods of detection of entanglement by operations and measurements performed solely on the system without accessing the environment. We use these methods to study the nature of the decoherence of a qubit-oscillator system. Qubit-oscillator entanglement is essential for developing bosonic quantum technology with quantum non-Gaussian states and its applications in quantum sensing and computing. The dominating bosonic platforms—trapped ions, electromechanics, and superconducting circuits—are based on the time-dependent gates that use such entanglement to achieve new quantum sensors and quantum error correction. The steplike time dependence of the Hamiltonian that is taken into account allows us to capture the complex interplay between the buildups of classical and quantum correlations, which could not be replicated in time-independent scenarios.

It is known that, by accounting for the multiboson interferences up to a finite order, the output distribution of noisy boson sampling, with distinguishability of bosons serving as noise, can be approximately sampled from in a time polynomial in the total number of bosons. The drawback of this approach is that the joint probabilities of completely distinguishable bosons, i.e., those that do not interfere at all, have to be computed also. In trying to restore the ability to sample from the distinguishable bosons with computation of only the single-boson probabilities, one faces the following issue: the quantum probability factors in a convex-sum expression, if truncated to a finite order of multiboson interference, have, on average, a finite amount of negativity in a random interferometer. The truncated distribution does become a proper one, while allowing for sampling from it in a polynomial time, only in a vanishing domain close to the completely distinguishable bosons. Nevertheless, the conclusion that the negativity issue is inherent to all efficient classical approximations to noisy boson sampling may be premature. I outline the direction for an enitre new program, which seems to point to a solution. However, its success depends on the asymptotic behavior of the symmetric group characters, which is not known.

We formulate a general three-dimensional semiclassical model for the study of correlated multielectron escape during fragmentation of molecules driven by intense infrared laser pulses. We do so in the context of triple ionization of strongly driven ${\mathrm{HeH}}_2{}^{+}$. Our model fully accounts for the singularity in the Coulomb potentials of a recolliding electron with the core and a bound electron with the core as well as for the interaction of a recolliding electron with a bound electron. This model also accounts for the magnetic field of the laser pulse. To avoid artificial autoionization, our model employs effective potentials to treat the interaction between bound electrons. We focus on triple and double ionization as well as frustrated triple and frustrated double ionization. In these processes, we identify and explain the main features of the sum of the kinetic energies of the final ion fragments. We find that frustrated double ionization is a major ionization process, and we identify the different channels and hence different final fragments that are obtained through frustrated double ionization. Also, we discuss the differences between frustrated double and triple ionization.

The emergence of transient checkerboard and stripe patterns in a stack of driven quasi-one-dimensional homogeneous dipolar condensates is studied. The parametric driving of the $s$-wave scattering length leads to the excitation of the lowest collective Bogoliubov mode. The character of the lowest mode depends critically on the orientation of the dipoles, corresponding to out-of-phase and in-phase density modulations in neighboring condensates, resulting in checkerboard and stripe patterns. Further, we show that a dynamical transition between the checkerboard and stripe patterns can be realized by quenching the dipole orientation either linearly or abruptly once the initial pattern is formed via periodic driving.

A recent experiment [H. S. Han et al., Phys. Rev. Lett. 127, 073604 (2021); see also accompanying online supplemental material] shows that when a three-level $V$-type atom with two closely lying upper states interacts with the same vacuum radiation field, its excited states enable vacuum-induced coupling (VIC) owing to the quantum interference between the spontaneous emission pathways. Here, we propose a feasible scheme for phase-engineered photon correlations in the presence of the VIC in an optical nanofiber (ONF) cavity quantum electrodynamics (QED) system. Specifically, we show that a phase-dependent strong photon antibunching with high brightness can be generated in the weak-coupling regime of light-atom interactions. This occurs because of the VIC, leading to both the destructive quantum interference between the different pathways for two-photon excitation and the total closed-loop coupling phase. Different types of purely quantum correlations, such as single- and two-photon blockades, can occur by properly tuning the total closed-loop coupling phase adhering on the VIC, and the switch from photon blockade to photon induced tunneling is revealed as well. On the other hand, the strong photon antibunching can be achieved in a broad driving frequency range, which relaxes the requirement for the driving frequency in the ONF cavity QED system. In addition, we compare the analytical and numerical results of the second- and third-order intensity correlation functions, and they are in good agreement. The present study may provide an alternative route to manipulate the few-photon states and have potential applications in single-photon sources and quantum communications.

The Thomas-Wigner rotation (TWR) results from the fact that a combination of boosts leads to a nontrivial rotation of a physical system. Its origin lies in the structure of the Lorentz group. In this article we discuss the idea that the TWR can be understood in the geometric manner, being caused by the nontrivially curved relativistic momentum space, i.e., the mass shell, seen as a Riemannian manifold. We show explicitly how the TWR for a massive spin-$1/2$ particle can be calculated as a holonomy of the mass shell. To reach this conclusion we recall how to construct the spin bundle over the mass shell manifold. Interpreting TWR as a holonomy means it belongs to the same family of phenomena as Berry's phase.

Quantum enhanced optimization of classical cost functions is a central theme of quantum computing due to its high potential value in science and technology. The variational quantum eigensolver (VQE) and the quantum approximate optimization algorithm (QAOA) are popular variational approaches that are considered the most viable solutions in the noisy-intermediate scale quantum (NISQ) era. Here, we study the scaling of the quantum resources, defined as the required number of circuit repetitions, to reach a fixed success probability as the problem size increases, focusing on the role played by measurement shot noise, which is unavoidable in realistic implementations. Simple and reproducible problem instances are addressed, namely, the ferromagnetic and disordered Ising chains. Our results show that: (1) VQE with the standard heuristic Ansatz scales comparably to direct brute-force search when energy-based optimizers are employed. The performance improves at most quadratically using a gradient-based optimizer. (2) When the parameters are optimized from random guesses, also the scaling of QAOA implies problematically long absolute runtimes for large problem sizes. (3) QAOA becomes practical when supplemented with a physically inspired initialization of the parameters. Our results suggest that hybrid quantum-classical algorithms should possibly avoid a brute force classical outer loop, but focus on smart parameters initialization.

The problem of decomposing an arbitrary Clifford element into a sequence of Clifford gates is known as Clifford synthesis. Drawing inspiration from similarities between this and the famous Rubik's cube twisty puzzle, we develop a machine learning approach for Clifford synthesis based on learning an approximation to the distance to the identity. This approach is probabilistic and computationally intensive. However, when a decomposition is successfully found, it often involves fewer gates than the decomposition methods used in the Qiskit decomposition protocol, which uses a combination of several well-known Clifford decomposition schemes. Additionally, our approach is much more flexible than existing algorithms in that arbitrary gate sets, device topologies, and gate fidelities may be incorporated, thus allowing for the approach to be tailored to a specific device.

Programmable quantum simulators based on Rydberg atom arrays are a fast-emerging quantum platform, bringing together long coherence times, high-fidelity operations, and large numbers of interacting qubits deterministically arranged in flexible geometries. Today's Rydberg array devices are demonstrating their utility as quantum simulators for studying phases and phase transitions in quantum matter. In this paper, we show that unprocessed and imperfect experimental projective measurement data can be used to enhance in silico simulations of quantum matter, by improving the performance of variational Monte Carlo simulations. As an example, we focus on data spanning the disordered-to-checkerboard transition in a $16\times 16$ square-lattice array [S. Ebadi et al., Nature (London) 595, 227 (2021)] and employ the data-enhanced variational Monte Carlo algorithm to train powerful autoregressive wave-function ansätze based on recurrent neural networks (RNNs). We observe universal improvements in the convergence times of our simulations with this hybrid training scheme. Notably, we also find that pretraining with experimental data enables relatively simple RNN ansätze to accurately capture phases of matter that are not learned with a purely variational training approach. Our work highlights the promise of hybrid quantum-classical approaches for large-scale simulation of quantum many-body systems, combining autoregressive language models with experimental data from existing quantum devices.

Starting with a product initial state, squeezed (squeezed coherent) state in one of the modes, and vacuum in the rest, we report that a circular waveguide comprising modes coupled with varying coupling strength is capable of producing genuine multimode entanglement (GME), quantified via the generalized geometric measure (GGM). We demonstrate that, for a fixed coupling and squeezing strength, the GME content of the resulting state increases as the range of couplings between the waveguides increases, although the GGM collapses and revives with the variation of coupling strength and time. The advantage of long-range coupling can be emphasized by measuring the area under the GGM curve, which clearly illustrates growing trends of GME with the increasing range of couplings. Moreover, long-range couplings help in generating a higher GGM for a fixed coupling strength. We analytically determine the exact expression of GGM for systems involving an arbitrary number of modes, when all the modes interact with each other equally. The entire analysis is performed in the phase-space formalism. We manifest the constructive effect of disorder in the coupling parameter, which promises a steady production of GME, independent of the coupling strength.

Laser frequency or phase noise is a main factor that limits the sensitivity of Rydberg microwave (MW) electrometry. In this study, we proposed a theoretical approach to estimate the effect of laser frequency noise on the atomic coherence of Rydberg sensors based on an atomic superheterodyne receiver. The noisy lasers were characterized using a phase-diffusion model. In particular, explicit formulas were derived to evaluate the noise-limited sensitivity for a given laser linewidth. Furthermore, the effect of Doppler broadening on the sensitivity of the Rydberg sensor at different atomic temperatures was estimated. Our theoretical results provide guidance for the development of Rydberg MW sensors and the method discussed herein can be applied to many other quantum systems.

The electronic stopping power and charge state of protons and helium ions irradiating iron under both channeling and off-channeling geometries are studied by first-principles molecular dynamics. Using real-time time-dependent density functional theory, the inner-electron excitation of iron under ion irradiation is explored. The calculated electronic stopping powers, which take into account the effects of inner-electron excitation, were found to be in good agreement with experimental data. To further investigate the charges attracted by the projectiles moving in iron, we introduced a modified method to decompose the charges into the occupations of the respective projectile orbitals. When the projectile ion moves along off-channeling trajectories, the orbital distribution of the charges attracted by protons is found to be unstable, while that of helium ions remains relatively stable. However, it is worth noting that the orbital distribution differs between the channeling and off-channeling cases for both protons and helium ions. Overall, our research provides insight into the electron capture behavior of protons and helium ions as they traverse through iron.

The vortex structure of wave packets on the transversal or spatiotemporal plane can significantly enhance their shifts during the reflection and refraction. These shifts are classified into six types: Goos-Hänchen shift within the plane of incidence, Imbert-Fedorov shift perpendicular to the plane of incidence, longitudinal shift, together with three counterparts of angular shifts. Here, we analytically derive the expressions for these shifts for three-dimensional spatiotemporal vortex wave packets that carry orbital angular momentum of arbitrary orientation. It is found that the influence of the optical configurations (e.g., incident angle, refractive index, etc.) on the shifts can be described by a deformation tensor, while that of the wave-packet structure is an isotropic one. Interestingly, the spatial and angular shifts induced by the topological charge can be attributed to two topologically distinct tangent vector fields on the spheres, respectively. Also, the conservation laws of transverse linear momentum and vertical angular momentum are revisited. Our studies may find potential in spatiotemporal pulse shaping and shed light on the recent controversy regarding the definition of intrinsic orbital angular momentum for a spatiotemporal vortex pulse.

Disorder in one-dimensional (1D) many-body systems facilitates abundant phases such as many-body localization (MBL) and thermalization. However, it remains unclear regarding their existence and behavior within hybrid quantum systems. Here, based on a simple bosonic-spin hybrid model, known as the Jaynes-Cummings-Hubbard (JCH) array, we investigate the effect of disorder compared to the phenomena in the clean system with the variation of atom-photon coupling strength. By using the level-spacing ratio, entanglement entropy, and the properties of observable diagonal and off-diagonal matrix elements, we find that strong disorder results in the appearance of a MBL phase in the JCH model that strongly violates the eigenstate thermalization hypothesis (ETH), while a conditional prethermal behavior can exist in the weak disorder regime. The conditional prethermal dynamics is based on the choice of initial product states. This work systematically reveals abundant many-body phases in the 1D JCH model and clarifies the discrepancies in the thermalization properties of systems with and without disorder.

We propose a scheme to control excitation transfer between an atom and the ends of the Su-Schrieffer-Heeger (SSH) chain. When the finite SSH chain is in the topological phase, and the frequency of the atom is resonant with the center frequency of the SSH chain, an effective transition between the atomic excited state and the band-gap states can be obtained. The system can be equal to the three-states model. Under this case, we can adiabatically transfer the atomic excitation to one of the ends of the SSH chain. Which end of the SSH chain is the receiver depends on which sublattice of the cell the atom is coupled to. Furthermore, the excitation can also be transferred from the atom to the ends of the chain by fixing the parameters under the dynamic evolution. Our paper provides a method for realizing controllable quantum information transfer based on the coupling of an atom with topological matter.

Quantum oscillators with nonlinear driving and dissipative terms have gained significant attention due to their ability to stabilize cat states for universal quantum computation. Recently, superconducting circuits have been employed to realize such long-lived qubits stored in coherent states. We present a generalization of these oscillators, which are not limited to coherent states. The key ingredient lies in the presence of different nonlinearities in driving and dissipation, beyond the quadratic one. Through an extensive analysis of the asymptotic dynamical features for different nonlinearities, we identify the conditions for the storage and retrieval of quantum states, such as squeezed states, in both coherent and incoherent superpositions. We explore their applications in quantum computing, where squeezing prolongs the lifetime of memory storage for qubits encoded in the superposition of two symmetric squeezed states, and in quantum associative memory, which has so far been limited to the storage of classical patterns.

Quantum channels, a subset of quantum maps, describe the unitary and nonunitary evolution of quantum systems. We study a generalization of the concept of Pauli maps to the case of multipartite high-dimensional quantum systems through the use of the Weyl operators. The condition for such maps to be valid quantum channels, i.e., complete positivity, is derived in terms of Fourier transform matrices. From these conditions, we find the extreme points of this set of channels and identify an elegant algebraic structure nested within them. In turn, this allows us to expand upon the concept of “component erasing channels” introduced in earlier work by the authors. We show that these channels are completely characterized by elements drawn of finite cyclic groups. An algorithmic construction for such channels is presented and the smallest subsets of erasing channels which generate the whole set are determined.

Quantum magnetometry uses quantum resources to measure magnetic fields with a precision and accuracy that cannot be achieved by its classical counterparts. In this paper, we propose a scheme for quantum magnetometry using discrete-time quantum walk (DTQW) where multipath interference plays a central role. The dynamics of a spin-half particle implementing DTQW on a one-dimensional lattice gets affected by magnetic fields and the controlled dynamics of DTQW help in estimating the fields' strength. To gauge the effects of the field, we study the variance of the particle's position probability distribution (PD) and use it to determine the direction of the magnetic field maximally affecting the quantum walk. We then employ statistical tools like quantum Fisher information (QFI) and Fisher information (FI) of the particle's position and spin measurements to assess the system's sensitivity to the magnetic fields. We find that one can use the position and spin measurements to estimate the strengths of the magnetic fields. Calculations for an electron implementing quantum walk of 50 time steps show that the estimate has a root-mean-square error of the order of 0.1 picoTesla. Moreover, the sensitivity of our system can be tuned to measure any desired magnetic field. Our results indicate that the system can be used as a tool for optimal quantum magnetometry.

High-order harmonic generation (HHG) through laser-molecule interaction provides a powerful means for detecting molecular structure and dynamics with subangström spatial and attosecond temporal resolution. However, accurately extracting molecular information from experimental harmonic spectra requires deconvolution of the angular average over the molecular rotational distribution, which is a challenging task due to the coherent nature of harmonic radiation. In this study we propose a deep-learning approach to disentangle the internal coupling between molecular alignment and single-molecule high-order harmonic radiation in experiments. With our method, the complex single-molecule dipole moments of high-order harmonics in both parallel and perpendicular directions, as well as the time-dependent molecular rotational distribution, can be simultaneously retrieved from the polarization-resolved angular distributions of HHG. From the retrieved harmonic dipole moments we can obtain comprehensive knowledge of the polarization states of the harmonics, including their ellipticity and helicity, without complicated experimental measurements. We demonstrate our method with two prototype molecules, ${\mathrm{N}}_2$ and ${\mathrm{CO}}_2$, in the experiment. Our approach provides an efficient way to disentangle single-molecule information from HHG experiments and will facilitate the study of molecular structure and dynamics imaging in complex polyatomic molecules.