The system of ultracold atoms with hyperfine spin $F=3/2$ might be unstable against the formation of quintet pairs if the interaction is attractive in the quintet channel. We have investigated the behavior of correlation functions in a model including only $s$-wave interactions at quarter filling by large-scale density-matrix renormalization-group simulations. We show that the correlations of quintet pairs become quasi-long-ranged when the system is partially polarized, leading to the emergence of various mixed superfluid phases in which BCS-like pairs carrying different magnetic moment coexist.

We investigate the performance of a single-element superconducting single-photon detector (SSPD) for quantum state reconstruction. We perform quantum state reconstruction, using the measured photon counting behavior of the detector. Standard quantum state reconstruction assumes a linear response; this simple model fails for SSPDs, which are known to show a nonlinear response intrinsic to the detection mechanism. We quantify the photon counting behavior of the SSPD by a sparsity-based detector tomography technique and use this to perform quantum state reconstruction of both thermal and coherent states. We find that the nonlinearities inherent in the detection process enhance the ability of the detector to do state reconstruction compared to a linear detector with similar efficiency for detecting single photons.

The robustness to different sources of error of the scheme for nonadiabatic holonomic gates proposed previously [New J. Phys. 14, 103035 (2012)] is investigated. Open system effects as well as errors in the driving fields are considered. It is found that the gates can be made error resilient by using sufficiently short pulses. The principal limit of how short the pulses can be made is given by the breakdown of the quasi-monochromatic approximation. A comparison with the resilience of adiabatic gates is carried out.

In this paper, we present a measure of multipartite entanglement ($k$-nonseparable), $k$-ME concurrence $C_{k-\mathrm{ME}}\left(\rho \right)$, that unambiguously detects all $k$-nonseparable states in arbitrary dimensions, where the special case 2-ME concurrence $C_{2-\mathrm{ME}}\left(\rho \right)$ is a measure of genuine multipartite entanglement. The measure $k$-ME concurrence satisfies important characteristics of an entanglement measure, including the entanglement monotone, vanishing on $k$-separable states, convexity, subadditivity, and being strictly greater than zero for all $k$-nonseparable states. Two powerful lower bounds on this measure are given. These lower bounds are experimentally implementable without quantum state tomography and are easily computable as no optimization or eigenvalue evaluation is needed. We illustrate detailed examples in which the given bounds perform better than other known detection criteria.

Currently, the theoretical uncertainty limits the interpretation of the atomic parity-nonconservation (PNC) measurements. We calculate the PNC $5s$-$6s$ electric dipole transition amplitude in rubidium and demonstrate that rubidium is a good candidate to search for new physics beyond the standard model since accuracy of the atomic calculations in rubidium can be higher than in cesium. PNC in cesium is currently the best low-energy test of the standard model; therefore, similar measurements for rubidium present a good option for further progress in the field. We also calculate the nuclear spin-dependent part of the PNC amplitude, which is needed for the extraction of the nuclear anapole moment from the PNC measurements.

The discrete-momentum-representation theory with short-range correlation and polarization approximated by local-density-functional theory has been augmented by connecting the short-range potential to an asymptotic polarization tensor. The capacity of this theory to correctly describe the coupling of a free electron with nuclear motion is tested by a detailed comparison of calculated and measured cross sections for vibrational excitation in cyclopropane. Absolute magnitudes of the cross sections, selectivity with respect to the vibrational modes excited, angular distributions in the entire angular range $0^{\circ }$–${180}^{\circ }$, and resonant structures dependent on the incident electron energy in the range from threshold to 16 eV are compared. The results are encouraging; the theory reproduces all these aspects quantitatively, even at low energies, down to about 1 eV. An important asset of this theory is that it is applicable to large molecules.

Using magnetic focusing in a supersonic jet, a beam of “normal” H${}_2$O molecules seeded in a krypton carrier gas is shown to provide a source of water molecules that is highly enhanced in the ortho-H${}_2$O ($o$-H${}_2$O) nuclear spin isomer over the high-temperature equilibrium 3:1 ortho:para ratio. Water from the magnetically focused beam is then isolated and stored within a Kr matrix at 13 K whereby the amplitude and lifetime of this strong nuclear spin polarization are quantified spectroscopically. Attempts to store the polarization in a colder Kr matrix reveal complex nuclear spin conversion processes.

Elastic scattering cross sections are measured for lithium atoms colliding with rare-gas atoms and SF${}_6$ molecules at tunable relative velocities down to $\sim$50 m/s. Our scattering apparatus combines a velocity-tunable molecular beam with a magneto-optic trap which provides an ultracold cloud of lithium atoms as a scattering target. Comparison with theory reveals the quantum nature of the collision dynamics in the studied regime, including rainbows as well as orbiting resonances.

We study superfluid transitions in bidimensional (2D) and tridimensional (3D) disordered and interacting Bose gases. We work in the limit of long-range correlated disorder such that it can be treated in the local density approximation. We present superfluid transition curves in both the disorder-temperature plane and the disorder-entropy plane in 2D and 3D Bose gases. Surprisingly, we find that a small amount of disorder is always favorable to the apparition of a superfluid. Our results offer a quantitative comparison with recent experiments in 2D disordered ultracold gases, for which no exact theory exists.

We establish a set of nonequilibrium quantum phase transitions in the Ising model driven under monochromatic nonadiabatic modulation of the transverse field. We show that besides the Ising-like critical behavior, the system exhibits an anisotropic transition which is absent in equilibrium. The nonequilibrium quantum phases correspond to states which are synchronized with the external control in the long-time dynamics.

We study multiphoton quantum-interference effects at a beam splitter and its connection to the prospect of attaining interferometric phase-shift measurements with noise levels below the standard quantum limit. Specifically, we consider the mixing of the most classical states of light coherent states with the most nonclassical states of light number states at a 50 : 50 beam splitter. Multiphoton quantum-interference effects from mixing photon-number states of small photon numbers with coherent states of arbitrary amplitudes are dramatic even at the level of a single photon. For input vacuum and coherent states, the joint photon-number distribution after the beam splitter is unimodal, a product of Poisson distributions for each of the output modes but with the input of a single photon, the original distribution is symmetrically bifurcated into a bimodal distribution. With a two-photon-number state mixed with a coherent state, a trimodal distribution is obtained, etc. The obtained distributions are shown to be structured so as to be conducive for approaching Heisenberg-limited sensitivities in photon-number parity-based interferometry. We show that mixing a coherent state with even a single photon results in a significant reduction in noise over that of the shot-noise limit. Finally, based on the results of mixing coherent light with single photons, we consider the mixing coherent light with the squeezed vacuum and the squeezed one-photon states and find the latter yields higher sensitivity in phase-shift measurements for the same squeeze parameter owing to the absence of the vacuum state.

Solid-state qubits hold the promise to achieve an unmatched combination of sensitivity and spatial resolution. To achieve their potential, the qubits need, however, to be shielded from the deleterious effects of the environment. While dynamical decoupling techniques can improve the coherence time, they impose a compromise between sensitivity and the frequency range of the field to be measured. Moreover, the performance of pulse sequences is ultimately limited by control bounds and errors. Here we analyze a versatile alternative based on continuous driving. We find that continuous dynamical decoupling schemes can be used for ac magnetometry, providing similar frequency constraints on the ac field and improved sensitivity for some noise regimes. In addition, the flexibility of phase and amplitude modulation could yield superior robustness to driving errors and a better adaptability to external experimental scenarios.

Quantum entanglement distillation is a probabilistic process which protects entanglement from environment-induced decoherence. In this paper, we investigate the distillation of a continuousvariable optic entangled state with noiseless linear amplification (NLA). NLA schemes perform better than the conventional photon-subtraction-based distillation scheme, particularly in distributing entanglement over extremely low efficiency quantum channels. Finally, a comparison between the NLA-based scheme and the local squeezing-enhanced photon subtraction scheme is also investigated.

For a system of three identical bosons interacting via short-range forces, when two of the atoms are about to form a two-body $s$-wave dimer, the Efimov effect takes place leading to the formation of an infinite number of three-body (Efimov) states. The lowest Efimov state crosses the three-body break-up threshold when the $s$-wave two-body scattering length is $a\approx -9.73r_{\mathrm{vdW}}$, $r_{\mathrm{vdW}}$ being the van der Waals length. This article focuses on a generalized version of this Efimov scenario, where two of the atoms are about to form a two-body $d$-wave dimer, resulting in strong $d$-wave interactions. In his paper [Phys. Rev. A 62, 050702(R) (2000)], Bo Gao predicted that for broad resonances the $d$-wave dimer is always formed when $a\approx 0.956r_{\mathrm{vdW}}$. Here we find that a single universal three-body state associated with the $d$-wave dimer is also formed near the three-body breakup threshold at $a\approx 1.09r_{\mathrm{vdW}}$, or alternatively $a_2=0.902r_{\mathrm{vdW}}$, where $a_2$ is the two-body $d$-wave scattering length. The signature of such a universal three-body state is signaled experimentally by an enhancement of the three-body recombination rate. The three-body effective potential curves that are crucial for understanding the recombination dynamics are also calculated and analyzed. An improved method to calculate the couplings, effective potential curves, and recombination rate coefficients is presented.

We study near a Feshbach resonance, as a function of the mass ratio, the fermion-dimer scattering amplitude in fermionic mixtures of two fermion species. When masses are equal the physical situation is known to be quite simple. We show that, when the mass ratio is increased, the situation becomes much more complex. For the $s$-wave contribution we obtain an analytical solution in the asymptotic limit of very large mass ratio. In this regime the $s$-wave scattering amplitude displays a large number of zeros, essentially linked to the known large value of the fermion-dimer scattering length in this regime. We find by an exact numerical calculation that a zero is still present for a mass ratio of 15. For the $p$-wave contribution we make our study below the mass ratio of 8.17, where a fermion-dimer bound state appears. We find that a strong $p$-wave resonance is present at low energy, due to a virtual bound state, in the fermion-dimer system, which is a forerunner of the real bound state. This resonance becomes prominent in the mass ratio range around the one corresponding to the ${}^{40}$K -${}^6$Li mixtures, much studied experimentally. This resonance should affect a number of physical properties. These include the equation of state of unbalanced mixtures at very low temperature but also the equation of state of balanced mixtures at moderate or high temperature. The frequency and the damping of collective modes should also provide a convenient way to evidence this resonance. Finally it should be possible to modify the effective mass of one of the fermionic species by making use of an optical lattice. This would allow one to study the strong dependence of the resonance as a function of the mass ratio of the two fermionic elements. In particular one could check if the virtual bound state is relevant for the instabilities of these mixtures.

The secondary electron yields as a result of slow highly charged ions (Ar${}^{4+},{\mathrm{Ar}}^{13+}$) impinging on clean Au(111), highly oriented pyrolitic graphite, and thin films of C${}_{60}$ on Au are presented. In order to investigate the dynamics of the neutralization of the highly charged ions in front of the surface, angular scans have been performed. The results give a clear indication that the observed increase in electron yield seen on C${}_{60}$ compared to Au stems from kinetic-energy-driven processes and not from processes driven by the potential energy carried by the highly charged ion.

Orthogonally polarized two-color sequential laser pulses are used to control the electron localization in the dissociation of H${}_2{}^{+}$. The first single attosecond pulse, whose polarization axis is perpendicular to the molecular axis, excites H${}_2{}^{+}$ from $1s{\sigma }_g$ to $2p{\pi }_u$, and the time-delayed infrared pulse, whose polarization axis is parallel to the molecular axis, steers the electron between two nuclei. The simulation of the time-dependent Schrödinger equation predicts the control degree of the electron localization can be up to 90$\%$ with the current laser technology. In this article, we reveal that the mechanism for this asymmetric localization is due to the mixture of $2p{\pi }_g$ and $2p{\pi }_u$, instead of $1s{\sigma }_g$and $2p{\sigma }_u$ in the previous studies.

We describe two-field optical techniques to control interactions in Feshbach resonances for two-body scattering in ultracold gases. These techniques create a molecular dark state in the closed channel of a magnetically tunable Feshbach resonance, greatly suppressing optical scattering compared to single optical field methods. The dark-state method enables control of the effective range, by creating narrow features that modify the energy dependence of the scattering phase shift, as well as control of the elastic and inelastic parts of the zero-energy $s$-wave scattering amplitude. We determine the scattering length and the effective range from an effective range expansion, by calculating the momentum-dependent scattering phase shift from the two-body scattering state.

We consider the propagation of ultrashort optical solitons in media described by a general Hamiltonian of multilevel atoms. Assuming that all transition frequencies of the medium are well below the typical wave frequency, i.e., only the contribution of infrared transitions is taken into account, we use a short-wave approximation and a rigorous application of the reductive perturbation formalism to derive a cumbersome coupled system of nonlinear partial differential equations describing ultrashort soliton evolution in such systems. The rather complicated set of coupled equations can be simplified to a generic double-sine-Gordon equation for a special case of identical three-level atoms, whereas for a special case of identical four-level atoms the system of coupled equations can be reduced to a generalized double-sine-Gordon equation. Numerical simulations showing the formation of robust breather-type solutions of both the standard double-sine-Gordon and of the generalized double-sine-Gordon equations from sinusoidal inputs with Gaussian envelopes are also presented.

By solving the two-level Maxwell-Bloch equations for resonant pulses numerically, we observe precursors in a regime where the area theorem is fulfilled. The precursors are $0\pi$ pulses traveling at a velocity close to the speed of light, and the main signal is a self-induced transparency (SIT) $2\pi$ soliton traveling at a much lower velocity. The manifestation of the precursors is strongest when the input pulse duration is on the same order as the inhomogeneous lifetime and the input area is close to $\pi$. Depending on the relationship between the input pulse duration and the inhomogeneous lifetime, the precursors can interact linearly or nonlinearly with the material. Experimental confirmation of these results should offer a direct and reasonably straightforward way of measuring optical precursors.