Using two-frequency driving in two dimensions opens up new possibilities for Floquet engineering, which range from controlling specific symmetries to tuning the properties of resonant gaps. In this work, we study two-band lattice models subject to two-tone Floquet driving and analyze the resulting effective Floquet band structures both numerically and analytically. On the one hand, we extend the methodology of Sandholzer et al. [Phys. Rev. Res. 4, 013056 (2022)] from one to two dimensions and find competing topological phases in a simple Bravais lattice when the two resonant drives at $1\omega$ and $2\omega$ interfere. On the other hand, we explore driving-induced symmetry breaking in the hexagonal lattice, in which the breaking of either inversion or time-reversal symmetry can be tuned independently via the Floquet modulation. Possible applications of our work include a simpler generation of topological bands for ultracold atoms and the realisation of nonlinear Hall effects as well as Haldane's parity anomaly in inversion-symmetric parent lattices.

We study a gas of interacting ultracold bosons held in a parabolic trap in the presence of an optical lattice potential. Treating the system as a discretized Gross-Pitaevskii model, we show how Floquet engineering, by rapidly “shaking” the lattice, allows the ground state of the system to be converted into a train of bright solitons by inverting the sign of the hopping energy. We study how the number of solitons produced depends on the system's nonlinearity and the curvature of the trap, show how the technique can be applied in both the high- and low-driving-frequency regimes, and demonstrate the phenomenon's stability against noise. We conclude that the Floquet approach is a useful and stable method of preparing solitons in cold-atom systems.

We analyze the formation of maximally entangled Rydberg atom pairs subjected to Landau-Zener sweeps of the atom-light detuning. Though the populations reach a steady value at longer times, the phases evolve continuously, leading to periodic oscillations in the entanglement entropy. The local unitary equivalence between the obtained maximally entangled states and the Bell states is verified by computing the polynomial invariants. Finally, we study the effect of spontaneous emission from the Rydberg state of rubidium atoms on the correlation dynamics and show that the oscillatory dynamics persists for high-lying Rydberg states. Our study may offer ways to generate maximally entangled states, quantum gates, and exotic quantum matter in arrays of Rydberg atoms through Landau Zener sweeps.

The Bell nonlocality is one of the most intriguing and counterintuitive phenomena displayed by quantum systems. Interestingly, such stronger-than-classical quantum correlations are somehow constrained, and one important question to the foundations of quantum theory is whether there is a physical, operational principle responsible for those constraints. One candidate is the information causality principle, which, in some particular cases, is proven to hold for quantum systems and to be violated by stronger-than-quantum correlations. In multipartite scenarios, though, it is known that the original formulation of the information causality principle fails to detect even extremal stronger-than-quantum correlations, thus suggesting that a genuinely multipartite formulation of the principle is necessary. In this work, we advance towards this goal, reporting a different formulation of the information causality principle in multipartite scenarios. By proposing a change of perspective, we obtain multipartite informational inequalities that work as necessary criteria for the principle to hold. We prove that such inequalities hold for all quantum resources and forbid some stronger-than-quantum ones. Finally, we show that our approach can be strengthened if multiple copies of the resource are available, or, counterintuitively, if noisy communication channels are employed.

We present a no-go result for postselection measurements where the conditional expectation value of a joint system-device observable under postselection is nothing else than the conventional expectation value. Such a no-go result relies on the rank-$m$ degenerate of the joint observable, where $m$ is the dimension of the device subspace. Remarkably, we show that the error and disturbance in quantum measurements obey the no-go result, which implies that the error-disturbance uncertainty is unaffected under postselection measurements.

We investigate the nonstationary phenomenon in a tripartite spin-1/2 system in the collision model (CM) framework. After introducing the dissipation through the system-environment collision for both Markovian and non-Markovian cases, we find the emergence of long-time oscillation in the dynamics of the system and the synchronization among subsystems. We connect the CM description and the quantum master equation in the continuous-time limit and explain the existence of the stable oscillation by means of Liouvillian spectrum analysis. We investigate the thermodynamics of persistent oscillations in our CM in both Markovian and non-Markovian regimes. In addition, we find that the imperfection of collective dissipation can be compensated by the randomness of the interaction sequence in our CM.

Quantum mutual information not only displays the mutual information in the system but also demonstrates some quantum correlation beyond entanglement. We explore here two alternatives of the multipartite quantum mutual information (MQMI) based on the von Neumann entropy according to the framework of the complete measure of the multiparticle quantum system. We show that these two MQMIs are complete and are monogamous on pure states, and one of them (we call it type-1) is not only completely monogamous but also tightly complete monogamous, while the other one (we call it type-2) is not. Moreover, we present another two MQMIs by replacing the von Neumann entropy with the Tsallis $q$-entropy from the former two ones. It is proved that one of them displays some degree of “completeness” as a measure of the multiparticle quantum system, but the other one is not even non-negative and thus it cannot be an alternative of MQMI. We also discuss the triangle relation for these three alternatives of MQMI. It is shown that the triangle inequalities hold for the former two MQMIs as that of entanglement measure but the later one fails. We thus can conclude that the type-1 von Neumann entropy MQMI is a “fine” measure of multipartite quantum correlation.

For some commonly used nonlocality quantifiers, the maximally nonlocal state may not coincide with the maximally entangled state, what became known as the anomaly of nonlocality. More recently, nonanomalous quantifiers have been defined, for which maximal entanglement and maximal nonlocality do coincide. In this work we investigate in detail how these quantifiers behave for nonmaximal-resource states and show that some mismatch remains between the studied figures of merit. Besides the nonlocal volume (a quantifier that counts the volume of the set of behaviors accessible to a specific state that gives rise to nonlocality), we present an alternative quantity referring to states and based on behaviors, the trace-weighted nonlocal volume. The construction is based on the nonlocal volume integral, but weighted by a quantifier of nonlocality for behaviors (the shortest distance between the behavior and the local set). Although in all investigated scenarios the anomaly is not present, the nonlocality quantifiers as compared to the entropy of entanglement are inversely related for some set of states within some Bell scenarios. In the investigated situations this phenomenon occurs when the considered states go through a change of rank. This fact is discussed in general and illustrated through computations for the (2,3,2) scenario.

Implementation of a twin-field quantum key distribution network faces limitations, including the low tolerance of interference errors for phase-matching-type protocols and the strict constraint regarding intensity and probability for sending-or-not-sending-type protocols. Here we propose a two-photon twin-field quantum key distribution protocol and achieve twin-field-type two-photon interference through postmatching phase-correlated single-photon interference events. We exploit the noninterference mode as the code mode to highly tolerate interference errors and the two-photon interference naturally removes the intensity and probability constraint. Therefore, our protocol can transcend the above-mentioned limitations while breaking the secret key capacity of repeaterless quantum key distribution. Simulations show that for a four-user network, under which each node with fixed system parameters can dynamically switch different attenuation links, the key rates of our protocol for all six links can either exceed or approach the secret key capacity. However, the key rates of all links are lower than the key capacity when using phase-matching-type protocols. Additionally, four of the links could not extract the key when using sending-or-not-sending-type protocols. We anticipate that our protocol can facilitate the development of practical and efficient quantum networks.

The scattering of charged particles by $N$ atoms is investigated theoretically. We derive the scattering equations by applying the approach of Faddeev that was originally developed to study quantum mechanically the scattering of three particles. We focus on the case in which the distance between the $N$ target atoms is sufficiently large to allow the interactions between the atoms to be neglected. The Faddeev equations can then be reduced to a linear system whose solution is valid from the low-energy diffusive scattering regime, where the Foldy-Lax approach is often applied, to the ballistic regime, where multiple-scattering effects are small. To illustrate the method, results of calculations of the scattering of electrons by random configurations of 250 atoms, modeled by spherical potential wells, are presented. As an additional application, the solution obtained by Mott for the inelastic scattering of $\alpha$ particles by two atoms is extended to $N$ atoms.

Reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) is a powerful photoelectron spectroscopy, offering direct access to internal dynamics of the target. It is being increasingly applied to molecular systems, but a general, computationally tractable theory of RABBITT spectra in molecules has so far been lacking. We show that under quite general assumptions, RABBITT spectra in molecules can be expressed as a convolution of the vibronic cross-correlation functions and two-electron photoionization matrix elements. We specialize the general expressions to the commonly encountered special cases. We expect our theory to enable accurate modeling and interpretation of molecular RABBITT spectra in most medium-sized molecules.

We visualize frustrated tunneling ionization (FTI) using the correlation function analysis outlined in our preceding works [I. Ivanov and K. T. Kim, J. Phys. B 55, 055001 (2022); Sci. Rep. 12, 19533 (2022)]. We apply this technique to the hydrogen atom subjected to a strong laser field. Our analysis supports the basic premises of the theory of FTI and demonstrates its sensitive dependence on the laser pulse duration and carrier envelope phase.

While the properties and the shape of the ground state of a gas of ultracold bosons are well understood in harmonic potentials, they remain for a large part unknown in the case of random potentials. Here we use localization-landscape (LL) theory to study the properties of the solutions to the Gross-Pitaevskii equation (GPE) in one-dimensional (1D) speckle potentials. In the cases of attractive interactions, we find that the LL allows one to predict the position of the localization center of the ground state (GS) of the GPE. For weakly repulsive interactions, we point out that the GS of the quasi-1D GPE can be understood as a superposition of a finite number of single-particle states, which can be computed by exploiting the LL. For intermediate repulsive interactions, we introduce a Thomas-Fermi-like approach for the GS which holds in the smoothing regime, well beyond the usual approximation involving the original potential. Moreover, we show that, in the Lifshitz glass regime, the particle density and the chemical potential can be well estimated by the LL. Our approach can be applied to any positive-valued random potential endowed with finite-range correlations and can be generalized to higher-dimensional systems.

Singular optics and metamaterials are both fascinating branches of modern optics. While the twist of vortex beams in phase and polarization leads to singularities, the twist of metasurfaces also leads to extraordinary properties in both optics and electronics. Here we present the spin-orbit interaction of a vortex beam reflected on twisted few-layer hyperbolic metasurfaces. As a result, the spatial Imbert-Fedorov (IF) shift and photonic spin Hall effect (PSHE) shift are investigated. We find that the spatial IF shift versus twist angle is symmetrical about the $l=0$ shift and it is symmetrical about the origin for opposite twist angle with opposite topological charge. In addition, the PSHE shift versus twist angle is also symmetrical about the $l=0$ shift. The shifts are enhanced with increasing topological charge and show a pretty different dependence on the twist angle when the incident beam is in different polarization state. These findings provide a different perspective for further investigations of twisted light and twisted metasurfaces.

We construct an uncertainty relation for arbitrary finite-dimensional $\mathcal{PT}$-invariant non-Hermitian quantum systems within a special inner product framework. This construction is led by “good observables,” which are a more general class of operators. We show that the cumulative gain in the quantum Fisher information when measuring two good observables for such non-Hermitian systems is much better than their Hermitian counterpart. Minimum uncertainty states being the best candidates for this gain near the exceptional point supports the intelligent or simultaneous non-Hermitian quantum sensors.

We study quantum thermodynamics in nonequilibrium reservoirs (NERs) that are prepared from initially thermal ones via unitary driving processes. Based on the formulation of entropy production in NERs, we establish a Landauer-like bound and show that the bound can be violated under a definite condition depending on the states of NERs. It is found that the breakdown of this Landauer-like bound implies occurrences of several anomalous phenomena in NERs, namely, the efficiency enhancement of quantum thermal machines beyond the Carnot limit, the extraction of work from a single reservoir, and the spontaneous heat flow from a cold reservoir to a hot one. The results are illustrated through a physical model taking the preparation processes of NERs into account. Our work sheds light on the reason and condition of some unusual phenomena occurring in NERs and is helpful for one to prepare effective NERs that can be used as quantum resources to realize certain thermodynamic tasks.

Kerr nonlinear oscillators driven by a two-photon process are promising systems to encode quantum information and to ensure a hardware-efficient scaling towards fault-tolerant quantum computation. In this paper, we show that an extra control parameter, the detuning of the two-photon drive with respect to the oscillator resonance, plays a crucial role in the properties of the defined qubit. At specific values of this detuning, we benefit from strong symmetries in the system, leading to multiple degeneracies in the spectrum of the effective confinement Hamiltonian. Overall, these degeneracies lead to a stronger suppression of bit-flip errors. We also study the combination of such Hamiltonian confinement with colored dissipation to suppress leakage outside of the bosonic code space. We show that the additional degeneracies allow us to perform fast and high-fidelity gates while preserving a strong suppression of bit-flip errors.

Harrow-Hassidim-Lloyd algorithm (HHL) allows for the exponentially faster solution of a system of linear equations. However, this algorithm requires the postselection of an ancilla qubit to obtain the solution. This postselection makes the algorithm result probabilistic. Here we show conditions when the HHL algorithm can work without postselection of the ancilla qubit. We derive expectation values for an observable $M$ on the HHL outcome state when ancilla qubit is measured in $|0\rangle$ and $|1\rangle$ and show a condition for postselection-free HHL running. We provide an explicit example of a practically interesting input matrix and an observable, which satisfies the postselection-free HHL condition. Our work can improve the performance of the HHL-based algorithms.

We address the task of estimating the ground-state energy of Hamiltonians coming from chemistry. We study numerically the behavior of a digital-analog variational quantum eigensolver for the ${\mathrm{H}}_2$, LiH, and ${\mathrm{BeH}}_2$ molecules, and we observe that one can estimate the energy to a few percent points of error leveraging on learning the atom register positions with respect to selected features of the molecular Hamiltonian and then an iterative pulse-shaping optimization, where each step performs a derandomization energy estimation.