We study the family of static and moving dark solitons in quasi-one-dimensional dipolar Bose-Einstein condensates, exploring their modified form and interactions. The density dip of the soliton acts as a giant antidipole which adds a nonlocal contribution to the conventional local soliton-soliton interaction. We map out the stability diagram as a function of the strength and polarization direction of the atomic dipoles, identifying both roton and phonon instabilities. Away from these instabilities, the solitons collide elastically. Varying the polarization direction relative to the condensate axis enables tuning of this nonlocal interaction between repulsive and attractive; the latter case supports unusual dark-soliton bound states. Remarkably, these bound states are themselves shown to behave like solitons, emerging unscathed from collisions with each other.

We investigate features in the single-particle spectral function beyond the Hubbard bands in the strongly correlated normal phase of the Bose-Hubbard model. There are two distinct classes of additional peaks generated by the bosonic statistics. The first type is thermally activated Hubbard “sidebands”, with the same physical origin as the zero-temperature Hubbard bands, but generated by excitations from thermally activated local occupation number states. The second class are two-particle fluctuation resonances driven by the lattice dynamics. In the unity filling Mott insulator, this takes the form of a localized triplon combined with a dispersing holon. Both types of resonances also manifest themselves in the structure factor and the interaction modulation spectra obtained from nonequilibrium bosonic dynamical mean-field theory calculations. Our findings explain experimental lattice modulation and Bragg spectroscopy results, and they predict a strong temperature dependence of the first sideband, thereby opening the door to precise thermometry of strongly correlated lattice bosons.

We investigate the ground-state and magnetic properties of a dipolar spin-1 Bose-Einstein condensate trapped in a symmetric double-well potential. In particular, we focus on the spin-vortex states by assuming that each potential well is highly pancake shaped. We show that the presence of the double-well potential gives rise to two different spin configurations for the spin-vortex pair states. We also study the response of the coupled spin-vortex pair to static transverse magnetic fields.

We study the influence of photons on the dynamics and the ground state of the atoms in a bosonic Josephson junction inside an optical resonator. The system is engineered in such a way that the atomic tunneling can be tuned by changing the number of photons in the cavity. In this setup the cavity photons are a means of control, which can be utilized both in inducing self-trapping solutions and in driving the crossover of the ground state from an atomic coherent state to a Schrödinger cat state. This is achieved, for suitable setup configurations, with interatomic interactions weaker than those required in the absence of a cavity. This is corroborated by the study of the entanglement entropy. In the presence of a laser, this quantum indicator attains its maximum value (which marks the formation of the catlike state and, at a semiclassical level, the onset of self-trapping) for attractions smaller than those of the bare junction.

Very sharp, low-energy structures observed in photoelectron spectra reveal the population of autoionizing states in krypton and argon in strong laser fields over a large range of different wavelengths. The energies of the electrons, emitted by autoionization in a field-free environment, provide direct information about the spectrum of states involved. Despite their ability to resist ionization by the populating laser pulse, we demonstrate the possibility to promote the excited electrons into the continuum by subsequent absorption of a single photon. Thus, applying a classical pump-probe scheme, we are able to manipulate the autoionization contribution on a picosecond time scale. Different scenarios for the creation of autoionizing states in strong laser fields are discussed.

The “synthetic dimension” proposal [A. Celi et al., Phys. Rev. Lett. 112, 043001 (2014)] uses atoms with $M$ internal states (“flavors”) in a one-dimensional (1D) optical lattice, to realize a hopping Hamiltonian equivalent to the Hofstadter model (tight-binding model with a given magnetic flux per plaquette) on an $M$-sites-wide square lattice strip. We investigate the physics of $\text{SU}\left(M\right)$ symmetric interactions in the synthetic dimension system. We show that this system is equivalent to particles [with $\text{SU}\left(M\right)$ symmetric interactions] experiencing an $\text{SU}\left(M\right)$ Zeeman field at each lattice site and a non-Abelian $\text{SU}\left(M\right)$ gauge potential that affects their hopping. This equivalence brings out the possibility of generating nonlocal interactions between particles at different sites of the optical lattice. In addition, the gauge field induces a flavor-orbital coupling, which mitigates the “baryon breaking” effect of the Zeeman field. For $M$ particles, concomitantly, the $\text{SU}\left(M\right)$ singlet baryon which is site localized in the usual 1D optical lattice, is deformed to a nonlocal object (“squished baryon”). We conclusively demonstrate this effect by analytical arguments and exact (numerical) diagonalization studies. Our study promises a rich many-body phase diagram for this system. It also uncovers the possibility of using the synthetic dimension system to laboratory realize condensed-matter models such as the $\text{SU}\left(M\right)$ random flux model, inconceivable in conventional experimental systems.

We investigate the non-Markovianity of continuous-variable Gaussian quantum channels through the evolution of an operational metrological quantifier, namely, the Gaussian interferometric power, which captures the minimal precision that can be achieved using bipartite Gaussian probes in a black-box phase estimation setup, where the phase shift generator is a priori unknown. We observe that the monotonicity of the Gaussian interferometric power under the action of local Gaussian quantum channels on the ancillary arm of the bipartite probes is a natural indicator of Markovian dynamics; consequently, its breakdown for specific maps can be used to construct a witness and an effective quantifier of non-Markovianity. In our work, we consider two paradigmatic Gaussian models, the damping master equation and the quantum Brownian motion, and identify analytically and numerically the parameter regimes that give rise to non-Markovian dynamics. We then quantify the degree of non-Markovianity of the channels in terms of Gaussian interferometric power, showing, in particular, that even nonentangled probes can be useful to witness non-Markovianity. This establishes an interesting link between the dynamics of bipartite continuous-variable open systems and their potential for optical interferometry. The results are an important supplement to the recent research on characterization of non-Markovianity in continuous-variable systems.

Weak measurements are a subset of measurement processes in quantum mechanics wherein the system, which is being measured, interacts very weakly with the measuring apparatus. Measurement values of observables undergoing a weak interaction and their amplification are concepts that have sharpened our understanding of interaction processes in quantum mechanics. Recent experiments show that naturally occurring processes such as resonance fluorescence from excited states of an atom can exhibit weak value amplification effect. In this paper we theoretically analyze the process of elastic resonance fluorescence from a $\mathsf{V}$-type three-level atomic system, using the well-known Weiskopff-Wigner (WW) theory of spontaneous emission. Within this theory we show that a weak interaction regime can be identified and for suitable choices of initial and final excited states the mean scattering time between these states show an amplification effect during interaction with the vacuum bath modes of the electromagnetic field. We thus show that a system-bath interaction can show weak value amplification. Using our theory we reproduce the published experimental results carried out in such a system. More importantly, our theory can calculate scattering time scales in elastic resonance scattering between multiple excited states of a single atom or between common excited state configurations of interacting multiatom systems.

The appearance of so-called exceptional points in the complex spectra of non-Hermitian systems is often associated with phenomena that contradict our physical intuition. One example of particular interest is the state-exchange process predicted for an adiabatic encircling of an exceptional point. In this work we analyze this and related processes for the generic system of two coupled oscillator modes with loss or gain. We identify a characteristic system evolution consisting of periods of quasistationarity interrupted by abrupt nonadiabatic transitions and we present a qualitative and quantitative description of this switching behavior by connecting the problem to the phenomenon of stability loss delay. This approach makes accurate predictions for the breakdown of the adiabatic theorem as well as the occurrence of chiral behavior observed previously in this context and provides a general framework to model and understand quasiadiabatic dynamical effects in non-Hermitian systems.

We present a general strategy to derive entanglement criteria through a dichotomization procedure which consists of performing a mapping from qudits to qubits that preserves the separability of the parties and the SU(2) rotational invariance. Consequently, it is possible to apply the well-known positive partial transpose criterion to reveal the existence of quantum correlations between qudits. We discuss some examples of entangled states that are detected using the proposed strategy. Finally, we demonstrate, using our scheme, how some variance-based entanglement witnesses can be generalized from qubits to higher dimensional spin systems.

We present a theoretical proposal to couple a single nitrogen-vacancy (NV) center to a superconducting flux qubit in the regime where both systems are off resonance. The coupling between both quantum devices is achieved through the strong driving of the flux qubit by a classical microwave field that creates dressed states with an experimentally controlled characteristic frequency. We discuss several applications such as controlling the NV center's state by manipulation of the flux qubit, performing the NV center full tomography and using the NV center as a quantum memory. The effect of decoherence and its consequences to the proposed applications are also analyzed. Our results provide a theoretical framework describing a promising hybrid system for quantum information processing, which combines the advantages of fast manipulation and long coherence times.

Quantum walk (QW) provides a versatile tool to study fundamental physics and also to make a variety of practical applications. We here start with the recent idea of nonlinear QW and show that introducing nonlinearity to QW can lead to a wealth of remarkable possibilities, e.g., simulating nonlinear quantum dynamics, thus enhancing the applicability of QW above the existing level for a universal quantum simulator. As an illustration, we show that the dynamics of a nonlinear Dirac particle can be simulated on an optical nonlinear QW platform implemented with a measurement-based-feedforward scheme. The nonlinear evolution induced by the feed-forward introduces a self-coupling mechanism to (otherwise linear) Dirac particles, which accordingly behave as a soliton. We particularly consider two kinds of nonlinear Dirac equations, one with a scalar-type self-coupling (Gross-Neveu model) and the other with a vector-type one (Thirring model), respectively. Using their known stationary solutions, we confirm that our nonlinear QW framework is capable of exhibiting characteristic features of a soliton. Furthermore, we show that the nonlinear QW enables us to observe and control an enhancement and suppression of the ballistic diffusion.

We introduce an aspect of nonlocality which arises when the task of quantum states distinguishability is considered under local operations and shared entanglement in the absence of classical communication. We find the optimal amount of entanglement required to accomplish the task perfectly for sets of orthogonal states and argue that it quantifies information nonlocality.

It has been proven by Rana et al. that the $n$-qubit generic Dicke states $\left|\mathrm{GD}\right(n,\ell )\rangle$ are uniquely determined, among arbitrary states, by their $(\ell +1)$-partite reduced density matrices, and $\left(\begin{array}{c}n-1\\ \ell \end{array}\right)$ number of them which have one party common to all are sufficient. We show that among arbitrary states, Dicke-type states are also uniquely determined by their $(\ell +1)$-partite reduced density matrices. Here, by Dicke-type states, we mean all multiqubit pure states that are stochastic local operations and classical communication equivalent to the $n$-qubit standard Dicke state $\left|D\right(n,\ell )\rangle$. Moreover, we find two kinds of subsets of the $(\ell +1)$-partite reduced density matrices set, both of which can determine the Dicke-type states. More importantly, the number of elements in these two subsets is much smaller than $\left(\begin{array}{c}n-1\\ \ell \end{array}\right)$.

We present a theoretical study on the recombination of a free electron into the ground state of a hydrogenlike ion in the presence of an external laser field. Emphasis is placed on the effects caused by the laser dressing of the residual ionic bound state. To investigate how this dressing affects the total and angle-differential cross section of laser-assisted radiative recombination (LARR) we apply first-order perturbation theory and the separable Coulomb-Volkov continuum ansatz. Using this approach, detailed calculations are performed for low-$Z$ hydrogenlike ions and laser intensities in the range from $I_L={10}^{12}$ to ${10}^{13}\text{W}/{\mathrm{cm}}^2$. It is seen that the total cross section as a function of the laser intensity is remarkably affected by the bound-state dressing. Moreover, the laser dressing becomes manifest as asymmetries in the angular distribution and the (energy) spectrum of the emitted recombination photons.

Quantum degenerate gases trapped in optical lattices are ideal test beds for fundamental physics because these systems are tunable, well characterized, and isolated from the environment. Controlled disorder can be introduced to explore suppression of quantum diffusion in the absence of conventional dephasing mechanisms such as phonons, which are unavoidable in experiments on electronic solids. Recent experiments use transport of degenerate Fermi gases in optical lattices [S. S. Kondov et al., Phys. Rev. Lett. 114, 083002 (2015)] to probe extreme regimes. These experiments find evidence for an intriguing insulating phase where quantum diffusion is completely suppressed by strong disorder. Quantitative interpretation of these experiments remains an open problem that requires inclusion of nonzero entropy, strong interaction, and trapping in an Anderson-Hubbard model. We argue that the suppression of transport can be thought of as localization of Hubbard-band quasiparticles. We construct a theory of dynamics of Hubbard-band quasiparticles tailored to trapped optical lattice experiments. We compare the theory directly with center-of-mass transport experiments of Kondov et al. with no fitting parameters. The close agreement between theory and experiments shows that the suppression of transport is only partly due to finite-entropy effects. We argue that the complete suppression of transport is consistent with short-time, finite-size precursors of Anderson localization of Hubbard-band quasiparticles. The combination of our theoretical framework and optical lattice experiments offers an important platform for studying localization in isolated many-body quantum systems.

We numerically investigate the expansion of clouds of hard-core bosons in the two-dimensional square lattice using a matrix-product-state–based method. This nonequilibrium setup is induced by quenching the trapping potential to zero and our work is specifically motivated by a recent experiment with interacting bosons in an optical lattice [Ronzheimer et al., Phys. Rev. Lett. 110, 205301 (2013)]. As the anisotropy of the amplitudes $J_x$ and $J_y$ for hopping in different spatial directions is varied from the one- to the two-dimensional case, we observe a crossover from a fast ballistic expansion in the one-dimensional limit $J_x\gg J_y$ to much slower dynamics in the isotropic two-dimensional limit $J_x=J_y$. We further study the dynamics on multileg ladders and long cylinders. For these geometries we compare the expansion of a cloud to the melting of a domain wall, which helps us to identify several different regimes of the expansion as a function of time. By studying the dependence of expansion velocities on both the anisotropy $J_y/J_x$ and the number of legs, we observe that the expansion on two-leg ladders, while similar to the two-dimensional case, is slower than on wider ladders. We provide a qualitative explanation for this observation based on an analysis of the rung spectrum.

We report a quantitative, analytical, and numerical comparison between two models of the interaction of a nonrelativistic quantum particle with a thin time-dependent absorbing barrier. The first model represents the barrier by a set of time-dependent discontinuous matching conditions, which are closely related to Kottler boundary conditions used in stationary-wave optics as a mathematical basis for Kirchhoff diffraction theory. The second model mimics the absorbing barrier with an off-diagonal $\delta$ potential with a time-dependent amplitude. We show that the two models of absorption agree in their predictions in a semiclassical regime, the regime readily accessible in modern experiments with ultracold atoms.

The ground-state phase diagram of attractively interacting Fermi gases in two dimensions with a population imbalance is investigated. We find the regime of stability for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, in which pairing occurs at a finite wave vector, and determine the magnitude of the pairing amplitude $\mathrm{\Delta }$ and FFLO wave vector $q$ in the ordered phase, finding that $\mathrm{\Delta }$ can be of the order of the two-body binding energy. Our results rely on a careful analysis of the zero-temperature gap equation for the FFLO state, which possesses nonanalyticities as a function of $\mathrm{\Delta }$ and $q$, invalidating a Ginzburg-Landau expansion in small $\mathrm{\Delta }$.