A spectrum of $\mathrm{\Delta }$-correlon quasiparticles is proposed to describe the excitation effect on electron correlation. The proposed quasiparticle description is based on the comparison of the pair-correlation functions of the excited and ground states at the special coalescence points of the configurational space. The complex form of the $\mathrm{\Delta }$-correlon wave function allows us to naturally separate regions with the excitation suppression of correlation (ESC $\mathrm{\Delta }$-correlon), where correlation is stronger in the ground state, from those with the excitation enhancement of correlation (EEC $\mathrm{\Delta }$-correlon), where correlation is stronger in the excited state. The proposed $\mathrm{\Delta }$-correlon approach is applied to characterize the important single excitations with the competition of “anticorrelation squeezing” and “enhanced-correlation spreading” of the two-electron distribution. The former feature characterizes single valence excitations of the ionic nature and this leads to the prevailing of the ESC effect in this type of excitations. In turn, the enhanced-correlation spreading characterizes single Rydberg excitations, which leads to the prevailing of the EEC effect in this type of excitations. The proposed $\mathrm{\Delta }$-correlon quasiparticles are studied both analytically and numerically. The analytical expressions for the on-top correlation functions are obtained with the two-determinantal two-electron model of the active site of an excitation. The two-dimensional contour plots of the ESC and EEC $\mathrm{\Delta }$-correlons obtained for the lowest vertical valence ionic and Rydberg excitations in the prototype molecules ${\mathrm{H}}_2, {\mathrm{N}}_2$, and ${\mathrm{C}}_2{\mathrm{H}}_4$ from the highly correlated wave functions confirm, in general, the trends in the relative correlation strength deduced from the model.

The Uhlmann process is built on the density matrix of a mixed quantum state and offers a way to characterize topological properties at finite temperatures. We analyze an ideal spin-$j$ quantum paramagnet in a magnetic field undergoing an Uhlmann process and derive general formulas of the Uhlmann phase and Loschmidt amplitude for arbitrary $j$ as the system traverses a great circle in the parameter space. A quantized jump of the Uhlmann phase signifies a topological quantum phase transition (TQPT) of the underlying process, which is accompanied by a zero of the Loschmidt amplitude. The exact results of $j=\frac{1}{2}$ and $j=1$ systems show topological regimes that survive only at finite temperatures but not at zero temperature, and the number of TQPTs is associated with the winding number in the parameter space. Our results pave the way for future studies on finite-temperature topological properties, and possible experimental protocols and implications for atomic simulators and digital simulations are discussed.

We show symmetric dynamics in dissipative quantum many-body systems. Given some conditions on the Hamiltonian and the jump operators, the time evolution of certain observables can be identical for ferromagnetic and antiferromagnetic interactions in an Ising model with external fields or for repulsive and attractive interactions in a Hubbard model. We present two theorems to determine the existence of such a dynamical symmetry in dissipative quantum systems. The symmetry under the steady state is present independently on the initial state. Further constraint on the initial state leads to a stronger dynamical symmetry in real-time evolutions. We demonstrate the application of our theorems in dissipative Ising and Hubbard models. In addition, for the Fermi-Hubbard model, our results also reveal a connection between spin and charge densities in dissipative dynamics. We validate the discussions with numerical simulations performed using tensor network algorithms.

We explore the edge defects induced by spin-spin interaction in a finite paradigmatic Heisenberg spin chain. By introducing a gradient magnetic field and a periodically modulated spin-exchange strength, the resonance between modulation frequency and magnetic-field gradient can also induce asymmetric defects at the edges, dubbed as Floquet-Wannier-Zeeman edge defects. The interplay between these two types of edge defects allows us to manipulate the magnon edge states from an isolated band into a continuum one. In the high-frequency regime, we analytically derive effective models to interpret the formation mechanisms for edge states by employing the multiscale perturbation analysis. Our study offers insights to understand and control the magnon edge states governed by the interplay between edge defects induced by the spin-spin interaction and the Floquet-Wannier-Zeeman manipulation.

We consider the system of three ${}^4\mathrm{He}$ atoms to assess whether a pure van der Waals potential can be used as a starting point for an effective field theory to describe three-body processes in ultracold atomic systems. Using a long-range van der Waals interaction in combination with short-distance two-body counterterms, we analyze the dependence of two- and three-body observables on the short-distance regulator that is required due to the singular nature of the van der Waals interaction. We benchmark our approach with results obtained with the realistic ${}^4\mathrm{He}-{}^4\mathrm{He}$ LM2M2 potential and find good agreement. We furthermore show that in this effective field theory approach no three-body force is required at leading order and that universal van der Waals physics leads to a universal correlation between three-body observables in the absence of an Efimov three-body parameter.

Results from higher order mean-field calculations of light interacting with atom arrays are presented for calculations of one- and two-time expectation values. The atoms are approximated as two levels and are fixed in space. Calculations were performed for mean-field approximations that include the expectation value of one operator (mean field), two operators (mean field 2), and three operators (mean field 3). For the one-time expectation values, we examined three different situations to understand the convergence with increasing order of mean field and some limitations of higher order mean-field approximations. As a representation of a two-time expectation value, we calculated the $g^{\left(2\right)}\left(\tau \right)$ for a line of atoms illuminated by a perpendicular plane wave at several emission angles and two different intensities. For many cases, the mean field 2 will be sufficiently accurate to quantitatively predict the response of the atoms as measured by one-time expectation values. However, the mean-field-3 approximation will often be needed for two-time expectation values.

The recent experimental realization of a programmable two-dimensional square superconducting qubit array opens a new avenue for quantum simulation with qubits [Science 372, 948 (2021)]. Here, we present an experimentally feasible method to achieve two-dimensional superconducting qubit lattice with tunable coupling strengths. A configuration with higher-order topological phases is constructed, featuring topologically protected boundary states in lower dimension (corner states). We show that the quadrupole topological phases can be effectively characterized by the dynamics of the single-excitation quantum state. Moreover, we also explore the detection and dynamics of the corner modes in our qubit lattice system. Particularly, we propose an effective scheme to realize quantum information transfer via the corner states. Our work suggests that superconducting circuits systems are a fertile platform to study novel topological quantum phases and may shed light on the ongoing exploration of topologically protected quantum information processing.

The average charge states of ${}^{20}\mathrm{Ne}^{7+}$ and ${}^{131}\mathrm{Xe}^{20+}$ ions penetrating through the gaseous argon targets with different areal densities are measured, and the velocities of projectile is close to the Bohr velocity regime ($v\sim 1$ a.u.). The results show that, with the increase of target density, the dynamic evolution of average charge occurs in three different stages. The dependence of the average charge state on the charge-changing cross section from the nonequilibrium stage to the equilibrium stage is revealed by the theoretical predictions that consider single-electron and additional double-electron processes. It is found that the multi-electron charge-changing process needs to be properly considered in the equilibrium stage. In addition, an analysis model is proposed to predict the average charge state of the projectile in the event of significant energy loss and to quantitatively evaluate the dependence of energy loss on the charge-changing cross section.

Cosmological reheating describes the transition of the postinflationary universe to a hot and thermal state. In order to shed light on the underlying dynamics of this process, we propose to quantum-simulate the reheating-like dynamics of a generic cosmological single-field model in an ultracold Bose gas. In our setup, the excitations on top of an atomic Bose-Einstein condensate play the role of the particles produced by the decaying inflaton field after inflation. Expanding spacetime as well as the background oscillating inflaton field are mimicked in the nonrelativistic limit by a time dependence of the atomic interactions, which can be tuned experimentally via Feshbach resonances. As we illustrate by means of classical-statistical simulations for the case of two spatial dimensions, the dynamics of the atomic system exhibits the characteristic stages of far-from-equilibrium reheating, including the amplification of fluctuations via parametric instabilities and the subsequent turbulent transport of energy towards higher momenta. The transport is governed by a nonthermal fixed point showing universal self-similar time evolution as well as a transient regime of prescaling with time-dependent scaling exponents. While the classical-statistical simulations can capture only the earlier stages of the dynamics for weak couplings, the proposed experiment has the potential of exploring the evolution up to late times even beyond the weak coupling regime.

Cooling the center-of-mass motion is an important tool for levitated optomechanical systems, but it is often not clear which method can practically reach lower temperatures for a particular experiment. We directly compare the parametric and velocity feedback damping methods, which are used extensively for cooling the motion of single trapped particles in a range of traps. By performing experiments on the same particle, and with the same detection system, we demonstrate that velocity damping cools the oscillator to a temperature an order of magnitude lower and is more resilient to imperfect experimental conditions. We show that these results are consistent with analytical limits as well as numerical simulations that include experimental noise.

Apart from stationary solitons originating from wave self-organization in ultrafast lasers, pulsating solitons have attracted lots of research interest due to their fantastic evolutionary behaviors, whose mechanisms are commonly dominated by dissipative effects such as gain, loss, and spectral filtering. Here, we demonstrate the trampolinelike pulsating soliton in the laser resonator originating from the conservative effects, i.e., Kerr effect and dispersion. It is found that the overdriven self-phase modulation induces the peak-to-dip transformation in the spectral domain, which is terminated by nonlinear spectral focusing due to the interplay between cross-phase modulation and anomalous dispersion. Such dynamic competition between the spectral broadening and focusing leads to the quasiperiodic peak-dip alternation with roundtrips like the trampoline game. A slow pulsation coexists with the trampolinelike fast pulsation due to the long-term soliton shaping. The simulation validates well experimental observations, fully revealing the underlying mechanism of the trampolinelike pulsating soliton. Having the potential to control laser stability, our results also show that the interplay between the Kerr effect and anomalous dispersion within laser cavities can trigger diverse transient phenomena in addition to the well-studied conventional soliton and modulation instability.

A scheme is presented for generation of a multipartite $W$ state for an arbitrary number of qubits. Based on a recent proposal of entanglement without touching, it serves to demonstrate the potential of particle indistinguishability as a useful resource of entanglement for practical applications. The devised scheme is efficient in design, meaning that it is built with linear optics without the need for auxiliary particles nor measurements. Yet, the success probability is shown to be highly competitive compared with the existing proposals (i.e., decreases polynomially with the number of qubits) and remains insensitive to particle statistics (i.e., has the same efficiency for bosons and fermions).

In a recent paper, Kravtsov and Molotkov [Phys. Rev. A 100, 042329 (2019)] propose a quantum key distribution protocol and claim “the protocol security is analyzed by an explicit construction of the most optimal unitary attack” and “we provide an extensive security analysis of the proposed protocol by considering a general symmetric unitary attack, and give succinct relations for the asymptotic secret key rate in the case of $N=8$.” We show that these claims are erroneous and propose two attacks which are more effective than the attacks considered in the paper of Kravtsov and Molotkov.

In reply to the Comment by D. A. Kronberg [Phys. Rev. A 104, 026401 (2021)], we would like to reassure readers that none of the critiques actually apply to our final result of the security analysis, namely, Eq. (30). The major part of the Comment, as well as the conclusions, are derived from the consideration of the multiphoton component, which does not appear in our final result. At the same time, we admit that our original interpretation of multiphoton terms in Eq. (26) as well as the potential extension of Eq. (30) with such terms were not correct. The single-photon component plays an essential role in quantum key distribution and was the only one to appear in the final key rate equation, which is valid against the most general collective attack. The Comment concludes that Eq. (30) “is incorrect ... and the security proof ... is not complete,” which, as we show, is not the case.

In this paper, we introduce the resource theory of unextendibility as a relaxation of the resource theory of entanglement. The free states in this resource theory are the $k$-extendible states, associated with the inability to extend quantum entanglement in a given quantum state to multiple parties. The free channels are $k$-extendible channels, which preserve the class of $k$-extendible states. We define several quantifiers of unextendibility by means of generalized divergences and establish their properties. By utilizing this resource theory, we obtain nonasymptotic upper bounds on the rate at which quantum communication or entanglement preservation is possible over a finite number of uses of an arbitrary quantum channel assisted by $k$-extendible channels at no cost. These bounds are significantly tighter than previously known bounds for both the depolarizing and erasure channels. Finally, we revisit the pretty strong converse for the quantum capacity of antidegradable channels and establish an upper bound on the nonasymptotic quantum capacity of these channels.

We study the dissipative dynamics and the formation of entangled states in a driven cascaded quantum network, where a cascaded double-cavity optomechanical system is coupled to a common unidirectional optical fiber. In terms of two coherent laser fields simultaneously driving the chiral coupling system, the entanglement can be effectively transferred from the light to two nanomechanical resonators (NMRs) through cavity modes based on the completely destructive interference of quantum noise. By adjusting the relative strength ratio of red- and blue-detuned pump lasers, it is found that the two NMRs can be driven into a mixed two-mode Gaussian entangled mechanical state rather than a non-Gaussian entangled dark state or a pure entangled dark state. If the frequency of the NMR is tuned to be much larger than the chiral coupled-cavity-chain damping rate, it is demonstrated that the chiral dissipative cascaded system can always realize one-way Einstein-Podolsky-Rosen (EPR) steering from one mechanical oscillator to the other, while the reverse one-way EPR steering is impossible to obtain, and the reason for achieving asymmetric Gaussian steering is analyzed. Furthermore, a simple one-way EPR steering criterion based on two-photon correlation can be achieved for an arbitrary bipartite Gaussian state under the standard form of EPR steering, and the effect of mechanical thermal noise on the steering is also discussed. Moreover, the present chiral system can be realized with currently available experimental technology; therefore, we hope that it can be very helpful for potential applications in quantum computation and quantum communication.

The kinetically and atomically balanced conditions have been extensively used in various basis-set expansion methods to remove the nonphysical spurious states appearing in the numerical calculation of the Dirac equation; however, they are generally not applicable for methods in discrete variable representation. In this paper we show that these conditions can be conveniently introduced into the generalized pseudospectral (GPS) method. Four types of balanced condition, the mono kinetic balance (MKB), dual kinetic balance, mono atomic balance (MAB), and dual atomic balance, are incorporated into the GPS method to eliminate the spurious states. Numerical calculations for a variety of bound states of H-like ions in point-charge models are compared with the analytical solutions to demonstrate the accuracy and efficiency of the developed methods. The application to highly charged ions with extended nuclear models is performed to show the flexibility of the balanced GPS methods in practical atomic structure calculations. It is concluded that the MAB-GPS and MKB-GPS methods, which are both free of any spurious states, show better performance and simpler implementation than the others in solving the Dirac equation with the potential in point-charge and extended nuclear models, respectively. The balanced GPS methods developed in this paper provide a useful tool for accurately solving the one-electron Dirac equation and efficiently constructing the multielectron relativistic wave functions.

By applying a sudden perturbation to one of the components of a mixture of two quantum fluids, we explore the effect on the motion of the second component on a short timescale. By implementing perturbation theory, we prove that for short times the response of the second component is fixed by the energy weighted moment of the crossed dynamic structure factor (crossed $f$-sum rule). We also show that by properly monitoring the time duration of the perturbation it is possible to identify peculiar fast spin drag regimes, which are sensitive to the interaction effects in the Hamiltonian. Special focus is given to the case of coherently coupled Bose-Einstein condensates, interacting Bose mixtures exhibiting the Andreev-Bashkin effect, normal Fermi liquids, and the polaron problem. The relevant excitations of the system contributing to the spin drag effect are identified and the contribution of the low-frequency gapless excitations to the $f$-sum rule in the density and spin channels is explicitly calculated employing the proper macroscopic dynamic theories. Both spatially periodic and Galilean boost perturbations are considered.

Integrated optical waveguides have been widely explored in the context of quantum simulation for various physical models based on paraxial diffraction of light, which are described by an equivalent Schrödinger equation. The physics in these systems can be formulated using the tight-binding models, in which the coupling between the waveguides can be tuned independently in a wide range, providing an excellent platform for simulation of various phenomena. In this work, we build a tight-binding model with parameters transported directly from two coupled waveguides, which are controlled by dielectric constant change, site distance, and geometries. This design principle can greatly save the simulation and experimental cost in real implementation. As compared with results from the exact simulation based on Maxwell equations, our numerical results demonstrate that the physics of the one- or two-dimensional large lattice systems could be well described by our tight-binding model, exhibiting the excellent transferability of the parameters in two coupled waveguides. In addition, some applications are further discussed: we show how to realize the topological Su-Schrieffer-Heeger (SSH) model, kinked SSH model, and study their associated topological phases and edge modes. Some two-dimensional models based on our tight-binding models are also discussed. Lastly, more intriguing applications, such as nonlinearity or disorder-induced effect and generation of the gauge potential are also briefly discussed. Our work intuitively provides an effective reference route for designing models for experiments and demonstrates the practicality of using the tight-binding approximation to solve complicated models. The design principle demonstrated in this work paves the foundation for the application of optical waveguides based on more complicated models, and will be readily verified experimentally in the near future.