We study the stability and dynamics of an ultracold bosonic gas trapped in a toroidal geometry and driven by rotation in the absence of dissipation. We first delineate, via the Bogoliubov mode expansion, the regions of stability and the nature of instabilities of the system for both repulsive and attractive interaction strengths. To study the response of the system to variations in the rotation rate, we introduce a “disorder” potential, breaking the rotational symmetry. We demonstrate the breakdown of adiabaticity as the rotation rate is slowly varied and find forced tunneling between the system's eigenstates. The nonadiabaticity is signaled by the appearance of a swallow-tail loop in the lowest-energy level, a general sign of hysteresis. Then, we show that this system is in one-to-one correspondence with a trapped gas in a double-well potential and thus exhibits macroscopic quantum self-trapping. Finally, we show that self-trapping is a direct manifestation of the behavior of the lowest-energy level.

The shifts in the four-body recombination peaks, due to an effective range correction to the zero-range model close to the unitary limit, are obtained and used to extract the corresponding effective range of a given atomic system. The approach is applied to an ultracold gas of cesium atoms close to broad Feshbach resonances, where deviations of experimental values from universal model predictions are associated with effective range corrections. The effective range correction is extracted with a weighted average given by $3.9\pm 0.8R_{\mathrm{vdW}}$, where $R_{\mathrm{vdW}}$ is the van der Waals length scale, which is consistent with the van der Waals potential tail for the Cs${}_2$ system. The method can be generally applied to other cold atom experimental setups to determine the contribution of the effective range to the tetramer dissociation position.

We construct dark solitons in the recently introduced model of the nonlinear dual-core coupler with the mutually balanced gain and loss applied to the two cores, which is a realization of parity-time symmetry in nonlinear optics. The main issue is stability of the dark solitons. The modulational stability of the continuous-wave background, which supports the dark solitons, is studied analytically, and the full stability is investigated in a numerical form via computation of eigenvalues for modes of small perturbations. Stability regions are thus identified in the parameter space of the system and verified in direct simulations. Collisions between stable dark solitons are briefly considered too.

It is shown that the spatial dispersion in composite systems formed by inclusion of finite size spheres may lead to negative group velocity and, subsequently, to negative refraction. Longitudinal and transverse electromagnetic modes exhibiting negative dispersion are found in the system of finite size charged clouds. It is also shown that similar mechanism due to the electric quadrupole coupling leads to excitation of longitudinal and transverse eigenmodes with negative dispersion in a random set of nonmagnetic metal spheres embedded in a dielectric host. It is shown that the negative refraction related to the spatial dispersion effects may alternatively be viewed as a result of simultaneously negative permittivity and permeability.

We present two-photon spectroscopy of a thermal rubidium vapor confined to the hollow core of a photonic-crystal fiber. Linewidths as narrow as 10 MHz were observed on the $5S_{1/2}\to 5D_{5/2}$ transition enabling the hyperfine splitting of the excited state to be resolved. Very strong nonlinear absorption ($>90\%$) was observed, with substantial absorption maintained over large detunings (9 GHz) from an intermediate state. These attributes make this system ideal for many frequency metrology and quantum optics applications.

Four-wave mixing of supercontinuum field wave forms with an ultrashort reference pulse is shown to retrieve the coherent components from multiple supercontinua and combine them into a beam whose spatial coherence is much higher than the spatial coherence of supercontinua in multiple filaments.

We study super–Bloch oscillations of ultracold atoms in a shaken lattice potential, subjected to a harmonically modulated mean-field interaction. Usually, any interaction leads to the decay of the wave packet and its super–Bloch oscillation. Here, we use the phases of interaction and shaking with respect to the free Bloch oscillation as control parameters. We find two types of long-living cases: (i) suppression of the immediate broadening of the wave packet, and (ii) dynamical stability of all degrees of freedom. The latter relies on the rather robust symmetry argument of cyclic time [Gaul et al., Phys. Rev. A 84, 053627 (2011)].

We experimentally determine the Gaussian quantum discord present in two-mode squeezed vacuum generated by a four-wave mixing process in hot rubidium vapor. The frequency spectra of the discord as well as the quantum and classical mutual information are also measured. In addition, the effects of symmetric attenuation introduced into both modes of the squeezed vacuum on the Gaussian quantum discord, and the quantum mutual information and the classical correlations are examined experimentally. Finally, we show that due to the multi-spatial-mode nature of the four-wave mixing process, the Gaussian quantum discord may exhibit sub- or superadditivity depending on which spatial channels are selected.

Loophole-free Bell tests for quantum nonlocality and long-distance secure communication require photodetection efficiencies beyond a threshold ${\eta }_{\mathrm{crit}}$ that depends on the Bell inequality and the noise affecting the entangled state received by the distant parties. Most calculations of ${\eta }_{\mathrm{crit}}$ assume that the noise is random and can be modeled as white noise. However, most sources suffer from colored noise. Indeed, since entangled states are usually created as a superposition of two possible deexcitation paths, a partial distinguishability between the two processes leads to the appearance of colored noise in the generated state. Recently, there was a proposal for a loophole-free Bell test [A. Cabello and F. Sciarrino, Phys. Rev. X 2, 021010 (2012)], where a specific colored noise appears as a consequence of the precertification of the photon's presence through single-photon spontaneous parametric down-conversion. Here we obtain ${\eta }_{\mathrm{crit}}$, the optimal quantum states, and the local settings for a loophole-free Bell test as a function of the amount of colored noise. We consider three bipartite Bell inequalities with $n$ dichotomic settings: Clauser-Horne-Shimony-Holt ($n=2$), $I_{3322}$ ($n=3$), and $A_5$ ($n=4$), both for the case of symmetric efficiencies, corresponding to photon-photon Bell tests, and for the totally asymmetric case, corresponding to atom-photon Bell tests. Remarkably, in all these cases, ${\eta }_{\mathrm{crit}}$ is robust against the colored noise. The present analysis can find application in any test of Bell inequalities in which the dominant noise is of the colored type.

We systematically study the non-Hermitian version of the one-dimensional anisotropic $XY$ model, which in its original form is a unique, exactly solvable, quantum spin model for understanding the quantum phase transition. The distinguishing features of this model are that it has a full real spectrum if all of the eigenvectors are intrinsic rotation-time-reversal symmetric rather than parity-time-reversal symmetric, and that its Hermitian counterpart is shown approximately to be an experimentally accessible system, i.e., an isotropic $XY$ spin chain with nearest-neighbor coupling. Based on the exact solution, exceptional points which separate the unbroken- and broken-symmetry regions are obtained and lie on a hyperbola in the thermodynamic limit. This provides a nice paradigm to elucidate the complex quantum-mechanics theory for a quantum spin system.

Sensors that harness exclusively quantum phenomena (such as entanglement) can achieve superior performance compared to those employing only classical principles. Recently, a technique based on postselected, weakly performed measurements has emerged as a method of overcoming technical noise in the detection and estimation of small interaction parameters, particularly in optical systems. The question of which other types of noise may be combated remains open. We here analyze whether the effect can overcome decoherence in a typical field-sensing scenario. Benchmarking a weak, postselected measurement strategy against a strong, direct strategy, we conclude that no advantage is achievable, and that even a small amount of decoherence proves catastrophic to the weak-value amplification technique.

We present experimental results on the preparation of a desired quantum state in a two-level system with the maximum possible fidelity using driving protocols ranging from generalizations of the linear Landau-Zener protocol to transitionless driving protocols that ensure perfect following of the instantaneous adiabatic ground state. We also study the minimum time needed to achieve a target fidelity and explore and compare the robustness of some of the protocols against parameter variations simulating a possible experimental uncertainty. In our experiments, we realize a two-level model system using Bose-Einstein condensates inside optical lattices, but the results of our investigation should hold for any quantum system that can be approximated by a two-level system.

We investigate the thermal entanglement between two end sites in spin chains and on fractal lattices by taking the negativity as a measure and using the decimation renormalization-group method. The effects of the temperature $T$, the anisotropy parameter $\Delta$, and the size of the system, $L$, on the entanglement are examined in detail. It is found that the entanglement decreases monotonically with increasing $T$ and vanishes beyond a critical value $T_c$. Our results also show that with increasing $\Delta$ from $-\infty$ to zero the entanglement first increases to the maximum and then decreases sharply to zero. Different from the cases of spin chains and Koch curves, the entanglement on the diamond-type hierarchical (DH) lattices presents some interesting behaviors. As the sizes of the DH lattices, $L$, become large, the entanglement is rather robust and there exists sizable entanglement between long-distance end sites. This result indicates that different fractal structures can result in various entanglement properties.

We show that only those composite quantum systems possessing nonvanishing quantum correlations have the property that any nontrivial local unitary evolution changes their global state. We derive the exact relation between the global state change induced by local unitary evolutions and the amount of quantum correlations. We prove that the minimal change coincides with the geometric measure of discord (defined via the Hilbert-Schmidt norm), thus providing the latter with an operational interpretation in terms of the capability of a local unitary dynamics to modify a global state. We establish that two-qubit Werner states are maximally quantum correlated, and are thus the ones that maximize this type of global quantum effect. Finally, we show that similar results hold when replacing the Hilbert-Schmidt norm with the trace norm.

A scaling for the total ionization cross sections of atomic and molecular targets by neutral to highly charged projectiles with energies ranging from a few keV/u to many MeV/u is presented, with few free parameters, all velocity independent, and whose values do not vary appreciably for the projectiles and atomic and molecular targets considered here. Another important feature of the proposed scaling is that it has been built considering the characteristics of the collision systems, taking into account the various physical processes that lead to target ionization and the velocity regions where each of them dominates, and performing a smooth transition between them. This scaling describes quite well the existent experimental data covering a broad range of collision systems as diverse as H${}^{+}$ on H, Xe${}^{30+}$ on He, or O${}^{6+}$ on uracil.

We study scattering of three equal-mass particles in one dimension. Integrable interactions are synonymous with nondiffractive scattering, meaning that the set of incoming momenta for any scattering event coincides with the set of outgoing momenta. A system is integrable if the two-particle scattering matrix obeys the Yang-Baxter equation. Nonintegrable interactions correspond to diffractive scattering, where the set of outgoing momenta may take on all values consistent with energy and momentum conservation. Such processes play a vital role in the kinetics of one-dimensional gases, where binary collisions are unable to alter the distribution function. When integrability is broken weakly, the result is a small diffractive scattering amplitude. Our main result is a simple formula for the diffractive part of the scattering amplitude, when the violation of the Yang-Baxter equation is small. Although the derivation is given for $\delta$-function interactions, the result depends only on the two-particle scattering matrix, and should therefore also apply to finite-range interactions close to integrable.

In this paper, we use steady-state measurements to obtain evidence of radiation trapping in an optically thick cloud of cold rubidium atoms. We investigate the fluorescence properties of our sample, pumped on opened transitions. This fluorescence exhibits a nontrivial dependence on the optical thickness of the media. A simplified model, based on rate equations, self-consistently coupled to a diffusive model of light transport, is used to explain the experimental observations in terms of incoherent radiation trapping on one spectral line. Measurements of the atomic populations and the fluorescence spectrum qualitatively agree with this interpretation.

Under negative feedback, the quality factor $Q$ of a radio-frequency atomic magnetometer can be decreased by more than two orders of magnitude, so that any initial perturbation of the polarized spin system can be rapidly damped, preparing the magnetometer for detection of the desired signal. We find that noise is also suppressed under such spin damping, with a characteristic spectral response corresponding to the type of noise; therefore magnetic, photon shot, and spin-projection noise can be measured distinctly. While the suppression of resonant photon shot noise implies the closed-loop production of polarization-squeezed light, the suppression of resonant spin-projection noise does not imply spin squeezing, rather simply the broadening of the noise spectrum with $Q$. Furthermore, the application of spin damping during phase-sensitive detection suppresses both signal and noise in such a way as to increase the sensitivity bandwidth. We demonstrate a threefold increase in the magnetometer's bandwidth while maintaining 0.3 fT/$\sqrt{\mathrm{Hz}}$ sensitivity.

Asymmetric photocurrent produced by ionizing laser pulses in a gas is known to be a highly efficient source of terahertz radiation. We examine the possibility of exploiting the asymmetry of the medium itself, rather than the properties of the laser field acting on it, to facilitate the generation of directional photocurrents. We show that the magnitude of directional current and the efficiency of its excitation in tunneling ionization of asymmetric molecules can be significantly enhanced compared to the case of symmetric systems. The results obtained both in a simple classical model and in quantum-mechanical numerical simulations favor the subcycle asymmetry of the ionization process in combination with the effect of the Coulomb potential on the escaping electron as a mechanism responsible for a high-efficiency generation of residual current in tunneling ionization of oriented asymmetric molecules.

An extreme ultraviolet (EUV) single attosecond pulse passing through a laser-dressed dense gas is studied theoretically. The weak EUV pulse pumps the helium gas from the ground state to the $2s2p\left({}^{1}P\right)$ autoionizing state, which is coupled to the $2s^2{(}^{1}S)$ autoionizing state by a femtosecond infrared laser with the intensity in the order of 10${}^{12}$ W/cm${}^2$. The simulation shows how the transient absorption and emission of the EUV are modified by the coupling laser. A simple analytical expression for the atomic response derived for $\delta$-function pulses reveals the strong modification of the Fano lineshape in the spectra, where these features are quite universal and remain valid for realistic pulse conditions. We further account for the propagation of pulses in the medium and show that the EUV signal at the atomic resonance can be enhanced in the gaseous medium by more than 50$\%$ for specifically adjusted laser parameters, and that this enhancement persists as the EUV propagates in the gaseous medium. Our result demonstrates the high-level control of nonlinear optical effects that are achievable with attosecond pulses.