In finite systems driven unitarily across topological phase transitions, the Chern number and the Bott index have been found to exhibit different behaviors depending on the boundary conditions and on the commensurability of the lattice. For periodic boundary conditions, the Chern number does not change for finite commensurate lattices (or in the thermodynamic limit). On the other hand, the Chern number can change for incommensurate lattices with periodic boundary conditions and the Bott index can change for lattices with open boundary conditions. Here we show that the scalings of the fields at which those two indices change exhibit Landau-Zener and near-adiabatic regimes depending on the speed at which the strength of the drive is ramped up and on the system size. Those regimes are preceded by a regime in which the topological indices do not change. The latter is the only regime that, for nonvanishing ramp speeds, survives in the thermodynamic limit. We then show that the dc Hall response can be used to detect topological phase transitions independently of the behavior of the topological indices.

We investigate theoretically the generation of microscopic atomic NOON states, corresponding to the coherent $|N,0\rangle +|0,N\rangle$ superposition with $N\sim 5$ particles, via collective tunneling of interacting ultracold bosonic atoms within a symmetric double-well potential in the self-trapping regime. We show that a periodic driving of the double well with suitably tuned amplitude and frequency parameters allows one to substantially boost this tunneling process without altering its collective character. The timescale to generate the NOON superposition, which corresponds to half the tunneling time and would be prohibitively large in the undriven double well for the atomic populations considered, can thereby be drastically reduced, which renders the realization of NOON states through this protocol experimentally feasible. Resonance- and chaos-assisted tunneling are identified as key mechanisms in this context. A quantitative semiclassical evaluation of their impact on the collective tunneling process allows one to determine the optimal choice for the driving parameters in order to generate those NOON states as fast as possible.

The momentum transfer from an electromagnetic field to reflecting and absorbing surfaces was asserted by the Maxwell equations. This phenomenon, with important implications for the development of the cosmos, has also been investigated in the discretized energy scale using optomechanical devices. With the quantization of the electromagnetic field, it was discovered that the vacuum field may influence the dynamics of some physical systems, such as in the Casimir effect and the radiative decay of an atom. Here, the effect of the radiation pressure by the electromagnetic vacuum on the surface of a compressible conductor is analyzed, and the model based on the Born-Markov master equation predicts a harmonic strain and momentum analog to the classical counterpart. A fundamental difference observed is the oscillating purity of the deformation state. In addition, it was demonstrated that the time-averaged force originated from the elastic reflection of the vacuum-field modes is comparable to the Casimir force for two ideal metallic plates separated by a distance proportional to the reflectivity cutoff wavelength.

We propose a scheme that can generate tunable optomechanically induced transparency in a hybrid piezo-optomechanical cavity system. The system is composed of a high-quality planar distributed Bragg reflector cavity modified with an embedded Gaussian-shaped defect. Moreover, interdigitated transducers are fabricated on the surface of the cavity to generate surface acoustic waves. Under the actuation of the surface acoustic wave, the upper Bragg mirrors can be vibrated as a bulk acoustic resonator, and the distributed Bragg reflector cavity becomes a standard three-level optomechanical system. In this situation, we show that when a strong pump optical field and a weak probe optical field are simultaneously applied to the hybrid optomechanical cavity system, optomechanically induced transparency occurs under the quantum interference between different energy-level pathways. Our scheme can be applied in the fields of optical switches and quantum information processing in solid-state quantum systems.

We investigate the different photon emission regimes created by a pre-excited and collimated atomic beam passing through a single mode of an optical cavity. In the regime where the cavity degrees of freedom can be adiabatically eliminated, we find that the atoms undergo superradiant emission when the collective linewidth exceeds the transit-time broadening. We analyze the case where the atomic beam direction is slanted with respect to the cavity axis. For this situation, we find that a phase of continuous light emission similar to steady-state superradiance is established providing the tilt of the atomic beam is sufficiently small. However, if the atoms travel more than half a wavelength along the cavity axis during one transit time we predict a dynamical phase transition to a bistable superradiant regime. In this phase the atoms undergo collective spontaneous emission with a frequency that can be either blue or red detuned from the free-space atomic resonance. We analyze the different superradiant regimes and the quantum critical crossover boundaries. In particular we find the spectrum of the emitted light and show that the linewidth exhibits features of a critical scaling close to the phase boundaries.

The physical origins of the transverse optical binding force and torque beyond the Rayleigh approximation have not been clearly expressed to date. Here, we present analytical expressions of the force and torque for a dual dipolar dielectric dimer illuminated by a plane wave propagating perpendicularly to the dimer axis. Using this analytical model, we explore the roles of the hybridized electric dipolar, magnetic dipolar, and electric-magnetic dipolar coupling interactions in the total force and torque on the particles. We find significant departures from the predictions of the Rayleigh approximation, particularly for high-refractive-index particles, where the force is governed by the magnetic interaction. This results in an enhancement of the dimer stability by one to four orders of magnitude compared to the predictions of the Rayleigh approximation. For the case of torque, this is dominated by the coupling interaction and increases by an order of magnitude. Our results will help to guide future experimental work in optical binding of high-refractive-index dielectric particles.

We approach the physical implications of the noncommutative nature of complementary observable algebras (COAs) from an information theoretic perspective. In particular, we derive a general entropic certainty principle stating that the sum of two relative entropies, naturally related to the COA, is equal to the so-called algebraic index of the associated inclusion. Uncertainty relations then arise by monotonicity of the relative entropies that participate in the underlying entropic certainty. Examples and applications are described in quantum field theories with global symmetries, where the COAs are formed by the charge-anticharge local operators (intertwiners) and the unitary representations of the symmetry group (twists), and in theories with local symmetries, where the COAs are formed by Wilson and t'Hooft loops. In general, the entropic certainty principle naturally captures the physics of order and disorder parameters, a feature that makes it a generic handle for the information theoretic characterization of quantum phases.

In the present work, the averaged fidelity is introduced as the steering parameter. According to the definitions of steering from Alice to Bob, a general scheme for designing linear steering criteria is developed for a high-dimensional system. For a given set of measurements on Bob's side, two quantities, the so-called nonsteering thresholds, can be defined. If the measured averaged fidelity exceeds these thresholds, the state shared by Alice and Bob is steerable from Alice to Bob, and the measurements performed by Alice are also verified to be incompatible. Within the general scheme, we also construct a linear steering inequality when the set of measurements performed by Bob has a continuous setting. Some applications are also provided.

Quantum-enhanced sensing has a goal of enhancing a parameter sensitivity with input quantum states, while quantum illumination has a goal of enhancing a target detection capability with input entangled states in a heavy noise environment. Here we propose a concatenation between quantum-enhanced sensing and quantum illumination that can take quantum advantage over the classical limit. First, phase sensing in an interferometry is connected to a target sensing via quantum Fisher information. Second, the target sensitivity is investigated in noisy quantum-enhanced sensing. Under the same input state energy, for example, $N$-photon entangled states can exhibit better performance than a two-mode squeezed vacuum state and a separable coherent state. Incorporating a photon-number difference measurement, finally, the noisy target sensitivity is connected to a signal-to-noise ratio which is associated with a minimum error probability of discriminating the presence and absence of the target. We show that both the target sensitivity and the signal-to-noise ratio can be enhanced with increasing thermal noise.

We consider discrete-modulation protocols for continuous-variable quantum key distribution (CV-QKD) that employ a modulation constellation consisting of a finite number of coherent states and that use a homodyne- or a heterodyne-detection receiver. We establish a security proof for collective attacks in the asymptotic regime, and we provide a formula for an achievable secret-key rate. Previous works established security proofs for discrete-modulation CV-QKD protocols that use two or three coherent states. The main constituents of our approach include approximating a complex, isotropic Gaussian probability distribution by a finite-size Gauss-Hermite constellation, applying entropic continuity bounds, and leveraging previous security proofs for Gaussian-modulation protocols. As an application of our method, we calculate secret-key rates achievable over a lossy thermal bosonic channel. We show that the rates for discrete-modulation protocols approach the rates achieved by a Gaussian-modulation protocol as the constellation size is increased. For pure-loss channels, our results indicate that in the high-loss regime and for sufficiently large constellation size, the achievable key rates scale optimally, i.e., proportional to the channel's transmissivity.

The variational quantum eigensolver (VQE) typically optimizes variational parameters in a quantum circuit to prepare eigenstates for a quantum system. Its application to many problems may involve a group of Hamiltonians, e.g., a Hamiltonian of a molecule is a function of the nuclear configurations. In this paper we incorporate derivatives of the Hamiltonian into the VQE and develop some hybrid quantum-classical algorithms, which explore both Hamiltonian and wave-function spaces for solving quantum chemistry problems more efficiently. For geometry optimization, we propose a mutual gradient-descent algorithm that updates the parameters of the Hamiltonian and wave function alternately, which can give a rapid convergence towards equilibrium structures of molecules as demonstrated in numerical simulations. Moreover, to speed up the calculation of the energy potential surface, we establish differential equations that govern how the optimized variational parameters of the wave function change with the intrinsic parameters of the Hamiltonian. Our study suggests a direction for a hybrid quantum-classical algorithm for solving quantum systems more efficiently by considering spaces of both the Hamiltonian and the wave function.

In this paper, we present experimental techniques to resolve the closely spaced hyperfine levels of a weak transition by eliminating the residual or partial two-photon Doppler broadening and crossover resonances in a wavelength-mismatched double-resonance spectroscopy. The elimination of the partial Doppler broadening is based on velocity-induced population oscillation (VIPO) and velocity-selective saturation (VSS) effects followed by the subtraction of the broad background of the two-photon spectrum. Since the VIPO and VSS effects are the phenomena for near-zero-velocity group atoms, the subtraction gives rise to Doppler-free peaks and the closely spaced hyperfine levels of the $6P_{3/2}$ state in Rb are well resolved. The double-resonance experiment is conducted on the $5S_{1/2}\to 5P_{3/2}$ strong transition (at 780 nm) and $5S_{1/2}\to 6P_{3/2}$ weak transition (at 420 nm) at room temperature.

The electron scattering process has been investigated by analyzing the interference structure in the photoelectron momentum distribution (PMD) of a hydrogen atom exposed to a single-cycle linearly polarized near-infrared laser field, based on the numerical solution of the full-dimensional time-dependent Schrödinger equation and the Coulomb correlative classical trajectory simulation. The interference pattern in the PMD is closely related to the form of the ultrashort pulse which is dominated by the carrier-envelope phase. A fish-bone-like pattern appears in the PMD using the sine electric field and a spider-like pattern appears using the cosine electric field. These interference structures reflect the scattering process. It is found that the stripe density of the spider-like pattern is mainly dominated by the recollision time of scattering electron trajectories, i.e., the longer the recollision time, the greater the stripe density. Therefore, the photoelectron interference pattern can be used to understand the ionization and scattering processes, and identify these processes on the attosecond time scale.

We investigate the rotational properties of a dipolar Bose-Einstein condensate trapped in a toroidal geometry. Studying the ground states in the rotating frame and at fixed angular momenta, we observe that the condensate acts in distinctly different ways depending on whether it is in the superfluid or in the supersolid phase. We find that intriguingly, the toroidal dipolar condensate can support a supersolid persistent current which occurs at a local minimum in the ground state energy as a function of angular momentum, where the state has a vortex solution in the superfluid component of the condensate. The decay of this state is prevented by a barrier that in part consists of states where a fraction of the condensate mimics solid-body rotation in a direction opposite to that of the vortex. Furthermore, we find that superfluid condensates may enter the supersolid phase when forced to take angular momentum. Last, the rotating toroidal supersolid shows hysteretic behavior that is qualitatively different depending on the superfluid fraction of the condensate.

The exact phase of one-dimensional quantum states with given Gaussian probability densities in the position and momentum representations is retrieved. The number of Pauli partners that are found depends on whether the Heisenberg uncertainty relation for position and momentum is saturated or not. Without saturation, two Pauli partners are found. They differ in the sign of the time derivative of their position uncertainties. The same problem is solved by an exact implementation of the Gerchberg-Saxton algorithm. Its convergence depends on uncertainty saturation as well.

A quantum digital signature (QDS) is utilized to authenticate classical messages and offers information theoretical security guaranteed by the laws of quantum mechanics. Focusing on signing a one-bit message, previous continuous-variable quantum digital signature (CV-QDS) protocols that shared secret keys are generated by quantum key distribution, and it is only mentioned to suitably iterate the one-bit protocol for a multibit message. Although the CV-QDS protocol is unconditionally secure while sending single-bit messages, iterating the one-bit protocol in a naive way will allow some forgery attacks. In this paper, for signing one-bit and multibit messages, a CV-QDS protocol is proposed, on the condition that nonrepudiation and no forging are guaranteed. For one-bit and multibit schemes, we compare the difference in the distribution stage and messaging stage, analyzing the security as well as summarizing some formulas for calculation. We hope the protocol with continuous variables could further improve the performance of QDS.

We report on a structural superfluid–Mott-insulator (SF-MI) quantum phase transition for an interacting one-dimensional Bose gas within permeable multirod lattices, where the rod lengths are varied from zero to the lattice period length. We use the ab initio diffusion Monte Carlo method to calculate the static structure factor, the insulation gap, and the Luttinger parameter, which we use to determine if the gas is a superfluid or a Mott insulator. For the Bose gas within a square Kronig-Penney (KP) potential, where barrier and well widths are equal, the SF-MI coexistence curve shows the same qualitative and quantitative behavior as that of a typical optical lattice with equal periodicity but slightly larger height. When we vary the width of the barriers from zero to the length of the potential period, keeping the height of the KP barriers, we observe a way to induce the SF-MI phase transition. Our results are of significant interest, given the recent progress on the realization of optical lattices with a subwavelength structure that would facilitate their experimental observation.

We study the problem of the stability of a two-component droplet. The standard solution [D. S. Petrov, Phys. Rev. Lett. 115, 155302 (2015)] is based on a particular form of the mean field energy functional, in particular on the assumption of vanishing of potentially large hard-mode-source energy contribution. This imposes a constraint on the densities of the two components. The problem is reduced to stability analysis of a one-component system. As opposed to this, we present a two-component approach including possible nonzero hard-mode-source energy. We minimize the energy under conditions corresponding to the experimentally relevant situation where volume is free and atoms can evaporate. For the specific case of a two-component Bose-Bose droplet we find approximate analytic solutions and compare them to the standard result. We show that the densities of a stable droplet are limited to a range depending on interaction strength, in contrast to the original unique solution.

Dissipative Kerr solitons are localized structures that exist in nonlinear optical cavities. They lead to the formation of microcombs—chip-scale frequency combs that could facilitate precision frequency synthesis and metrology by capitalizing on advances in silicon photonics. Previous demonstrations have mainly focused on anomalous dispersion cavities. Notwithstanding, localized structures also exist in the normal dispersion regime in the form of circulating dark pulses, but their physical dynamics is far from being understood. Here, we explore dark-pulse Kerr combs generated in normal dispersion optical microresonators and report the discovery of reversible switching between coherent dark-pulse combs, whereby distinct states can be accessed deterministically. Furthermore, we reveal that the formation of dark-pulse Kerr combs is associated with the appearance of another resonance, a feature that has never been observed for dark pulses and is ascribed to soliton behavior. These results contribute to understanding the nonlinear physics in normal dispersion nonlinear cavities and provide insight into the generation of microcombs with high conversion efficiency.

It is known that spontaneous Raman scattering is accompanied by the appearance of nonclassical cross correlations of photons at Stokes and anti-Stokes frequencies. Such correlations occur due to the internal nonlinear interaction between electronic and nuclear vibration degrees of freedom. Since the properties of a given media are unchangeable, the possibilities for controlling the cross correlations are limited. This, in turn, restricts the possible applications of the correlated light obtained during the spontaneous Raman scattering. In this work, we investigate the influence of statistical properties of an incident light on the cross correlations of Stokes and anti-Stokes light. We show that the incoherence of the incident light results not only in the incoherence of the scattered light but also in the enhancement of the interbeam coherence at different frequencies. We demonstrate that the cross correlations of Stokes and anti-Stokes light intensities are enhanced when the incident light with high second-order autocorrelation function is applied. Our results pave the way to control correlated light obtained in the process of spontaneous Raman scattering.