We show that the momentum distribution of a nonlinear matter wave suddenly released with a finite velocity in a speckle potential converges, after an out-of-equilibrium evolution, to a universal Rayleigh-Jeans thermal distribution. By exploring the complete phase diagram of the equilibrated wave, we discover that for low but nonzero values of the disorder strength, a large-scale structure—a condensate—appears in the equilibrium distribution.

We explore the mechanism of below-threshold even-harmonic generation of the ${\mathrm{H}}_2{}^{+}$ molecular ion subject to intense near-infrared linearly and circularly polarized laser fields. The task is accomplished by accurate treatment of the time-dependent Schrödinger equation in prolate spheroidal coordinates within the generalized pseudospectral formalism. Even harmonics are detected in the harmonic spectra of stretched molecules, mostly with the internuclear separations of 5 to 9 a.u. Consecutive dynamic localization of the electron density near each of the nuclei is responsible for the broken inversion symmetry, which in turn leads to emission of low-order even harmonics. An intuitive picture of the process is provided by the analysis of the time evolution of the electron density and time-frequency spectra of the dipole acceleration. A clear theoretical explanation of the phenomenon is given within the Floquet formalism.

Einstein-Podolsky-Rosen steering manifests a type of quantum correlations intermediate between entanglement and Bell nonlocality. In this paper we propose a scheme for realizing hybrid atom-mechanical quantum steering in the steady-state regime. In our scheme, an optomechanical two-mode cavity is coupled to a distant ensemble of double-$\lambda$ atoms in a cascade way, which induces the dissipative interaction between the mechanics and the internal atomic states. We show that strong cavity dissipation can lead to two-way atom-mechanical steering and the optimal steering exhibited in an approximate two-mode squeezed-vacuum atom-mechanical state. Moreover, we find that for the present cascade coupling scheme, the one-way steering from the mechanical oscillator to the atoms is achievable under certain conditions, while the reverse one-way steering is impossible to obtain. The reason for achieving the asymmetric steering is analyzed and the effect of mechanical thermal noise on the steering is also studied.

We examine the effect of adding $\mathcal{PT}$-symmetric gain and loss terms to quasi-one-dimensional lattices (ribbons) that possess flat bands. We focus on three representative cases: the Lieb ribbon, the kagome ribbon, and the stub ribbon. In general, we find that the effect on the flat band depends strongly on the geometrical details of the lattice being examined. One interesting result that emerges from an analytical calculation of the band structure of the Lieb ribbon including gain and loss is that its flat band survives the addition of $\mathcal{P}\mathcal{T}$ symmetry for any amount of gain and loss and also survives the presence of anisotropic couplings. For the other two lattices, any presence of gain and loss destroys their flat bands. For all three ribbons, there are finite stability windows whose size decreases with the strength of the gain and loss parameter. For the Lieb and kagome cases, the size of this window converges to a finite value. The existence of finite stability windows plus the constancy of the Lieb flat band are in marked contrast to the behavior of a pure one-dimensional lattice.

We present an analysis of the different types of noise from the detection and interrogation laser in our strontium lattice clock. We develop a noise model showing that in our setup quantum projection noise-limited detection is possible if more than 130 atoms are interrogated. Adding information about the noise spectrum of our clock laser with sub-${10}^{-16}$ fractional frequency instability allows one to infer the clock stability for different modes of operation. Excellent agreement with experimental observations for the instability of the difference between two interleaved stabilizations is found. We infer a clock instability of $1.6\times {10}^{-16}/\sqrt{\tau }$ as a function of averaging time $\tau$ expressed in seconds for normal clock operation.

The existence of the collective electronic excitation in polycyclic aromatic hydrocarbon (PAH) molecules has been predicted before on the basis of the presence of a large delocalized $\pi$ electron cloud around the carbon skeleton. Here, we present a manifestation of energy and angular distributions of electron emission upon deexcitation of the collective plasmon resonance in coronene, a PAH molecule, under fast ion impact. The angular distributions of these electrons show an unusually enhanced forward-backward angular asymmetry, in contrast to the observed uniform distributions for simpler atomic (Ne) or molecular (${\mathrm{CH}}_4$) targets. A simple model of photoelectron angular distribution from an oscillating dipolar plasmon, calculated including the first retardation term in the transition matrix element, provides excellent agreement with the observed distribution. The ratio of forward-to-backward electron emission intensity clearly exhibits a broad peak which is in excellent agreement with the theoretical prediction of the plamson peak. This observation may provide some new inputs towards the astrophysical problem of UV photon absorption by PAHs in the interstellar medium, or in the search for materials suitable for UV plasmonics.

Periodically driven quantum systems can be used to realize quantum pumps, ratchets, artificial gauge fields, and novel topological states of matter. Starting from the Keldysh approach, we develop a formalism, the Floquet-Boltzmann equation, to describe the dynamics and the scattering of quasiparticles in such systems. The theory builds on a separation of time scales. Rapid, periodic oscillations occurring on a time scale $T_0=2\pi /\mathrm{\Omega }$ are treated using the Floquet formalism and quasiparticles are defined as eigenstates of a noninteracting Floquet Hamiltonian. The dynamics on much longer time scales, however, is modeled by a Boltzmann equation which describes the semiclassical dynamics of the Floquet quasiparticles and their scattering processes. As the energy is conserved only modulo $\hslash \mathrm{\Omega }$, the interacting system heats up in the long-time limit. As a first application of this approach, we compute the heating rate for a cold-atom system, where a periodical shaking of the lattice was used to realize the Haldane model [G. Jotzu et al., Nature (London) 515, 237 (2014)].

Dynamical decoupling protocols are one of the most used tools for efficient quantum error corrections and for reservoir engineering. In this paper we study the effect of dynamical decoupling pulses on the preservation of both quantum and classical correlations, and their influence on the intriguing phenomenon of time-invariant discord. We study two qubits experiencing local dephasing with an Ohmic class spectrum and subject to dynamical decoupling protocols. We investigate the connection between the dynamics of both classical and quantum correlations and the behavior of information flow between system and environment. Finally, we establish a set of necessary conditions for which time-invariant discord can be created using dynamically decoupling techniques.

We apply the concept of quantum speed limit (QSL)—the minimal time needed to perform a driven evolution—to complex interacting many-body systems where the effects of interactions have to be taken into account. We introduce a general strategy to eliminate the detrimental effects of the interparticle repulsion and drive the system at the QSL by applying a compensating control pulse (CCP). To prove the principles we consider a prototypical many-body system, a bosonic Josephson junction, and investigate a transfer of atoms from the ground state of one well to the ground state of the neighboring well, at increasing levels of complexity—from a textbook two-level approximation to full many-body treatment. By tracing the efficiency of the CCP protocol we show that the driven dynamics does follow the geodetic pathway and, therefore, it is optimal. The CCP strategy, applicable for a general interacting quantum many-body system with strong driving, can be of a practical relevance for the experimental implementation of quantum technology protocols, as quantum simulations or matter-wave metrology.

Einstein-Podolsky-Rosen (EPR) steering describes how different ensembles of quantum states can be remotely prepared by measuring one particle of an entangled pair. Here, we investigate quantum steering for single quantum $d$-dimensional systems (qudits) and devise efficient conditions to certify the steerability therein, which we find are applicable both to single-system steering and EPR steering. In the single-system case our steering conditions enable the unambiguous ruling out of generic classical means of mimicking steering. Ruling out “false-steering” scenarios has implications for securing channels against both cloning-based individual attack and coherent attacks when implementing quantum key distribution using qudits. We also show that these steering conditions also have applications in quantum computation, in that they can serve as an efficient criterion for the evaluation of quantum logic gates of arbitrary size. Finally, we describe how the nonlocal EPR variant of these conditions also function as tools for identifying faithful one-way quantum computation, secure entanglement-based quantum communication, and genuine multipartite EPR steering.

The generation of quantum entanglement of macroscopic or mesoscopic bodies in mechanical motion is generally bounded by the thermal fluctuation exerted by their environments. Here we propose a scheme to establish stationary entanglement between two mechanically oscillating mirrors of a cavity. It is revealed that, by applying a broadband squeezed laser acting as a squeezed-vacuum reservoir to the cavity, a stable entanglement between the mechanical mirrors can be generated. Using the adiabatic elimination and master equation methods, we analytically find that the generated entanglement is essentially determined by the squeezing of the relative momentum of the mechanical mirrors, which is transferred from the squeezed reservoir through the cavity. Numerical verification indicates that our scheme is within the present experimental state of the art of optomechanics.

Quantum physics allows for unconditionally secure communication through insecure communication channels. The achievable rates of quantum-secured communication are fundamentally limited by the laws of quantum physics and in particular by the properties of entanglement. For a lossy communication line, this implies that the secret-key generation rate vanishes at least exponentially with the communication distance. We show that this fundamental limitation can be violated in a realistic scenario where the eavesdropper can store quantum information for only a finite, yet arbitrarily long, time. We consider communication through a lossy bononic channel (modeling linear loss in optical fibers) and we show that it is in principle possible to achieve a constant rate of key generation of one bit per optical mode over arbitrarily long communication distances.

High-resolution photoelectron momentum distributions of Xe atoms ionized by 800-nm linearly polarized laser fields have been traced at intensities from $1.1\times {10}^{13}$ to $3.5\times {10}^{13}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$ using velocity-map imaging techniques. At certain laser intensities, the momentum spectrum exhibits a distinct double-ring structure for low-order above-threshold ionization, which appears to be absent at lower or higher laser intensities. By investigating the intensity-resolved photoelectron energy spectrum, we find that this double-ring structure originates from resonant multiphoton ionization involving multiple Rydberg states of atoms. Varying the laser intensity, we can selectively enhance the resonant multiphoton ionization through certain atomic Rydberg states. The photoelectron angular distributions of multiphoton resonance are also investigated for the low-order above-threshold ionization.

Strong field ionization by circularly polarized laser fields from initial states with internal orbital momentum has an interesting propensity rule: electrons counter-rotating with respect to the laser field can be liberated more easily than co-rotating electrons [Barth and Smirnova, Phys. Rev. A 84, 063415 (2011)]. Here we show that application of few-cycle IR pulses allows one to use this propensity rule to detect ring currents associated with such quantum states, by observing angular shifts of the ejected electrons. Such shifts present the main observable of the attoclock method. We use time-dependent analytical $R$-matrix theory to show that the attoclock measured angular shifts of an electron originating from two counter-rotating orbitals ($p^{+}$ and $p^{-}$) are noticeably different. Our work opens opportunities for detecting ring currents excited in atoms and molecules, using the attoclock setup.

Two well-known mechanisms, one-axis twisting (OAT) and two-axis countertwisting (TACT), generate spin-squeezed states dynamically. The latter provides better spin squeezing (SS) but has not been demonstrated as the form of its interaction does not occur naturally in known physical systems. Several proposals for realizing effective TACT transformed from OAT require stringent experimental conditions in order to overcome the resulting nonstationary (oscillating) SS and continuously varying mean spin directions. This work presents a simple scheme that solves both problems by freezing SS at an optimal point and realizing effectively persistent SS by inhibiting further squeezing dynamics. Explicit procedures are outlined for persistent SS of the TACT limit. Protocols based on our scheme favorably relax experimental demands, which significantly brighten the prospects for realizing TACT.

Performing a systematic Bogoliubov–de Gennes spectral analysis, we illustrate that stationary vortex lines, vortex rings, and more exotic states, such as hopfions, are robust in three-dimensional atomic Bose-Einstein condensates, for large parameter intervals. Importantly, we find that the hopfion can be stabilized in a simple parabolic trap, without the need for trap rotation or inhomogeneous interactions. We supplement our spectral analysis by studying the dynamics of such stationary states; we find them to be robust against significant perturbations of the initial state. In the unstable regimes, we not only identify the unstable mode, such as a quadrupolar or hexapolar mode, but we also observe the corresponding instability dynamics. Furthermore, deep in the Thomas-Fermi regime, we investigate the particlelike behavior of vortex rings and hopfions.

We describe a collective-state atomic interferometer (COSAIN) with the signal fringe as a function of phase difference or rotation narrowed by $\sqrt{N}$ compared to a conventional interferometer, $N$ being the number of atoms, without entanglement. This effect arises from the interferences among collective states, and is a manifestation of interference at a Compton frequency of $10\times {10}^{30}$ Hz, or a de Broglie wavelength of 4.5 femtometer, for $N={10}^6$ and $v=1$ m/s. The population of the collective state of interest is detected by a null measurement scheme, in which an event corresponding to detection of zero photons corresponds to the system being in that particular collective state. The signal is detected by collecting fluorescence through stimulated Raman scattering of Stokes photons, which are emitted predominantly against the direction of the probe beam, for a high enough resonant optical density. The sensitivity of the ideal COSAIN is found to be given by the standard quantum limit. However, when detection efficiency and collection efficiency are taken into account, the detection scheme of the COSAIN increases the quantum efficiency of detection significantly in comparison to a typical conventional Raman atomic interferometer employing fluorescence detection, yielding a net improvement in stability by as much as a factor of 10. We discuss how the inhomogeneities arising from the nonuniformity in experimental parameters affect the COSAIN signal. We also describe an alternate experimental scheme to enhance resonant optical density in a COSAIN by using cross-linearly polarized counterpropagating Raman beams.

High-sensitivity differential atom interferometers (AIs) are promising for precision measurements in science frontiers in space, including gravity-field mapping for Earth science studies and gravitational wave detection. Difficulties associated with implementing long-baseline differential AIs have previously included the need for a high optical power, large differential Doppler shifts, and narrow dynamic range. We propose a configuration of twin AIs connected by a laser-ranging interferometer (LRI-AI) to provide precise information of the displacements between the two AI reference mirrors and also to phase-lock the two independent interferometer lasers over long distances, thereby drastically improving the practical feasibility of long-baseline differential AI measurements. We show that a properly implemented LRI-AI can achieve equivalent functionality to the conventional differential AI measurement configuration.

A system of two coupled semiconductor-based resonators is studied when lasing around an exceptional point. We show that the presence of nonlinear saturation effects can have important ramifications on the transition behavior of this system. In sharp contrast with linear $\mathcal{PT}$-symmetric configurations, nonlinear processes are capable of reversing the order in which the symmetry breaking occurs. Yet, even in the nonlinear regime, the resulting non-Hermitian states still retain the structural form of the corresponding linear eigenvectors expected above and below the phase-transition point. The conclusions of our analysis are in agreement with experimental data.