It is shown that electron backscattering can enhance high-order-harmonic generation in periodic systems with broken translational symmetry. Paradigmatically, we derive for a finite chain of atoms the harmonic cutoff due to electrons backscattered from the edges of the chain and demonstrate a maximum in the harmonic yield if twice the quiver amplitude of the driven electrons equals the chain length. For an intuitive understanding of our quantum results we develop a refined semiclassical trajectory model with finite electron-hole separation after tunneling. We demonstrate that the same “tunnel exit” also holds for interband harmonics in conventional periodic solid-state systems.

We propose an ultracold atom analog of false-vacuum decay using all three states of a spin-1 Bose gas. We consider a one-dimensional system with both radio-frequency and optical Raman couplings between internal states. An advantage of our proposal is the lack of a time-modulated coupling, which can lead to instabilities. Within the elaborate phase structure of the system, we identify an effective Klein-Gordon field and use Gross-Pitaevskii simulations within the truncated Wigner approximation to model the decay of a metastable state. We examine the dependence of the rate of vacuum decay on particle density for ${}^7\mathrm{Li}$ and ${}^{41}\mathrm{K}$ and find reasonable agreement with instanton methods.

We derive a stochastic hierarchy of matrix product states (HOMPS) for non-Markovian dynamics in an open quantum system at finite temperature, which is numerically exact and efficient. HOMPS is obtained from the recently developed stochastic hierarchy of pure states (HOPS) by expressing HOPS in terms of formal creation and annihilation operators. The resulting stochastic first-order differential equation is then formulated in terms of matrix product states and matrix product operators. In this way the exponential complexity of HOPS can be reduced to scale polynomial with the number of particles. The validity and efficiency of HOMPS is demonstrated for the spin-boson model and long chains where each site is coupled to a structured, strongly non-Markovian environment.

Realizing universal topological quantum computers requires the manipulation of non-Abelian topological orders in a physical system, which presents great challenges. Conversely, the rapid development in circuit-based quantum computing offers a reliable quantum simulation approach to study these topological orders. The preliminary problem is how to identify distinct topological orders. Here, we develop a framework based on the quantum scattering circuit to directly and efficiently measure the modular transformation matrix, which is widely deemed as the fingerprint of a given topological order. The information of the modular transformation matrix is encoded in the probe qubit, and the readout merely requires single-qubit Pauli measurements. We further implement the scheme in a nuclear magnetic resonance quantum simulator to emulate the string-net model, where an Abelian ${\mathbb{Z}}_2$ toric code and a non-Abelian Fibonacci order emerge. In particular, the latter is predicted to be the simplest candidate for universal topological quantum computers. The two topological orders are unambiguously distinguished by the experimentally measured modular transformation matrices. As an experimental demonstration of a non-Abelian topological order with efficient readout, our work may open avenues toward investigating topological orders in circuit-based quantum simulators.

In electron-electron interactions in electromagnetic systems, retardation in the exchange of a virtual photon is essentially important as the first-order quantum electrodynamics correction. However, the retardation effect is generally so small that it is buried in unretarded electric and magnetic interactions and thus has yet to be directly probed. Here, we present a giant contribution of the retardation effect in an electron-electron interaction via observing strong electric-dipole-allowed radiative transition rates. The relative transition rates are obtained for two dominant radiative transitions from the $1s2s^{2}2p_{1/2}2p_{3/2}$ inner-shell excited state of boronlike tungsten and bismuth ions to $1s^{2}2s^{2}2p_{1/2}$ and $1s^{2}2s^{2}2p_{3/2}$, and it was found that the transition rate ratio between the two transitions is affected by the retardation effect up to more than 100%.

We report a precision measurement of a tune-out wavelength for ${}^{87}\mathrm{Rb}$ using circularly polarized light. A tune-out wavelength characterizes a zero in the electric polarizability of the atom. For circularly polarized light, the total polarizability depends on both the scalar and vector polarizability components. This shifts the location of the tune-out wavelength and makes it sensitive to different combinations of atomic dipole matrix elements than the scalar polarizability alone. Using ${\sigma }_{-}$ polarized light with a purity of 0.9931(1), we observe a tune-out wavelength of 785.1522(3) nm, which agrees with theoretical expectations when small contributions from the core electrons and off-resonant valence states are taken into account.

A long-standing problem in quantum-orbit theory has been exactly which solutions of the saddle-point equations to include in the decomposition of the ionization or harmonic-generation amplitude. Up to now, solutions corresponding to a negative travel time have always been discarded. For the case of an elliptically polarized driving laser field, we show that certain solutions with a negative travel time are relevant and have to be included, in addition to the customary orbits with positive travel times, in order to achieve good agreement with the result of a numerical evaluation. In fact, these solutions are responsible for a pronounced qualitative effect in the high-order above-threshold ionization amplitude: a feature with the shape of a coffee bean split along the direction of the major polarization axis, which dominates the velocity map especially for long wavelength. We also discuss the electron trajectories in complex space and time that correspond to these orbits.

We consider a general version of the phenomenon of more nonlocality with less entanglement within the framework of the unambiguous (i.e., conclusive) quantum state discrimination problem under local quantum operations and classical communication. We show that although the phenomenon was obtained before for two qutrits, it can also be observed for two qubits, whereas still being at the single-copy level. We establish that the phenomenon is intrinsically connected to the concept of unextendible entangled bases in the two-qubit case. In the process, we demonstrate a hierarchy of nonlocality among sets of two-qubit orthogonal pure states where the “nonlocality” is in the sense of a difference between global and local abilities of quantum state discrimination. We present a complete characterization of two-qubit pure orthogonal state sets of cardinality three with respect to their nonlocality in terms of unambiguous local distinguishability, the status for other cardinalities being already known. The results are potentially useful for secure quantum communication technologies with an optimal amount of resources.

We propose an approach based on random matrix theory to calculate the temporal second-order intensity correlation function $g^{\left(2\right)}\left(t\right)$ of the radiation emitted by random lasers and random fiber lasers. The multimode character of these systems, with a relevant degree of disorder in the active medium, and a large number of random scattering centers substantially hinder the calculation of $g^{\left(2\right)}\left(t\right)$. Here, we apply in a photonic system the universal statistical properties of Ginibre's non-Hermitian random matrix ensemble to obtain $g^{\left(2\right)}\left(t\right)$. Excellent agreement is found with time-resolved measurements for several excitation powers of an erbium-based random fiber laser. We also discuss the extension of the random matrix approach to address the statistical properties of general disordered photonic systems with various Hamiltonian symmetries.

The nondipole under-the-barrier dynamics of the electron during strong-field tunneling ionization is investigated, examining the role of the Coulomb field of the atomic core. The common analysis in the strong-field approximation is consequently generalized to include the leading light-front nondipole Coulomb corrections and demonstrates the counterintuitive impact of the sub-barrier Coulomb field. Despite its attractive nature, the sub-barrier Coulomb field increases the photoelectron nondipole momentum shift along the laser propagation direction, involving a strong dependence on the laser field. The scaling of the effect with respect to the principal quantum number and angular momentum of the bound state is found. We demonstrate that the signature of Coulomb induced sub-barrier effects can be identified in the asymptotic photoelectron momentum distribution via a comparative study of the field-dependent longitudinal momentum shift for different atomic species with state-of-the-art experimental techniques of midinfrared lasers.

We have realized the optical excitation of krypton to a metastable level with an efficiency as high as $23\%$ using near-resonant 124-nm light produced by four-wave mixing in mercury vapor. Self-absorption in mercury is circumvented by adjusting the detuning and phase matching according to experimental and theoretical characterizations. Density-matrix calculations of the metastable krypton generation agree with the measured dependencies of the excitation efficiency, indicating pathways towards a further improvement. The obtained excitation efficiency, being one order of magnitude higher than that of previously demonstrated techniques, enables an extension of the metastable density regime for atomic physics applications, including magnetometry, atom lithography, and radioisotope dating.

Temperature is a deceptively simple concept that still raises deep questions at the forefront of quantum physics research. The observation of thermalization in completely isolated quantum systems, such as cold-atom quantum simulators, implies that a temperature can be assigned even to individual, pure quantum states. Here, we propose a scheme to measure the temperature of such pure states through quantum interference. Our proposal involves interferometry of an auxiliary qubit probe, which is prepared in a superposition state and subsequently decoheres due to weak coupling with a closed, thermalized many-body system. Using only a few basic assumptions about chaotic quantum systems, namely, the eigenstate thermalization hypothesis and the emergence of hydrodynamics at long times, we show that the qubit undergoes pure exponential decoherence at a rate that depends on the temperature of its surroundings. We verify our predictions by numerical experiments on a quantum spin chain that thermalizes after absorbing energy from a periodic drive. Our Letter provides a general method to measure the temperature of isolated, strongly interacting systems under minimal assumptions.

Due to strong photon-phonon interaction, coupled microcavities have provided a fruitful platform for chaos investigation. In particular, the parity-time symmetric microcavities pair has attracted unprecedented attention since the exceptional point (EP) in the non-Hermitian system offers more intriguing and abundant dynamics. Motivated by achieving a robust EP, a novel exceptional surface (ES) notion is proposed in the coupled microcavities. However, the chaotic dynamics on the ES remain yet unexplored. This study explores the chaos generation on the ES. We discover that the ES could dramatically decrease the threshold of chaos generation by >50% compared to the eigenvalues escaping from the ES. Unlike conventional chaotic dynamics, chaotic signals on the ES possess a surprising globally flattened, but locally chirped spectrum. Such an advantage provides an additional degree of freedom to implement chaotic applications on the physical layer. In addition, a coupling phase-induced chaos is found, which demonstrates an accumulated chaos generation beyond the conventional wisdom. More importantly, such an arrangement enables chaotic signals to achieve the temporal Vernier and harmonic Vernier effects, expanding the scope of chaos. This study of chaos generation on the ES further stimulates applications in compact chaotic devices and potentially facilitates fundamental chaotic dynamics research.

We propose a complete architecture for deterministic generation of entangled multiphoton states. Our approach utilizes periodic driving of a quantum-dot emitter and an efficient light-matter interface enabled by a photonic crystal waveguide. We assess the quality of the photonic states produced from a real system by including all intrinsic experimental imperfections. Importantly, the protocol is robust against the nuclear spin bath dynamics due to a naturally built-in refocusing method reminiscent to spin echo. We demonstrate the feasibility of producing Greenberger-Horne-Zeilinger and one-dimensional cluster states with fidelities and generation rates exceeding those achieved with conventional “fusion” methods in current state-of-the-art experiments. The proposed hardware constitutes a scalable and resource-efficient approach towards implementation of measurement-based quantum computing and quantum communication.

We theoretically study dynamical formation of a quantum droplet in a two-component Bose-Hubbard system with an external trap potential. Specifically, the superfluid in the central region surrounded by the Mott insulator with double filling forms a quantum droplet, which is self-bound thanks to the discontinuous nature of the quantum phase transition between the two phases. We show how to induce the characteristic behavior of the droplet through the control of the trap potential by using the time-dependent Gutzwiller simulations in a two-dimensional system. The static and dynamical properties of the droplet can be described qualitatively by the effective Ginzburg-Landau field theory with cubic-quintic nonlinearities where the attractive cubic nonlinearity emerges, although all the bare interparticle interactions are repulsive.

Synchronization in quantum systems has been largely driven by specific examples of frequency entrainment as well as mutual synchronization. Here we study quantum synchronization as a Liouville space perturbation theory. We begin by clarifying the role of centers, symmetries, and oscillating coherences in the context of quantum synchronization. We then analyze the eigenspectrum of the Liouville superoperator generating the dynamics of the quantum system and determine the conditions under which synchronization arises. We apply our framework to derive a powerful relationship between energy conservation, degeneracies, and synchronization in quantum systems. Finally, we demonstrate our approach by analyzing two mutually coupled thermal machines and prove that nondegenerate thermal networks cannot be simultaneously energy conserving and synchronous.

We consider tight-binding single-particle lattice Hamiltonians which are invariant under an antiunitary antisymmetry: the anti-$\mathcal{PT}$ symmetry. The Hermitian Hamiltonians are defined on $d$-dimensional non-Bravais lattices. For an odd number of sublattices, the anti-$\mathcal{PT}$ symmetry protects a flatband at energy $E=0$. We derive the anti-$\mathcal{PT}$ constraints on the Hamiltonian and use them to generate examples of generalized kagome networks in two and three lattice dimensions. Furthermore, we show that the anti-$\mathcal{PT}$ symmetry persists in the presence of uniform DC fields and ensures the presence of flatbands in the corresponding irreducible Wannier-Stark band structure. We provide examples of the Wannier-Stark band structure of generalized kagome networks in the presence of DC fields, and their implementation using Floquet engineering.

Radiation reaction (RR) is the oldest still-unsolved problem in electrodynamics. In addition to conceptual difficulties in its theoretical formulation, the requirement of exceedingly large charge accelerations has thus far prevented its unambiguous experimental identification. Here, we show how measurable RR effects in a laser-electron interaction can be achieved through the use of flying focus pulses (FFPs). By allowing the focus to counterpropagate with respect to the pulse phase velocity, a FFP overcomes the intrinsic limitation of a conventional laser Gaussian pulse (GP) that limits its focus to a Rayleigh range. For an electron initially also counterpropagating with respect to the pulse phase velocity, an extended interaction length with the laser peak intensity is achieved in a FFP. As a result, the same RR deceleration factors are obtained, but at FFP laser powers orders of magnitude lower than for ultrashort GPs with the same energy. This renders the proposed setup much more stable than those using GPs and allows for more accurate in situ diagnostics. Using the Landau-Lifshitz equation of motion, we show numerically and analytically that the capability of emerging laser systems to deliver focused FFPs will allow for a clear experimental identification of RR.

We catalog known optical moiré lattices and uncover exotic lattice configurations following a geometric analog of the ancient sieve of Eratosthenes algorithm for finding prime numbers. Rich dynamics of Bose-Einstein condensates loaded into these optical lattices is revealed from numerical simulations of time-of-flight interference patterns. What sets this method apart is the ability to tune the periodicity of the optical lattices without changing the wavelength of the laser, yet maintaining the local potential at the individual lattice sites. In addition, we discuss the ability to spatially translate the optical lattice through applying a structured phase only.

Anderson localization confines the wave function of a quantum particle in a one-dimensional random potential to a volume of the order of the localization length $\xi$. Nonlinear add-ons to the wave dynamics mimic many-body interactions on a mean-field level, and result in escape from the Anderson cage and in unlimited subdiffusion of the interacting cloud. We address quantum corrections to that subdiffusion by (i) using the ultrafast unitary Floquet dynamics of discrete-time quantum walks, (ii) an interaction strength ramping to speed up the subdiffusion, and (iii) an action discretization of the nonlinear terms. We observe the saturation of the cloud expansion of $N$ particles to a volume $\sim N\xi$. We predict and observe a universal intermediate logarithmic expansion regime which connects the mean-field diffusion with the final saturation regime and is entirely controlled by particle number $N$. The temporal window of that regime grows exponentially with the localization length $\xi$.