We numerically model the evolution of a pair of coherently split quasicondensates. A truly one-dimensional case is assumed, so that the loss of the (initially high) coherence between the two quasicondensates is due to dephasing only, but not due to the violation of integrability and subsequent thermalization (which are excluded from the present model). We confirm the subexponential time evolution of the coherence between two quasicondensates $\propto \exp [-(t/t_0){}^{2/3}]$, experimentally observed by Hofferberth et al. [Nature 449, 324 (2007)]. The characteristic time $t_0$ is found to scale as the square of the ratio of the linear density of a quasicondensate to its temperature, and we analyze the full distribution function of the interference contrast and the decay of the phase correlation.

We consider a semiconductor quantum well in a microcavity driven by coherent light and coupled to a squeezed vacuum reservoir. By systematically solving the pertinent quantum Langevin equations in the strong-coupling and low-excitation regimes, we study the effect of exciton-photon detuning, external coherent light, and the squeezed vacuum reservoir on vacuum Rabi splitting and on quantum statistical properties of the light emitted by the quantum well. We show that the exciton-photon detuning leads to a shift in polariton resonance frequencies and a decrease in fluorescence intensity. We also show that the fluorescent light exhibits quadrature squeezing, which predominately depends on the exciton-photon detuning and the degree of the squeezing of the input field.

We describe how electromagnetically induced transparency can arise in quadratically coupled optomechanical systems. Due to quadratic coupling, the underlying optical process involves a two-phonon process in an optomechanical system, and this two-phonon process makes the mean displacement, which plays the role of atomic coherence in traditional electromagnetically induced transparency (EIT), zero. We show how the fluctuation in displacement can play a role similar to atomic coherence and can lead to EIT-like effects in quadratically coupled optomechanical systems. We show how such effects can be studied using the existing optomechanical systems.

The observation of the so-called dispersion cancellation of paired photons is generally attributed to the presence of frequency entanglement between two frequency anticorrelated photons. In this paper, it is shown that by introducing the appropriate amount of chromatic dispersion or phase modulation between nonentangled photons, it is also possible to observe these effects. Indeed, it is found that the relevant characteristic required for the observation of dispersion cancellation or the cancellation of temporal phase modulation is the presence of certain frequency correlations between the photons and not necessarily the presence of entanglement.

We study the instability suppression of vector-necklace-ring soliton clusters carrying zero, integer, and fractional angular momentums in nonlocal nonlinear media with an arbitrary degree of nonlocality. We show that the combination of nonlocality and mutual trapping of soliton constituent components can completely stabilize the vector-necklace-ring soliton clusters which are otherwise only quasistable in local media. Our results may be useful to studies of the novel soliton states in Bose-Einstein with dipolar long-range interactions.

We study the excitation dynamics of an inhomogeneously broadened spin ensemble coupled to a single cavity mode. The collective excitations of the spin ensemble can be described in terms of generalized spin waves, and, in the absence of the cavity, the free evolution of the spin ensemble can be described as a drift in the wavenumber without dispersion. In this article we show that the dynamics in the presence of coupling to the cavity mode can be described solely by a modified time evolution of the wavenumbers. In particular, we show that collective excitations with a well-defined wavenumber pass without dispersion from negative to positive-valued wavenumbers without populating the zero wavenumber spin wave mode. The results are relevant for multimode collective quantum memories where qubits are encoded in different spin waves.

Detection and quantification of entanglement in quantum resources are two key steps in the implementation of various quantum-information processing tasks. Here, we show that Bell-type inequalities are not only useful in verifying the presence of entanglement but can also be used to bound the entanglement of the underlying physical system. Our main tool consists of a family of Clauser-Horne-like Bell inequalities that cannot be violated maximally by any finite-dimensional maximally entangled state. Using these inequalities, we demonstrate the explicit construction of both lower and upper bounds on the concurrence for two-qubit states. The fact that these bounds arise from Bell-type inequalities also allows them to be obtained in a semi-device-independent manner, that is, with assumption of the dimension of the Hilbert space but without resorting to any knowledge of the actual measurements being performed on the individual subsystems.

A scheme is proposed for the generation of a three-dimensional entangled state for two atoms trapped in a cavity via quantum Zeno dynamics. Because the scheme is based on the resonant interaction, the time required to produce entanglement is very short compared with the dispersive protocols. We show that the resulting effective dynamics allows for the creation of robust qutrit-qutrit entanglement. The influence of various decoherence processes such as spontaneous emission and photon loss on the fidelity of the entangled state is investigated. Numerical results show that the scheme is robust against the cavity decay since the evolution of the system is restricted to a subspace with null-excitation cavity fields. Furthermore, the present scheme has been generalized to realize $N$-dimensional entanglement for two atoms.

We present a general formalism to the problem of perfect state transfer (PST), where the state involves multiple excitations of the quantum network. A key feature of our formalism is that it allows for inclusion of nontrivial interactions between the excitations. Hence, it is perfectly suited to addressing the problem of PST in the context of various types of physical realizations. The general formalism is also flexible enough to account for situations where multiple excitations are “focused” onto the same site.

We investigate the entanglement properties of finite spin rings, with noncollinear Ising interaction between nearest neighbors. The orientations of the Ising axes are determined either by the spin position within the ring (model $A$) or by the direction of the bond (model $B$). In both cases, the considered spin Hamiltonians have a point-group symmetry instead of the translation invariance that characterizes spin rings with collinear Ising interaction. The ground state of these models exhibit remarkable entanglement properties, resembling Greenberger-Horne-Zeilinger (GHZ)-like states in the absence of an applied magnetic field (model $B$). Besides, the application of a homogeneous magnetic field allows to modify qualitatively the character of the ground-state entanglement, switching from multipartite to pairwise quantum correlations (both models $A$ and $B$).

A single-party strategy in a multiround quantum protocol can be implemented by sequential networks of quantum operations connected by internal memories. Here, we provide an efficient realization in terms of computational-space resources.

A new type of tubular inorganic metal-carbon sandwich molecule based on cyclic ${\mathrm{C}}_6$ is proposed. These consist of multiple layers of cyclic ${\mathrm{C}}_6$ with intercalated metal atoms. Structures and electronic properties of these molecules have been calculated using first-principles density functional techniques. We have evaluated all metals in the first six periods of the periodic table, except for the lanthanides, as possible components of such molecules. Our calculations show high bond energy and small energy gaps in many of these structures. We suggest that the high structural stability and high conductivity in these sandwich compounds make them very promising for use in nanoelectronic applications.

It is shown that the electromagnetic Casimir force acting on a conducting body (e.g., a realistic metallic piston) sliding in a background formed by cut silver nanorods (with the body perforated by the nanorods) is repulsive at distances larger than the separation of the nanorods, even if the host material of the nanorods is air. It is demonstrated that the physical origin of this effect is in essence related to Boyer’s prediction that magnetic and conducting walls repel each other. Indeed, we show that from the point of view of an observer inside the nanowire structure, the interface formed by severing the nanowires mimics accurately the behavior of a magnetic wall for $P$-polarized waves. In contrast to other piston configurations reported in the literature, the Casimir interaction in the nanowire background is an ultralong-range force that decays with the distance to the nearby interface as $1/a^2$.

Bose-Einstein condensation (BEC) of an ideal gas is investigated, beyond the thermodynamic limit, for a finite number $N$ of particles trapped in a generic three-dimensional power-law potential. We derive an analytical expression for the condensation temperature $T_c$ in terms of a power series in $x_0={\varepsilon }_0/k_B{T}_c$, where ${\varepsilon }_0$ denotes the zero-point energy of the trapping potential. This expression, which applies in Cartesian, cylindrical, and spherical power-law traps, is given analytically at infinite order. It is also given numerically for specific potential shapes as an expansion in powers of $x_0$ up to the second order. We show that, for a harmonic trap, the well-known first-order shift of the critical temperature $\Delta T_c/T_c\propto N^{-1/3}$ is inaccurate when $N⩽{10}^5$, the next order (proportional to $N^{-1/2}$) being significant. We also show that finite-size effects on the condensation temperature cancel out in a cubic trapping potential, e.g., $V(r)\propto r^3$. Finally, we show that in a generic power-law potential of higher order, e.g., $V(r)\propto r^{\alpha }$ with $\alpha >3$, the shift of the critical temperature becomes positive. This effect provides a large increase of $T_c$ for relatively small atom numbers. For instance, an increase of about $+$40% is expected with ${10}^4$ atoms in a $V(r)\propto r^{12}$ trapping potential.

We consider a laser-driven and dissipative system of two coupled cavities with Jaynes-Cummings nonlinearity. In particular, we investigate both incoherent and coherent laser driving, corresponding to different experimental situations. We employ Arnoldi time evolution as a numerical tool to solve exactly the many-body master equation describing the nonequilibrium quantum system. We evaluate the fluorescence spectrum and the spectrum of the second-order correlation function of the emitted light field. Finally, we relate the measured spectra of the dissipative quantum system to excitations of the corresponding nondissipative quantum system. Our results demonstrate how to interpret spectra obtained from dissipative quantum systems and specify what information is contained therein.

We examine the recently introduced measure of chirality of a monochromatic optical field [Y. Tang and A. E. Cohen, Phys. Rev. Lett. 104, 163901 (2010)] using the momentum (plane-wave) representation and helicity basis. Our analysis clarifies the physical meaning of the measure of chirality and unveils its close relation to the polarization helicity, spin angular momentum, energy density, and Poynting energy flow. We derive the operators of the optical chirality and of the corresponding chiral momentum, which acquire remarkably simple forms in the helicity representation.

We produce YbF molecules with a density of ${10}^{18}$ m${}^{-3}$ using laser ablation inside a cryogenically cooled cell filled with a helium buffer gas. Using absorption imaging and absorption spectroscopy we study the formation, diffusion, thermalization, and optical pumping of the molecules. The absorption images show an initial rapid expansion of molecules away from the ablation target followed by a much slower diffusion to the cell walls. We study how the time constant for diffusion depends on the helium density and temperature and obtain values for the YbF-He diffusion cross section at two different temperatures. We measure the translational and rotational temperatures of the molecules as a function of time since formation, obtain the characteristic time constant for the molecules to thermalize with the cell walls, and elucidate the process responsible for limiting this thermalization rate. Finally, we make a detailed study of how the absorption of the probe laser saturates as its intensity increases, showing that the saturation intensity is proportional to the helium density. We use this to estimate collision rates and the density of molecules in the cell.

We study the quantum anomalous Hall states in the $p$-orbital bands of the honeycomb optical lattices loaded with single-component fermions. Such an effect has not yet been realized in both condensed-matter and cold-atom systems. By applying the available experimental techniques to rotate each lattice site around its own center, the band structures become topologically nontrivial. At a certain rotation angular velocity $\Omega$, a flat band structure appears with localized eigenstates carrying chiral current moments. By imposing the soft confining potential, the density profile exhibits a wedding-cake-shaped distribution with insulating plateaus at commensurate fillings. Moreover, the inhomogeneous confining potential induces dissipationless circulation currents, the magnitudes and chiralities of which vary with the distance from the trap center. In the insulating regions, the Hall conductances are quantized, and in the metallic regions, the directions and magnitudes of chiral currents can not be described by the usual local-density approximation. The quantum anomalous Hall effects are robust at temperature scales that are small compared to band gaps, which increase the feasibility of experimental realizations.

We propose a scheme to generate high-dimensional self-trapped laser beams at a very low light intensity via atomic coherence. The system we consider is a resonant four-level atomic ensemble, working in an active Raman gain regime and at room temperature. We derive a high-dimensional nonlinear envelope equation for a signal field with a specific saturable nonlinearity. We show that because of the quantum interference effect induced by a control field, the imaginary part of the coefficients of the signal-field envelope equation can be much smaller than their real part. We demonstrate that the system supports gain-assisted, stable, high-dimensional spatial optical solitons and long-lifetime vortices, which can be produced with light power at the microwatt level.

We address surface solitons at the interface between linear media and nonlocal nonlinear media in the presence of a discontinuity in refractive index at the surface of these two materials. We investigated the influence of the degree of nonlocality on the stability, energy flow, and full width at half-maximum of the surface wave solitons. It is shown that surface solitons will be stable only if the degree of nonlocality exceeds a critical value. We find that the refractive index difference can affect the power distribution of the surface solitons in the two media. Also, different boundary values at the interface can lead to different relative peak positions of the surface solitons. However, neither the refractive index nor the boundary conditions can affect the stability of the solitons, for a given degree of nonlocality.