We use the time-dependent Gross-Pitaevskii equation to study the motion of a vortex ring produced by phase imprinting on an elongated cloud of cold atoms. Our approach models the experiments of Yefsah et al. [Nature (London) 499, 426 (2013)] on ${}^6$Li in the Bose-Einstein-condensate regime where the fermions are tightly bound into bosonic dimers. We find ring oscillation periods which are much larger than the period of the axial harmonic trap. Our results lend further strength to Bulgac et al.'s arguments (arXiv:1306.4266) that the “heavy solitons” seen in those experiments are actually vortex rings. We numerically calculate the periods of oscillation for the vortex rings as a function of interaction strength, trap aspect ratio, and minimum vortex ring radius. In the presence of axial anisotropies the rings undergo complicated internal dynamics where they break into sets of vortex lines, then later combine into rings. These structures oscillate with a similar frequency to simple axially symmetric rings.

We consider a one-dimensional lattice of atoms with laser excitation to a Rydberg state and spontaneous emission. The atoms are coupled due to the dipole-dipole interaction of the Rydberg states. This driven-dissipative system has a broad range of nonequilibrium phases, such as antiferromagnetic ordering and bistability. Using the quantum trajectory method, we calculate the spatial correlation function throughout the parameter space for up to 18 lattice sites. We show that bistability significantly strengthens the spatial correlations and the entanglement.

Epitaxially grown quantum dots (QDs) are promising sources of nonclassical states of light such as single photons and entangled photons. However, in order for them to be used as a resource for long-distance quantum communication, distributed quantum computation, or linear optics quantum computing, these photons must be coupled efficiently to long-lived quantum memories as part of a quantum repeater network. Here, we theoretically examine the prospects for efficient storage and retrieval of a QD-generated single photon with a 1-ns lifetime in a multilevel atomic system. We calculate using an experimentally demonstrated optical depth of 150 that the storage (total) efficiency can exceed 46$\%$ (28$\%$) in a dense, ultracold ensemble of ${}^{87}$Rb atoms. Furthermore, we find that the optimal control pulse required for storage and retrieval can be obtained using a diode laser and an electro-optic modulator rather than a mode-locked, pulsed laser source. Increasing the optical depth, for example, by using Bose-condensed ensembles or an optical cavity, can increase the efficiencies to near unity. Aside from enabling a high-speed quantum network based on QDs, such an efficient optical interface between an atomic ensemble and a QD can also lead to entanglement between collective spin-wave excitations of atoms and the spin of an electron or hole confined in the QD.

The positive effects of various perturbations introduced in a bidimensional photonic-crystal patterned membrane on its integrated absorption are investigated numerically and theoretically. Two phenomena responsible for the enhanced absorption observed are identified: an increase of the spectral density of modes, obtained thanks to folding mechanisms in the reciprocal lattice, and a better coupling of the modes with the incident light. By introducing a proper pseudodisordered pattern, we show that those two effects can be exploited so as to overcome the integrated absorption obtained for an optimized and single pattern unit cell photonic crystal.

We introduce a scheme for odd-order aberration-free transmission of two-photon beams through the turbulent atmosphere. We show that a two-photon beam with a Gaussian fourth-order correlation profile can be transmitted through a turbulence cell with very low signal loss. Our scheme relies on the spatial correlations present in two-photon states generated by spontaneous parametric down conversion.

We show that the interaction of polar alkali-metal dimers in the quintet spin state leads to the formation of a deeply bound reaction complex. The reaction complex can decompose adiabatically into homonuclear alkali-metal dimers (for all molecules except KRb) and into alkali-metal trimers (for all molecules). We show that there are no barriers for these chemical reactions. This means that all alkali-metal dimers in the $a^3{\Sigma }^{+}$ state are chemically unstable at ultracold temperature, and the use of an optical lattice to segregate the molecules and suppress losses may be necessary. In addition, we calculate the minimum-energy path for the chemical reactions of alkali-metal hydrides. We find that the reaction of two molecules is accelerated by a strong attraction between the alkali-metal atoms, leading to a barrierless process that produces hydrogen atoms with large kinetic energy. We discuss the unique features of the chemical reactions of ultracold alkali-metal dimers in the $a^3{\Sigma }^{+}$ electronic state.

Intracavity second-harmonic generation is one of the simplest of the quantum optical processes and is well within the expertise of most optical laboratories. It is well understood and characterized, both theoretically and experimentally. We show that it can be a source of continuous-variable asymmetric Gaussian harmonic steering with fields which have a coherent excitation, hence combining the important effects of harmonic entanglement and asymmetric steering in one easily controllable device, adjustable by the simple means of tuning the cavity loss rates at the fundamental and harmonic frequencies. We find that whether quantum steering is available via the standard measurements of the Einstein-Podolsky-Rosen correlations can depend on which quadrature measurements are inferred from output spectral measurements of the fundamental and the harmonic. Altering the ratios of the cavity loss rates can be used to tune the regions where symmetric steering is available, with the results becoming asymmetric over all frequencies as the cavity damping at the fundamental frequency becomes significantly greater than at the harmonic. This asymmetry and its functional dependence on frequency is a potential new tool for experimental quantum information science, with possible utility for quantum key distribution. Although we show the effect here for Gaussian measurements of the quadratures, and cannot rule out a return of the steering symmetry for some class of non-Gaussian measurements, we note here that the system obeys Gaussian statistics in the operating regime investigated and Gaussian inference is at least as accurate as any other method for calculating the necessary correlations. Perhaps most importantly, this system is simpler than any other methods we are aware of which have been used or proposed to create asymmetric steering.

Separate addressing of individual qubits is a challenging requirement for scalable quantum computation, and crosstalk between operations on neighboring qubits remains a significant source of error for current experimental implementations of multiqubit platforms. We propose a scheme based on spatial refocusing from interference of several coherent laser beams to significantly reduce the crosstalk error for any type of quantum gate. A general framework is developed for the spatial refocusing technique, in particular with practical Gaussian beams, and we show that the crosstalk-induced infidelity of quantum gates can be reduced by several orders of magnitude with a moderate cost of a few correction laser beams under typical experimental conditions.

Resource trade-offs can often be established by solving an appropriate robust optimization problem for a variety of scenarios involving constraints on optimization variables and uncertainties. Using an approach based on sequential convex programming, we demonstrate that quantum gate transformations can be made substantially robust against uncertainties while simultaneously using limited resources of control amplitude and bandwidth. Achieving such a high degree of robustness requires a quantitative model that specifies the range and character of the uncertainties. Using a model of a controlled one-qubit system for illustrative simulations, we identify robust control fields for a universal gate set and explore the trade-off between the worst-case gate fidelity and the field fluence. Our results demonstrate that, even for this simple model, there exists a rich variety of control design possibilities. In addition, we study the effect of noise represented by a stochastic uncertainty model.

Unambiguous-nonorthogonal-state discrimination has a fundamental importance in quantum information. Moreover, it can be used for entanglement distillation and secure communication. The discrimination is carried out by a positive operator-valued measure (POVM) generalized measurement, which is typically implemented by coupling the system to an ancilla. We find a trade-off between the needed energy resources and the evolution time needed to implement the POVM and express it in terms of an actionlike cost inequality. We find the realization that minimizes this actionlike cost and show that, in this case, the cost is determined by the maximal population transfer from the system to the ancilla. We demonstrate our findings in an example of a three-level system coupled to a laser.

We report measurements of the absorption strength of the cesium $6s{}^2{S}_{1/2}\to 7p{}^2{P}_{3/2}$ and the $6s{}^2{S}_{1/2}\to 7p{}^2{P}_{1/2}$ transitions at $\lambda =456$ and 459 nm, respectively, in an atomic vapor cell. By simultaneously measuring the absorption strength on the Cs $D_1$ line ($6s{}^2{S}_{1/2}\to 6p{}^2{P}_{1/2}$), for which the electric dipole transition moment is precisely known, we are able to determine the reduced dipole matrix elements for these two lines. We report $\langle 7P_{3/2}\left|\right|r\left|\right|6S_{1/2}\rangle =0.5780\left(7\right)a_0$ and $\langle 7P_{1/2}\left|\right|r\left|\right|6S_{1/2}\rangle =0.2789\left(16\right)a_0$, with fractional uncertainties of 0.12% and 0.6%, respectively. Through these measurements, we can reduce the discrepancy between values of the vector polarizability for the Cs $6s{}^2{S}_{1/2}\to 7s{}^2{S}_{1/2}$ transition that have been determined through two different methods.

We present a theoretical study on stimulated electronic Raman scattering in neon by resonant excitation with an x-ray free electron laser (XFEL). This study is in support of the recent experimental demonstration [C. Weninger et al., Phys. Rev. Lett. (to be published)] of stimulated x-ray Raman scattering. Focusing the broadband XFEL pulses into a cell of neon gas at atmospheric pressure a strong inelastic x-ray scattering signal in the forward direction was observed, as the x-ray energy was varied across the region of core-excited Rydberg states and the $K$ edge. The broadband and intrinsically incoherent x-ray pulses from the XFEL lead to a rich, structured line shape of the scattered radiation. We present a generalized Maxwell-Liouville–von Neumann approach to self-consistently solve for the amplification of the scattered radiation along with the time evolution of the density matrix of the atomic and residual ionic system. An in-depth analysis of the evolution of the emission spectra as a function of the Raman gain is presented. Furthermore, we propose the use of statistical methods to obtain high-resolution scattering data beyond the lifetime broadening despite pumping with incoherent x-ray pulses.

We propose a scheme for inverse engineering control in open quantum systems. Starting from an undetermined time evolution operator, a time-dependent Hamiltonian is derived in order to guide the system to attain an arbitrary target state at a predefined time. We calculate the fidelity of our inverse engineering control protocol in the presence of the noise with respect to the stochastic fluctuation of the linear parameters of the Hamiltonian during the time evolution. For a special family of Hamiltonians for two-level systems, we show that the control evolution of the system under noise can be categorized into two standard decohering processes: dephasing and depolarization, for both Markovian and non-Markovian conditions. In particular, we illustrate our formalism by analyzing the robustness of the engineered target state against errors. Moreover, we discuss the generalization of the inverse protocol for higher-dimensional systems.

We design fast trajectories of a trap to transport two ions using a shortcut-to-adiabaticity technique based on invariants. The effects of anharmonicity are analyzed first perturbatively, with an approximate, single relative-motion mode, description. Then, we use classical calculations and full quantum calculations. This allows us to identify discrete transport times that minimize excitation in the presence of anharmonicity. An even better strategy to suppress the effects of anharmonicity in a continuous range of transport times is to modify the trajectory using an effective trap frequency shifted with respect to the actual frequency by the coupling between relative and center-of-mass motions.

We study a system of hard-core bosons at half-filling in a one-dimensional optical superlattice. The bosons are allowed to hop to nearest- and next-nearest-neighbor sites. We obtain the ground-state phase diagram as a function of microscopic parameters using the finite-size density-matrix renormalization-group method. Depending on the sign of the next-nearest-neighbor hopping and the strength of the superlattice potential the system exhibits three different phases, namely the bond-order (BO) solid, the superlattice induced Mott insulator (SLMI), and the superfluid (SF) phase. When the signs of both hopping amplitudes are the same (the unfrustratedase), the system undergoes a transition from the SF to the SLMI at a nonzero value of the superlattice potential. On the other hand, when the two amplitudes differ in sign (the frustrated case), the SF is unstable to switching on a superlattice potential and also exists only up to a finite value of the next-nearest-neighbor hopping. This part of the phase diagram is dominated by the BO phase which breaks translation symmetry spontaneously even in the absence of the superlattice potential and can thus be characterized by a bond-order parameter. The transition from BO to SLMI appears to be first order.

We study the linear absorption and dispersion properties of a four-level double-V-type quantum system near a plasmonic nanostructure. The quantum system interacts with two orthogonal circularly polarized laser fields with the same frequency and different phases and electric field amplitudes. We find that the presence of the plasmonic nanostructure leads to strong modification of the absorption and dispersion spectra for one of the laser fields, in the presence of the other, and show that one can use the phase difference and the relative electric field amplitudes of the two laser fields for efficient control of the optical properties of the system. Effects such as complete optical transparency, zero absorption with nonzero dispersion, and gain without inversion are obtained.

Next-generation integrated photonic circuits (PCs) require the combination of on-chip coherent light sources and modulators to increase the integration density and reduce the energy consumption. While significant advances have been accomplished in direct laser modulation and channeling the modulated emissions into waveguides, there is still an enormous challenge in distributing the signals to their designed ends. Here we show that the outputs from microcavities can be directly modulated by local perturbations. We demonstrate this concept in a waveguides connected quadruple cavity, where the generated signals can be optionally distributed to one or multiple designed waveguides by slightly modulating the refractive index on the order of 0.01. This concept should help the advances of high-density on-chip photonic circuits.

We examine the validity of the rotating-wave approximation (RWA) in nonadiabatic holonomic single-qubit gates [New J. Phys. 14, 103035 (2012)]. We demonstrate that the adoption of RWA may lead to a sharp decline in fidelity for rapid gate implementation and small energy separation between the excited and computational states. The validity of the RWA in the recent experimental realization [Nature (London) 496, 482 (2013)] of nonadiabatic holonomic quantum computation for a superconducting qubit is examined.

Recording the transmitted spectrum of a weak attosecond pulse through a medium, while a strong femtosecond pulse copropagates at variable delay, probes the strong-field dynamics of atoms, molecules, and solids. Usually, the interpretation of these measurements is based on the assumption of a thin medium. Here, the propagation through a macroscopic medium of helium atoms in the region of fully allowed resonances is investigated both theoretically and experimentally. The propagation has dramatic effects on the transient spectrum even at relatively low pressures (50 mbar) and short propagation lengths (1 mm). The absorption does not evolve monotonically with the product of propagation distance and pressure, but regions with characteristics of Lorentz line shapes and characteristics of Fano line shapes alternate. Criteria are deduced to estimate whether macroscopic effects can be neglected or not in a transient absorption experiment. Furthermore, the theory in the limit of single-atom response yields a general equation for Lorentz- and Fano-type line shapes at variable pulse delay.

We propose an operational quasiprobability function for qudits, enabling a comparison between quantum and hidden-variable theories. We show that the quasiprobability function becomes positive semidefinite if consecutive measurement results are described by a hidden-variable model with locality and noninvasive measurability assumed. Otherwise, it is negative valued. The negativity depends on the observables to be measured as well as a given state, as the quasiprobability function is operationally defined. We also propose a marginal quasiprobability function and show that it plays the role of an entanglement witness for two qudits. In addition, we discuss an optical experiment of a polarization qubit to demonstrate its nonclassicality in terms of the quasiprobability function.