Motivated by the goal of realizing spin-orbit coupling (SOC) beyond one dimension, we propose and analyze a method to generate effective two-dimensional SOC in bilayer BECs with laser-assisted interlayer tunneling. We show that interplay among the interlayer tunneling, SOC, and intralayer atomic interaction can give rise to diverse ground-state configurations. In particular, the system undergoes a transition to a different type of stripe phase which spontaneously breaks the time-reversal symmetry. Different from the ordinary Rashba-type SOC, a fractionalized skyrmion lattice emerges spontaneously in the bilayer system without external traps. Furthermore, we predict the occurrence of a tetracritical point in the phase diagram of the bilayer BECs, where four phases merge. The origin of the different emerging phases is elucidated.

The Talbot effect is a near-field phenomenon of a coherent wave transmitted through a grating to form self-images at distances of integer multiples of the so-called Talbot length. However, at long distances, the higher-order diffraction waves fail to converge and spatial walk-off aberration occurs. Here, we describe a scheme for measuring the optical Talbot effect including the spatial walk-off aberration. Our approach utilizes a quantum dot fluorescent film as a tomographic means to probe the Talbot intensity patterns. Since the Talbot effect itself is based on diffraction and interference, measuring it with conventional microscopy systems with a limited acceptance angle might not allow for covering aberrations introduced by walk-off effects. The experiment demonstrates that, by utilizing the fluorescent film as an intermediate step and imaging the fluorescence instead, rather than imaging the intensity patterns directly, higher resolution can be achieved and effects not covered by the numerical aperture of the microscope objectiveused for imaging can be observed.

Motional ground-state cooling and quantum-coherent manipulation of mesoscopic mechanical systems are crucial goals in both fundamental physics and applied science. We demonstrate that the motional ground state can be achieved in the highly unresolved sideband regime, through coherent auxiliary cavity interferences. We further illustrate coherent strong Rabi coupling between indirectly coupled and individually optimized mechanical resonators and optical cavities through effective dark-mode interaction. The proposed approach provides a platform for quantum manipulation of mesoscopic mechanical devices beyond the resolved sideband limit.

We study the ground state of a finite-size ensemble of interacting qubits driven by a quantum field. We find a maximally entangled $W$ state in the ensemble part of the system for a certain region of the coupling parameters. The area of this region decreases as the ensemble size increases and, in the classical limit, becomes the line in parameter space that defines the phase transition of the system. In the classical limit, we also study the dynamics of the system and its transition from order to disorder for initial energies close to the ground-state energy. We find that a critical energy providing this transition is related to the minimum of the projection of the total angular momentum of the quantum system in the $z$ direction.

Unlike a conventional optical cavity laser, a random laser system is generally characterized by a nondirectional output emission. In this work we report an experimental and theoretical study on the angular properties of the random laser emission and its dependence on the diffusive properties of the sample and the spatial gain profile, showing the possibility of controlling it by the total amount of available energy. We show that the directional characteristics are associated with the statistical regime of fluctuations in the emission spectrum and, in particular, with a Lévy statistical regime. A model based on a phase-insensitive feedback mechanism has been used in order to explain the experimental data and two different Lévy subregimes have been identified.

Electric dipole polarizability $\left({\alpha }_d\right)$ and the electric dipole moment (EDM) due to the electron-nucleus tensor-pseudotensor (T-PT) and the nuclear Schiff moment (NSM) interactions are accurately determined for ${}^{199}\mathrm{Hg}$. Relativistic many-body methods at different levels of approximations are employed to gain insights into the passage of the electron correlation effects in the evaluation of these properties. Significant changes in the results compared to previous calculations are observed. We estimate our final EDM results to be within 2% accuracy, which, in combination with the measured value of ${}^{199}\mathrm{Hg}$ EDM, yield limits on the T-PT coupling constant as $C_T<2.09\times {10}^{-9}$ and on the NSM as $S<1.45\times {10}^{-12}\left|e\right|{\text{fm}}^3$. Using these values, together with the latest nuclear structure and quantum chromodynamics calculations, we get limits for the strong CP-violating parameter as $|\overline{\theta }|<1.1\times {10}^{-9}$ and for the combined up- and down-quark chromo-EDMs as $|{\tilde{d}}_u-{\tilde{d}}_d|<2.8\times {10}^{-26}\left|e\right|\text{cm}$.

We discuss the nonlocality of $W$ and Dicke states subject to losses. We consider two noise models, namely, loss of excitations and loss of particles, and investigate how much loss can be tolerated such that the final state remains nonlocal. This leads to a measure of robustness of the nonlocality of Dicke states, with a clear physical interpretation. Our results suggest that the relation between nonlocality and entanglement of Dicke states is not monotonic.

We propose a potentially practical scheme for quantum state transfer between a Cooper-pair box circuit and an electron spin ensemble of diamond nitrogen-vacancy (NV) centers. Both subsystems are placed into a common circuit QED and can be modeled as effective three-level subsystems under the appropriate external biases. Due to significant suppression of the photon decay, the robust state transfer between the two subsystems can be accomplished by using the technique of stimulated Raman adiabatic passage by individual microwave pulses, where a superconducting coplanar resonator serves as the quantum data bus. Numerical simulations show that the present scheme could offer a viable route towards robust quantum state transfer in hybrid solid-state systems.

The interfacial profiles and interfacial tensions of phase-separated binary mixtures of Bose-Einstein condensates are studied theoretically. The two condensates are characterized by their respective healing lengths ${\xi }_1$ and ${\xi }_2$ and by the interspecies repulsive interaction $K$. An exact solution to the Gross-Pitaevskii (GP) equations is obtained for the special case ${\xi }_2/{\xi }_1=1/2$ and $K=3/2$. Furthermore, applying a double-parabola approximation (DPA) to the energy density featured in GP theory allows us to define a DPA model, which is much simpler to handle than GP theory but nevertheless still captures the main physics. In particular, a compact analytic expression for the interfacial tension is derived that is useful for all ${\xi }_1,{\xi }_2$, and $K$. An application to wetting phenomena is presented for condensates adsorbed at an optical wall. The wetting phase boundary obtained within the DPA model nearly coincides with the exact one in GP theory.

We have previously shown that quantum-enhanced atom interferometry can be achieved by mapping the quantum state of squeezed optical vacuum to one of the atomic inputs via a beamsplitter-like process [Phys. Rev. A 90, 063630 (2014)]. Here we ask the question: is a better phase sensitivity possible if the quantum state transfer (QST) is described by a three-mode-mixing model, rather than a beamsplitter? The answer is yes, but only if the portion of the optical state not transferred to the atoms is incorporated via information recycling. Surprisingly, our scheme gives a better sensitivity for lower QST efficiencies and with a sufficiently large degree of squeezing can attain near-Heisenberg-limited sensitivities for arbitrarily small QST efficiencies. Furthermore, we use the quantum Fisher information to demonstrate the near optimality of our scheme.

We show that an ultracold atomic cloud bouncing on an oscillating mirror can reveal spontaneous breaking of a discrete time-translation symmetry. In many-body simulations, we illustrate the process of the symmetry breaking that can be induced by atomic losses or by a measurement of particle positions. The results pave the way for understanding and realization of the time crystal idea where crystalline structures form in the time domain due to spontaneous breaking of continuous time-translation symmetry.

We propose a conditional implementation of superpositions of Gaussian operations on different modes of light. These operations are conditional entanglers generating non-Gaussian entanglement between two modes of propagating light beams. The all-optical operations emulate weak effects of highly nonlinear operations only available when light interacts with individual atoms. In particular, we propose schemes to implement superposition of coherent displacements and squeezings on two beams of light and discuss physical properties of generated non-Gaussian entanglement. These methods together with high quality of optical tomography of highly nonclassical states allow one to understand non-Gaussian entangling processes.

We study a one-dimensional binary optical lattice in the presence of diagonal disorder and alternating gain and loss, and examine the light transport phenomena for localized and extended input beams. In the pure $\mathcal{PT}$-symmetric case, we derive an exact expression for the behavior of light localization in terms of typical parameters of the system. Within the $\mathcal{PT}$-symmetric region light localization becomes constant as a function of the strength of the gain and loss parameter, but outside the $\mathcal{PT}$-symmetric window, light localization increases as the gain and loss parameter increases. When disorder is added, we observe that the presence of gain and loss inhibits (favors) the transport for localized (extended) excitations.

We investigate the two-photon scattering properties of a Jaynes-Cummings (JC) nonlinearity consisting of a two-level system (qubit) interacting with a single-mode cavity, which is coupled to two waveguides, each containing a single incident photon wave packet initially. In this scattering setup, we study the interplay between the Hong-Ou-Mandel (HOM) effect arising due to quantum interference and effective photon-photon interactions induced by the presence of the qubit. We calculate the two-photon scattering matrix of this system analytically and identify signatures of interference and interaction in the second-order auto- and cross-correlation functions of the scattered photons. In the dispersive regime, when qubit and cavity are far detuned from each other, we find that the JC nonlinearity can be used as an almost linear, in situ tunable beam splitter giving rise to ideal Hong-Ou-Mandel interference, generating a highly path-entangled two-photon NOON state of the scattered photons. The latter manifests itself in strongly suppressed waveguide cross-correlations and Poissonian photon number statistics in each waveguide. If the two-level system and the cavity are on resonance, the JC nonlinearity strongly modifies the ideal HOM conditions leading to a smaller degree of path entanglement and sub-Poissonian photon number statistics. In the latter regime, we find that photon blockade is associated with bunched autocorrelations in both waveguides, while a two-polariton resonance can lead to bunched as well as antibunched correlations.

The unitary Fermi gas (UFG) offers a unique opportunity to study quantum turbulence both experimentally and theoretically in a strongly interacting fermionic superfluid with the highest vortex line density of any known superfluid. It yields to accurate and controlled experiments and admits the only dynamical microscopic description via time-dependent density-functional theory, apart from dilute bosonic gases, of the crossing and reconnection of superfluid vortex lines conjectured by Feynman [R. P. Feynman, Prog. Low Temp. Phys. 1, 17 (1955)] to be at the origin of quantum turbulence in superfluids at zero temperature. We demonstrate how various vortex configurations can be generated by using well-established experimental techniques: laser stirring and phase imprinting. New imaging techniques demonstrated by Ku et al. [M. J. H. Ku et al., Phys. Rev. Lett. 113, 065301 (2014)] should be able to directly visualize these crossings and reconnections in greater detail than performed so far in liquid helium. We demonstrate the critical role played by the geometry of the trap in the formation and dynamics of a vortex in the UFG and how laser stirring and phase imprint can be used to create vortex tangles with clear signatures of the onset of quantum turbulence.

The “folding algorithm” [M. C. Bañuls, M. B. Hastings, F. Verstraete, and J. I. Cirac, Phys. Rev. Lett. 102, 240603 (2009)] is a matrix product state algorithm for simulating quantum systems that involves a spatial evolution of a matrix product state. Hence, the computational effort of this algorithm is controlled by the temporal entanglement. We show that this temporal entanglement is, in many cases, equal to the spatial entanglement of a modified Hamiltonian. This inspires a modification to the folding algorithm, which we call the “hybrid algorithm.” We find that this leads to improved accuracy for the same numerical effort. We then use these algorithms to study relaxation in a transverse plus parallel field Ising model, finding persistent quasiperiodic oscillations for certain choices of initial conditions.

Light thermalized at room temperature in an optically pumped, dye-filled microcavity resembles a model system of noninteracting Bose-Einstein condensation in the presence of dissipation. We have experimentally investigated some of the steady-state properties of this unusual state of light and found features which do not match the available theoretical descriptions. We have seen that the critical pump power for condensation depends on the pump beam geometry, being lower for smaller pump beams. Far below threshold, both intracavity photon number and thermalized photon cloud size depend on pump beam size, with optimal coupling when the pump beam matches the thermalized cloud size. We also note that the critical pump power for condensation depends on the cavity cutoff wavelength and longitudinal mode number, which suggests that energy-dependent thermalization and loss mechanisms are important.

Many of the standard Bell inequalities (e.g., CHSH) are not effective for detection of quantum correlations which allow for steering, because for a wide range of such correlations they are not violated. We present Bell-like inequalities which have lower bounds for nonsteering correlations than for local causal models. The inequalities involve all possible measurement settings at each side. We arrive at interesting and elegant conditions for steerability of arbitrary two-qubit states.

We propose an alternative scheme of shortcuts to a quantum controlled phase gate in a much shorter time based on the approach of Lewis-Riesenfeld invariants in cavity quantum electrodynamics systems. This scheme can be used to perform a one-qubit phase gate, a two-qubit controlled phase gate, as well as a multiqubit controlled phase gate. The strict numerical simulations demonstrate that the total operation time for implementing controlled phase gates is much shorter than previous schemes and very robust against decoherence.

In this paper, we analyze several critical issues in semi-device-independent quantum information processing protocol. In practical experimental realization, randomness generation in that scenario is possible only if the efficiency of the detectors used is above a certain threshold. Our analysis shows that the critical detection efficiency is $\frac{\sqrt{2}}{2}$ in the symmetric setup, while in the asymmetric setup if one of the bases has perfect critical detection efficiency then the other one can be arbitrarily close to 0. We also analyze the semi-device-independent random-number-generation efficiency based on different averages of guessing probability. To generate more randomness, the proper averaging method should be applied. Its choice depends on the value of a certain dimension witness. More importantly, the general analytical relationship between the maximal average guessing probability and dimension witness is given.