The multiple collisions between an optical pulse and a temporal boundary in highly noninstantaneous Kerr media are investigated. The pulse exhibits parabolic-like motion from its acceleration phase to its collision with the temporal boundary. The collision location is influenced by the position of the temporal boundary, the noninstantaneous response of the media, and the power of the pulse. Additionally, the evolution of the wave-form profile after the collision is analyzed. The findings indicate that the pulse splits into reflected and transmitted components, with the evolution of these profiles being proportional to the ratio of the pulse power to the medium's relaxation time. Finally, simulations of the pulse's temporal evolution over ten dispersion lengths show that the reflected pulse undergoes a secondary collision with the temporal boundary.

Nonreciprocal optical devices play a pivotal role in modern optical systems and advanced quantum technologies. Nonreciprocal devices based on optical nonlinearity have garnered significant attention due to their compatibility with on-chip photonics fabrication techniques. However, when the opposite-propagating continuous waves are incident to such nonlinear nonreciprocal devices simultaneously, achieving dynamic nonreciprocity has remained a formidable challenge. In addition, the devices relying on nonlinear resonators suffer from limited bandwidths. In this paper, we theoretically propose a method for achieving an on-chip isolator and quasicirculator by employing a nonlinear Mach-Zehnder interferometer. Harnessing the chirality between self- and cross-Kerr nonlinearities in the nonlinear arm of the interferometer, our device achieves remarkable dynamic nonreciprocity. We obtain an isolation contrast of $⩾0.9$ for the optical isolator and fidelity of $⩾0.97$ for the optical quasicirculator, and a broad bandwidth of $20\text{nm}$ for both. Our protocol paves a way to realize a broadband on-chip nonlinear nonreciprocal device.

We present a numerical study of the spatial coherence of light that is radiated from a dilute ensemble of atoms. The spatial coherence is established as a result of the collective (cooperative) coupling of the atoms to the light and is qualitatively different from the coherence of a laser. Specifically, the coherence in collective spontaneous emission does not rely on population inversion and stimulated emission, is governed by antiphasing of the dipoles (subradiance), and the key figure of merit for the observed coherence is $\approx N/(L/\lambda )$. Here, $N$ is the number of atoms in the ensemble, $L$ is the size of the sample, and $\lambda$ is the wavelength of the emitted light.

As the main medium of quantum information, the frequency difference between optical and microwave photons is five orders of magnitude. Therefore, we generally improve the transmission efficiency of optical and microwave photons through coherent and bidirectional measures. The studies of the frequency conversion of optical and microwave photons in recent decades are all limited to Markovian approximations. In this paper we propose a scheme to achieve a two-stage frequency conversion between optical and microwave photons based on a hybrid of magnetic and mechanical excitations in non-Markovian environments. We estimate the interacting strengths under the optimal conditions of yttrium iron garnet and derive the conversion efficiency of optical and microwave photons through the scattering matrix. In all resonance modes, adjusting the defined complex cooperativities can get the conversion efficiency close to unity. The frequency conversion efficiency demonstrates a transition from Markovian to non-Markovian regimes by controlling the environmental spectral widths. This transition leads to enhancements of both the frequency conversion efficiency and conversion bandwidths, which originates from the excitation backflow obtained through the interaction between the systems and non-Markovian environments. In non-Markovian regimes, we predict that in areas with a large order of magnitude of optical pump power, the conversion bandwidths can be increased, but at the cost of a decrease in conversion efficiency, while decreasing the spectral widths can also achieve the same maximum conversion efficiency as in Markovian environments without requiring such a large optical pump power in non-Markovian regimes. Our scheme may provide an opportunity for the investigation of long-distance quantum communication for other non-Markovian quantum systems in quantum optics.

We explore a method for achieving photon blockade (PB) through a two-photon Jaynes-Cummings (JC) model that includes both cavity and atom drivings. Analytical expressions for the steady state are derived to calculate photon-number distribution and correlation functions. Based on this analysis, we establish the theoretical conditions required for generating PB by controlling both drivings and propose a scheme to identify conventional photon blockade (CPB) and unconventional photon blockade (UPB) by analyzing the second- and third-order correlation functions. We validate our findings through numerical simulations conducted in an artificial atom system. We discuss two cases, one in which the transition frequency of the two-level atom is in two-photon resonance with the cavity frequency and the other in which it is off-resonance. We observe a significant interplay between CPB and UPB effects in the resonant case. In the off-resonance case, we demonstrate that UPB can be achieved by manipulating the strengths of atom driving and cavity driving to satisfy a specific relationship while meeting the requirements for optimal detuning relations. This work provides essential insights into controlling the PB effect using the two-photon JC model with both the atom and cavity drivings.

The Bell-state measurement (BSM) is the projection of two qubits onto four orthogonal maximally entangled states. Here we first propose how to appropriately define more general BSMs, which have more than four possible outcomes, and then study whether they exist in quantum theory. We observe that nonprojective BSMs can be defined in a systematic way in terms of equiangular tight frames of maximally entangled states, i.e., a set of maximally entangled states, where every pair is equally, and in a sense maximally, distinguishable. We show that there exists a five-outcome BSM through an explicit construction and find that it admits a simple geometric representation. Then we prove that there exists no larger BSM on two qubits by showing that no six-outcome BSM is possible. We also determine the most distinguishable set of six equiangular maximally entangled states and show that it falls only somewhat short of forming a valid quantum measurement. Finally, we study the nonprojective BSM in the contexts of both local state discrimination and entanglement-assisted quantum communication. Our results put forward natural forms of nonprojective joint measurements and provide insight into the geometry of entangled quantum states.

In an atom-photon interacting system described by the Tavis-Cummings-Hubbard (TCH) model, we demonstrate the emergence of a quasi-steady state in a dissipative environment that exhibits intriguing ergodic behavior. The TCH model undergoes a dissipative transition from normal to superradiant phase hosting a gapped Higgs and gapless Goldstone modes. However, in a large region of the phase diagram, the instability of the Goldstone mode leads to the disappearance of the stable superradiant phase. In this regime, the decorrelator dynamics reveals light cone spreading of the perturbations and positive Lyapunov exponent, indicating enhanced fluctuations. Remarkably, a quasi-steady state emerges under quench dynamics in this unstable regime; in this state, a class of collective quantities such as site-averaged photon number and atomic excitations approach a steady value, in spite of the large temporal fluctuations in corresponding microscopic variables. This quasi-steady state describes an incoherent fluid of photons with significant phase fluctuation. The phase space dynamics reveals a fascinating ergodic behavior in the presence of dissipation leading to the characterization of the dynamical variables into two distinct classes. The first class includes site-averaged photon numbers and atomic excitations; these exhibit a stationary distribution regardless of the initial condition indicating ergodic behavior. The second class of variables, particularly those related to photon or atom phases, in contrast, retain information about the initial conditions, resulting in a violation of ergodicity for finite size systems. Additionally, the dynamical variables of the ergodic class exhibit a fascinating collective scarring phenomenon as the peak of their distribution is attracted towards the unstable steady state, analogous to the single-particle quantum scar. We discuss the relevance of our findings in the current experiments.

Photonic integrated circuits (PICs) are key to advancing quantum technologies for secure communications. They offer inherent stability, low losses, and compactness compared to standard fiber-based and free-space systems. Our research demonstrates PICs' effectiveness in enhancing quantum communications; we implement a three-state BB84 protocol with a decoy-state method, and we employ an integrated receiver and superconducting nanowire single-photon detectors to achieve technological advancements. One of the most notable results is the extraction of a secret key over a 45 dB channel attenuation. This achievement in the maximum covered distance signifies the potential of integrated photonics to advance the field of quantum communication.

The signature of an unatomic system is revealed by a continuous scale invariance that appears during a progressive dimensional squeezing of a resonantly interacting trimer. The unatomic regime is reached at the dimension $\overline{D}$, which for three identical atoms is found to be $\overline{D}=2.292$—below this value, the trimer wave function at short distances displays a power-law behavior. The fingerprint of this crossover is a sharp evolution of the contacts that characterizes the trimer momentum distribution tail.

Interparticle Coulombic electron capture (ICEC) is an environment-enabled electron capture process by means of which a free electron can be efficiently attached to a system (i.e., ion, atom, molecule, or quantum dot). The excess electron attachment energy is simultaneously transferred to a neighboring system (the environment) which undergoes ionization. ICEC has been theoretically predicted in van der Waals and hydrogen bonded systems as well as in quantum dot arrays. The theoretical approaches employed in these works range from analytical models to ab initio electronic structure and dynamical calculations. A common assumption in these approaches is that nuclei remain fixed during ICEC. In this paper, we use full explicit electron-nuclear dynamics simulations to show that the relative nuclear motion between the two species involved in ICEC enables the electron attachment at kinetic energies of the incoming electron far below the vertical energy threshold (i.e., at the equilibrium geometry of the system). ICEC is therefore more efficient than expected.

Networks of Rydberg atoms provide a powerful basis for quantum simulators and quantum technologies. Inspired by matter-wave atomtronics, here we engineer switches, diodes, and universal logic gates composed of Rydberg networks. Our schemes control the dynamics of a Rydberg excitation via the antiblockade or facilitation mechanism, allowing for much faster devices compared to cold atom systems. Our approach is robust to noise and can be applied to individually trapped atoms and extensive three-dimensional gases. In analogy to electronics, Rydberg atomtronic devices promise to enhance quantum information processors and quantum simulators.

In this work we describe a technique for numerical exact diagonalization. The method is particularly suitable for cold bosonic atoms in optical lattices, in which multiple atoms can occupy a lattice site. We describe the use of the method for Bose-Hubbard model of a two-dimensional square lattice system as an example; however, the method is general and can be applied to other lattice models and can be adapted to three-dimensional systems. The proposed numerical technique focuses in detail on how to construct the basis states as a hierarchy of wave functions. Starting from single-site Fock states, we construct the basis set in terms of row states and multirow states. This simplifies the application of constraints and calculation of the Hamiltonian matrix. The approach simplifies the calculation of the reduced density matrices, and this has applications in characterizing the topological entanglement of the state. Each step of the method can be parallelized to accelerate the computation. As a case study, we discuss the computation of the spatial bipartite entanglement entropy in the correlated $\nu =1/2$ fractional quantum Hall state.

Synchronization manifests itself in oscillators adjusting their frequencies and phases with respect to an external signal or another oscillator. In the quantum case, new features appear such as destructive interferences that can result in the suppression of phase locking. A three-level (spin-1) oscillator with equal gain and damping rates and subject to an external drive does not exhibit any 1:1 phase locking but 2:1 phase locking, i.e., its phase distribution features two maxima. This bistable locking at two opposite phases is a signature of the quantum interference synchronization blockade. An analogous behavior was found for two identical coupled spin-1 oscillators. In this work, we consider two coupled spin-1 oscillators and a drive applied to the first spin. This leads to two interference blockades between the drive and the first spin as well as between both spins. Although both interference blockades persist for strong drive and coupling strengths, remarkably, the undriven spin does show a 1:1 phase locking to the external drive. The magnitude of the locking is proportional to the drive strength if the drive strength is small. In other words, the undriven oscillator synchronizes to the external drive through both interference blockades while the blockades persist. For a chain of three coupled spin-1 oscillators, we find synchronization between the first and third spins mediated via the blockaded, second spin.

Entropy production is a fundamental concept that plays a crucial role in the second law of thermodynamics and the measure of irreversibility. It imposes rigorous constraints on the kinds of transformations allowed in thermodynamic processes. Using an optical setup, here we experimentally demonstrate the division of entropy production of an open quantum system into a population-related component and a coherence-related component, supporting previous theoretical predictions. The coherence-related component represents a genuine quantum contribution with no classical counterpart. By adjusting bath temperatures and initial coherences of the system, we first derive the total entropy production due to both populations and coherences, then remove all the coherences of the system to solely obtain the population-related contribution. The difference between these two results permits to isolate the coherence-related term. Based on this division, our experiment ultimately provides proof of principle that irreversibility at the quantum level can be reduced through properly harnessing the two contributions to entropy production.

The multifractal critical phase (MCP) fundamentally differs from extended and localized phases, exhibiting delocalized distributions in both position and momentum spaces. The investigation on the MCP has largely focused on one-dimensional quasiperiodic systems. Here, we introduce a two-dimensional (2D) quasiperiodic model with a MCP. We present its phase diagram and investigate the characteristics of the 2D system's MCP in terms of wave packet diffusion and transport based on this model. We further investigate the movement of the phase boundary induced by the introduction of next-nearest-neighbor hopping by calculating the fidelity susceptibility. Finally, we consider how to realize our studied model in superconducting circuits. Our work opens the door to exploring MCP in 2D systems.

We employ two-band density-matrix equations and time-dependent density-functional theory to calculate the high-order harmonic generation (HHG) in monolayer hexagonal boron nitride (hBN) under irradiation by a linearly polarized laser. We mainly focus on the parallel interband harmonics when the laser is along the zigzag and armchair directions of monolayer hBN. To gain insight into the electron dynamics during HHG, we compare the lattice momenta that dominate the specific harmonic radiation with the saddle-point momenta predicted by the current semiclassical recombination models for the corresponding harmonics, revealing an evident discrepancy between them. To address this discrepancy, we develop a semiclassical recombination model that accounts for the tunneling displacement of electron-hole pairs, which is closely related to the energy gap and relative velocity of the electron-hole pair at the ionization moment. Furthermore, we elucidate the underlying mechanism dominating the orientation dependence of parallel even harmonics and attribute the enhanced harmonic generation to caustic effects. This work sheds light on the electron dynamics during HHG in monolayer hBN.

We consider a “symmetric” quantum droplet in two spatial dimensions, which rotates in a harmonic potential, focusing mostly on the limit of “rapid” rotation. We examine this problem using a purely numerical approach, as well as a semianalytic Wigner-Seitz approximation (first developed by Baym, Pethick, and their co-workers) for the description of the state with a vortex lattice. Within this approximation we assume that each vortex occupies a cylindrical cell, with the vortex-core size treated as a variational parameter. Working with a fixed angular momentum, as the angular momentum increases and depending on the atom number, the droplet accommodates none, few, or many vortices, before it turns to center-of-mass excitation. For the case of a “large” droplet, working with a fixed rotational frequency of the trap $\mathrm{\Omega }$, as $\mathrm{\Omega }$ approaches the trap frequency $\omega$, a vortex lattice forms, the number of vortices increases, the mean spacing between them decreases, while the “size” of each vortex increases as compared to the size of each cell. In contrast to the well-known problem of contact interactions, where we have melting of the vortex lattice and highly correlated many-body states, here no melting of the vortex lattice is present, even when $\mathrm{\Omega }=\omega$. This difference is due to the fact that the droplet is self-bound. For $\mathrm{\Omega }=\omega$, the “smoothed” density distribution becomes a flat top, very much like the static unconfined droplet. When $\mathrm{\Omega }$ exceeds $\omega$, the droplet maintains its shape and escapes to infinity, via center-of-mass motion.

We theoretically investigate the impact of weak static perturbations of a flat potential on the density of a quasi-two-dimensional dipolar Bose gas. We consider the perturbative effects of potential perturbations at first order and restrict to the mean-field stable regime. We first study cosinusoidal potential perturbations at a given spatial frequency; this allows us to understand the effects of optical lattices as well as of isolated momentum contributions in the Fourier decomposition of an arbitrary potential. We then study potential perturbations characterized by a static white-noise spectrum over a given momentum range; this captures the effects of inherent optical aberrations in setups that create uniform optical dipole traps.

Dissipative phase transitions in quantum systems have been largely studied under the so-called Markovian approximation, where the environments to which the systems are coupled are memoryless. Here, we present a generalization of the spectral theory of dissipative phase transitions to non-Markovian systems, encompassing a much broader class of quantum materials and experiments and opening many possibilities for non-Markovian engineering of matter phases such as, as explored in the companion Letter [Debecker et al., Phys. Rev. Lett. 133, 140403 (2024)], reshaping of phase boundaries and triggering of phase transitions. We first prove several statements about the connections between the spectrum of the generator of the non-Markovian dynamics of general systems and dissipative phase transitions. Then, as a benchmark, we show that our framework can capture all the expected signatures of the superradiant phase transition appearing in a challenging $\mathbb{U}\left(1\right)$-symmetric two-mode Dicke model from a reduced description of the dynamics of the atoms only, a task for which all other methods have failed so far.

The Goos-Hänchen (GH) shift is a specifical optical phenomenon that describes a shift parallel to the reflected light inside the plane of incidence, when a finite-width light undergoes total internal reflection at the interface of medium. Although the GH shift in optics has been widely observed experimentally, its generalization remains uncovered completely in relativistic quantum mechanics for the spinor structure and the possible existence of Klein's paradox. Through approximation and angular spectrum analysis, we calculate the GH shift for Dirac fermions under relativistic conditions when they are incident obliquely on a three-dimensional infinite potential barrier. Furthermore, we find that the relativistic quantum GH shift will degenerate into the result based on Schrödinger equation under nonrelativistic limit, while the GH shift can be negative in Dirac's framework, which is different from the nonrelativistic case.