We study the dynamics of two-component atomic Bose gases initially in a mixture encountering a sudden quench of the interspecies interactions. The dynamics above the critical temperature $T_c$ is studied using a leading-order large-$N$ approximation that predicted a phase transition from mixing to phase separation as a function of the interspecies coupling. Here, we explore the dynamics of this phase transition following a quench and compare our results to those found at zero temperature using the time-dependent Gross-Pitaevskii equations which ignore quantum and thermal fluctuations. In the regime above $T_c$ where no condensate is present, however, the time evolution of the densities following the quench exhibits features similar to those found at zero temperature where only the condensates contribute to the densities. When the interspecies interaction jumps above the critical value, we observe dynamical transitions from a homogeneous mixture to a phase-separated structure for both cases. Our simulations suggest that at temperatures above $T_c$ where no condensate is present this dynamical transition should still be observable in experiments.

We show that the observable rate of tunneling ionization of a molecule in an intense low-frequency laser field is affected by nuclear motion and can essentially differ from a bare electronic characteristic calculated for fixed nuclei. Both the absolute value of the rate and the shape of its orientation dependence are affected. The effect is significant for $I\sim {10}^{14}$ W/cm${}^2$ and becomes more pronounced at lower intensities. An isotope effect in tunneling ionization of H${}_2$ and D${}_2$ is predicted. The results are compared with available experiments.

We study theoretically light focusing at subwavelength scale inside a disordered strongly scattering open medium. We show that broadband time reversal at a single point antenna, in conjunction with near-field interactions and multiple scattering, produces spatial focusing with a quality comparable to that obtained in an ideal closed cavity. This provides different perspectives for super-resolved optical imaging and coherent control of single nanosources or absorbers in complex media.

Evolution of the $z$ component of a single spin in a finite cyclic $XY$ spin-$1/2$ chain is studied. Initially one selected spin is polarized while other spins are completely unpolarized and uncorrelated. Polarization of the selected spin as a function of time is proportional to the autocorrelation function $g_0^{zz}\left(t\right)$ at infinite temperature. Initialization of the selected spin gives rise to two wave packets moving in opposite directions and winding over the circle. We express $g_0^{zz}\left(t\right)$ as a series in winding number and derive tractable approximations for each term. This allows us to give qualitative explanations and quantitative descriptions of various finite-size effects such as partial revivals and transition from regular to erratic behavior.

Few studies explored the dynamics of nonclassical correlations besides entanglement in open multipartite quantum systems. Here, we address the time evolution of quantum discord and entanglement in a model of three noninteracting qubits subject to a classical random telegraph noise in common and separated environments. Two initial entangled states of the system are examined, namely the GHZ- and W-type states. The dynamics of quantum correlations results to be strongly affected by the input configuration of the qubits, the type of the system-environment interaction, and the memory properties of the environmental noise. When the qubits are nonlocally coupled to the random telegraph noise, the GHZ-type states partially preserve, at long times, both discord and entanglement, regardless of the correlation time of the environmental noise. The survived entangled states turn out to be also detectable by means of suitable entanglement witnesses. On the other hand, in the same conditions, the decohering effects suppress all the quantum correlation of the W-type states which are thus less robust than the GHZ-type ones. The long-time survival of tripartite discord and entanglement opens interesting perspectives in the use of multipartite entangled states for practical applications in quantum information science.

We present a statistical analysis on the performance of a protocol for the faithful transfer of a quantum state in finite qubit or spin chains, in the presence of diagonal and off-diagonal disorder. It is shown that the average-state fidelity, typically employed in the literature for the quantification of the transfer, may overestimate considerably the performance of the protocol in a single realization, leading to faulty conclusions about the success of the transfer.

We compare the effect of different noise scenarios on the achievable rate of an ${\varepsilon }_{}$-secure key for the BB84 and the six-state protocol. We study the situation where quantum noise is added deliberately, and investigate the remarkable benefit for the finite key rate. We compare our results to the known case of added classical noise and the asymptotic key rate, i.e., in the limit of infinitely many signals. As a complementary interpretation, we show that under the realistic assumption that the noise which is unavoidably introduced by a real channel is not fully dedicated to the eavesdropper, the secret key rate increases significantly.

Efficient quantum-state transfer is achieved in a uniformly coupled spin-$1/2$ chain, with open boundaries, by application of local magnetic fields on the second and last-but-one spins, respectively. These effective barriers induce the appearance of two eigenstates, bilocalized at the edges of the chain, which allow a high-quality transfer also at relatively long distances. The same mechanism may be used to send an entire e-bit (e.g., an entangled qubit pair) from one to the other end of the chain.

In this paper, taking into account the effect of the induced interaction, we calculate the superfluid-transition temperature of ultracold Fermi gases within the framework of the non-self-consistent $T$-matrix approximation. The result shows that the transition temperature is reduced by about $10\%$ at unitarity due to the induced interaction. In the Bardeen-Cooper-Schrieffer (BCS) limit, the critical temperature is about $0.45T_c^{\text{BCS}}$ and it is the same as the result of the usual Gorkov and Melik-Barkhudarov correction, whereas in the Bose-Einstein condensate (BEC) limit, the influence of the induced interaction can be neglected and the critical temperature turns out to be the transition temperature of ideal Bose gases. Our results show that the induced interaction cannot be neglected in the study of the BCS-BEC crossover.

We derive analytically the phase diagram of a two-component Bose gas confined in an anharmonic potential, which becomes exact and universal in the limit of weak interactions and small anharmonicity of the trapping potential. The transitions between the different phases, which consist of vortex states of single and multiple quantization, are all continuous because of the addition of the second component.

We study a spinor fermionic system under the effect of a spin-exchange interaction. We focus on the interplay between the spin-exchange interaction and the effective quadratic Zeeman shift. We examine the static and the dynamic properties of both a two- and many-body system. We find that the spin-exchange interaction induces coherent Rabi oscillation in the two-body system, but the oscillation is quickly damped when the system is extended to the many-body case.

We apply time-dependent Gutzwiller mean-field theory to provide a qualitative understanding for bosons in optical lattices that approach states corresponding to negative absolute temperatures. We perform the dynamical simulations to relate to the recent experiments by Braun et al. [S. Braun, J. P. Ronzheimer, M. Schreiber, S. S. Hodgman, T. Rom, I. Bloch, and U. Schneider, Science 339, 52 (2013)]. Time-of-flight images calculated from the two-dimensional numerical simulations reproduce characteristics of the experimental observations, in particular the emergence of the four peaks at the corners of the Brillouin zone.

We derive analytical expressions for the frequency and damping of the lowest collective modes of a two-dimensional Fermi gas using kinetic theory. For strong coupling, we furthermore show that pairing correlations overcompensate the effects of Pauli blocking on the collision rate for a large range of temperatures, resulting in a rate which is larger than that of a classical gas. Our results agree well with experimental data, and they recover the observed crossover from collisionless to hydrodynamic behavior with increasing coupling for the quadruple mode. Finally, we show that a trap anisotropy within the experimental bounds results in a damping of the breathing mode which is comparable to what is observed, even for a scale-invariant system.

A unified theory is developed to investigate both thermal ghost imaging and ghost diffraction through turbulent atmosphere. A very simple analytical imaging formula is derived and contains several previously known results as particular cases. Numerical examples are given to show the effects of the turbulence strength, source parameters, and detector size on the quality of ghost imaging and ghost diffraction. Specifically, we find ghost imaging is more robust against the turbulence than ghost diffraction. A comparison of ghost imaging with a conventional lens imaging system is given to show the advantages or disadvantages of ghost imaging through turbulent atmosphere.

We present the theory for retarded resonance interaction between two identical atoms at arbitrary positions near a metal surface. The dipole-dipole resonance interaction force that binds isotropically excited atom pairs together in free space may turn repulsive close to an ideal (totally reflecting) metal surface. On the other hand, close to an infinitely permeable surface it may turn more attractive. We illustrate numerically how the dipole-dipole resonance interaction between two oxygen atoms near a metal surface may provide a repulsive energy of the same order of magnitude as the ground-state binding energy of an oxygen molecule. As a complement we also present results from density-functional theory.

The photoionization cross sections for the 6$p$ ${}^2$$P_{3/2}$ excited state of potassium have been measured using a two-step selective excitation and ionization technique in conjunction with an atomic-beam source and a time-of-flight (TOF) mass spectrometer. The cross sections of photoionization have been measured at three ionizing laser wavelengths, 1064, 689.53, and 532 nm, as 4.80±0.31, 3.59±0.18, and 2.69±0.22 Mb, respectively, and the number density has been determined as 2×10${}^{11}$ cm${}^{-3}$. The presently measured experimental values of the photoionization cross sections are in excellent agreement with the theoretical calculations.

We show that the mistakes pointed out by Rana and Parashar [Phys. Rev. A 87, 016301 (2013)] do not invalidate the main conclusion of our work [Phys. Rev. A 86, 024302 (2012)]. We show that the errors affected only a particular application of our general results, and present the correction.

Trace distance is available to capture the dynamical information of the unital aspect of a quantum process. However, it cannot reflect the nonunital part. So the nondivisibility originating from the nonunital aspect cannot be revealed by the corresponding measure based on the trace distance. We provide a measure of nonunital non-Markovianity of quantum processes, which is a supplement to Breuer-Laine-Piilo (BLP) non-Markovianity measure. A measure of the degree of the nonunitality is also provided.

Understanding the relation between the different forms of inseparability in quantum mechanics is a longstanding problem in the foundations of quantum theory and has implications for quantum information processing. Here we make progress in this direction by establishing a direct link between quantum teleportation and Bell nonlocality. In particular, we show that all entangled states which are useful for teleportation are nonlocal resources, i.e., lead to deterministic violation of Bell's inequality. Our result also extends the phenomenon of superactivation of quantum nonlocality, recently proven by C. Palazuelos [Phys. Rev. Lett. 109, 190401 (2012)], and suggests that the latter might in fact be more general than initially thought.

Active protection of quantum states is an essential prerequisite for the implementation of quantum computing. Dynamical decoupling (DD) is a promising approach that applies sequences of control pulses to the system in order to reduce the adverse effect of system-environment interactions. Since every hardware device has finite precision, the errors of the DD control pulses can themselves destroy the stored information rather than protect it. We experimentally compare the performance of different DD sequences in the presence of an environment that was chosen such that all relevant DD sequences can equally suppress its effect on the system. Under these conditions, the remaining decay of the qubits under DD allows us to compare very precisely the robustness of the different DD sequences with respect to imperfections of the control pulses.