We report on an improved scheme to generate Bose-Einstein condensates (BECs) and degenerate Fermi gases of strontium. This scheme allows us to create quantum gases with higher atom number, a shorter time of the experimental cycle, or deeper quantum degeneracy than before. We create a BEC of ${}^{84}$Sr exceeding ${10}^7$ atoms, which is a 30-fold improvement over previously reported experiments. We increase the atom number of ${}^{86}$Sr BECs to $2.5\times {10}^4$ (a fivefold improvement) and refine the generation of attractively interacting ${}^{88}$Sr BECs. We present a scheme to generate ${}^{84}$Sr BECs with a cycle time of 2 s. We create deeply degenerate ${}^{87}$Sr Fermi gases with $T/T_F$ as low as 0.10(1), where the number of populated nuclear spin states can be set to any value between one and ten. Furthermore, we report on a total of five different double-degenerate Bose-Bose and Bose-Fermi mixtures. These studies prepare an excellent starting point for applications of strontium quantum gases anticipated in the near future.

It is generally believed that the BCS-BEC evolution in fermionic systems with $s$-wave pairing is a smooth crossover. However, for nonzero orbital-angular-momentum pairing such as $p$- or $d$-wave pairing, the system undergoes a quantum phase transition at the point where the chemical potential $\mu$ vanishes. In this paper, we study the BCS-BEC quantum phase transition and the collective excitations associated with the order-parameter fluctuations in two-dimensional fermionic systems with $p$- and $d$-wave pairings. We show that the quantum phase transition in such systems can be generically traced back to the infrared behavior of the fermionic excitation at $\mu =0$: $E_{\mathbf{k}}\sim k^l$, where $l=1,2$ is the quantum number of the orbital angular momentum. The nonanalyticity of the thermodynamic quantities is due to the infrared divergence caused by the fermionic excitation at $\mu =0$. As a result, the evolution of the Anderson-Bogoliubov mode is not smooth: Its velocity is nonanalytical across the quantum phase transition.

Modulation theory, based on a Lagrangian formulation of the governing equations, is used to investigate the propagation of a nonlinear, nonlocal optical vortex solitary wave in a finite nematic-liquid-crystal cell. The nematic response to the vortex is calculated using the approach of the method of images (MOI). It is demonstrated that the MOI is a reliable alternative to the usual Fourier series solution as it requires an order of magnitude fewer terms to obtain excellent agreement with numerical solutions. It is found that the cell walls, in addition to repelling the optical vortex solitary wave, as for an optical solitary wave, can destabilize it due to the fixed director orientation at the walls. A linearized stability analysis is used to explain and analyze this instability. In particular, the minimum distance of approach of a stable vortex to the wall is determined from the stability analysis. Good agreement is found with numerical minimum approach distances.

Continuous change of the propagation direction of a classical control field in the process of its off-resonant Raman interaction with a weak signal field in a three-level atomic medium is suggested for quantum storage of a single-photon wave packet. It is shown that due to the phase-matching condition such an angular control allows one to reversibly map the single-photon wave packet to the Raman spatial coherence grating. Thus, quantum storage and retrieval can be realized without using inhomogeneous broadening of the atomic transitions or manipulating the amplitude of the control field. Under some conditions the proposed scheme proves to be mathematically analogous to the quantum storage scheme based on controlled reversible inhomogeneous broadening of the atomic states.

We report the existence, stability, and rich dynamics of dissipative lattice solitons in optical media described by the cubic-quintic complex Ginzburg-Landau model with parity-time ($\mathcal{PT}$) symmetric potentials. We focus on studying the generic spatial soliton propagation scenarios by changing (a) the linear loss coefficient in the complex Ginzburg-Landau model, (b) the amplitudes, and (c) the periods of real and imaginary parts of the complex-valued $\mathcal{PT}$-symmetric optical lattice potential. Generically, it is found that if the period of the real part of the $\mathcal{PT}$-symmetric optical lattice potential is close to $\pi ,$ the spatial solitons are tightly bound and they can propagate straightly along the lattice, while if the period of the real part of the $\mathcal{PT}$-symmetric optical lattice potential is larger than $\pi ,$ the launched solitons are loosely bound and they can exhibit either a transverse (lateral) drift or a persistent swing around the input launching point due to gradient force arising from the spatially inhomogeneous loss. These latter features are intimately related to the dissipative nature of the system under consideration because they do not arise in the conservative counterpart of the dynamical model. These generic propagation scenarios can be effectively managed by properly changing the profile of the spatially inhomogeneous loss.

The depolarizing quantum operation plays an important role in studying the quantum noise effect and implementing general quantum operations. In this work, we report a scheme which implements a fully controllable input-state-independent depolarizing quantum operation for a photonic polarization qubit.

We report the observation of interspecies Feshbach resonances in an optically trapped mixture of ${}^{85}$Rb and ${}^{133}$Cs. We measure nine resonances in the lowest spin channel for a magnetic field range from 0 to 700 G and show that they are in good agreement with coupled-channel calculations. The interspecies background scattering length is close to zero over a large range of magnetic fields, permitting the sensitive detection of Feshbach resonances through interspecies thermalization. Our results confirm the quality of the Rb-Cs potential curves [Phys. Rev. A 85, 032506 (2012)] and offer promising starting points for the production of ultracold polar molecules.

For a bosonic (fermionic) open system in a bath with many bosonic (fermionic) modes, we derive the exact non-Markovian master equation in which the memory effect of the bath is reflected in the time-dependent decay rates. In this approach, the reduced density operator is constructed from the formal solution of the corresponding Heisenberg equations. As an application of the exact master equation, we study the active probing of the non-Markovianity of the quantum dissipation of a single bosonic mode of an electromagnetic field in a cavity-QED system. The non-Markovianity of the bath of the cavity is explicitly reflected by the atomic decoherence factor.

The momentum distribution is a powerful probe of strongly interacting systems that are expected to display universal behavior. This is contained in the contact parameters which relate few- and many-body properties. Here we consider a Bose gas in two dimensions and explicitly show that the two-body contact parameter is universal and then demonstrate that the momentum distribution at next-to-leading order has a logarithmic dependence on momentum which is vastly different from the three-dimensional case. Based on this, we propose a scheme for measuring the effective dimensionality of a quantum many-body system by exploiting the functional form of the momentum distribution.

We study abrupt changes in the dynamics and/or steady state of fermionic noise-driven systems produced by small changes in the system parameters. Specifically, we consider fermionic systems whose dynamics is described by master equations that are quadratic (and, under certain conditions, quartic) in creation and annihilation operators. We analyze phase transitions in the steady state as well as “dynamical transitions.” The latter are characterized by abrupt changes in the rate at which the system asymptotically approaches the steady state. We illustrate our general findings with relevant examples of fermionic (and, equivalently, spin) systems and show that they can be realized in ion chains.

Mutually unbiased bases determine an optimal set of measurements to extract complete information about the quantum state of a system. However, quite often a priori information about the state exists, making some of the measurement bases superfluous. This is, for example, the case when multiqubit states belong to the permutationally invariant subspace. In this paper we derive the minimal sets of mutually unbiased bases needed to tomographically reconstruct such states.

We investigate the amount of entanglement and quantum discord extractable from a two-mode squeezed state as considered from the viewpoint of two observers, Alice (inertial) and Rob (accelerated). We find that using localized modes produces qualitatively different correlation properties for large accelerations than do Unruh modes. Specifically, the entanglement undergoes a sudden death as a function of acceleration, and the discord asymptotes to zero in the limit of infinite acceleration. We conclude that the previous Unruh mode analyses do not determine the acceleration-dependent entanglement and discord degradation of a given quantum state.

We present an optical scheme to conditionally generate even or odd squeezed superpositions of coherent states (SSCSs). The optical setup consists of an unbalanced beam splitter whose transmittance tends to unity, and additional balanced beam splitters and photodetectors in auxiliary modes. Squeezed coherent states with different amplitudes are the input states in the optical scheme. The single-qubit operations are probabilistic and employ two- and three-photon subtractions from initial beams as the driving force. Generation of the even or odd SSCSs is observed in a wide diapason of values of used parameters. We consider a possibility to realize a one-way Hadamard gate for the squeezed coherent states when the base states are transformed into superposition states. States approximating the output states of a Hadamard gate with high fidelity can be realized by imposing restrictions on the values of used parameters. Higher-order subtractions from input beams are necessary to generate the SSCSs with larger amplitudes and higher fidelities. The problem is resolved in a Wigner representation to take into account imperfections of the optical devices.

We consider a group covariant quantum state set with a group in which each element corresponds to a unitary or antiunitary operator. This type of quantum state set can express a broad class of quantum state sets, including geometrically uniform state sets and self-symmetric state sets. We derive that for any quantum measurement for a group-covariant state set (both with and without a certain fraction of inconclusive results), a group covariant quantum measurement exists with respect to the same group with the same performance, that is, the same probability of correct detection and the same rate of inconclusive results. We then show that a group covariant optimal measurement exists for any group-covariant state set. In some cases, we derive an optimal measurement for a group-covariant state set.

The transition-matrix ($T$-matrix) approach provides a general formalism to study scattering problems in various areas of physics, including acoustics (scalar fields) and electromagnetics (vector fields), and is related to the theory of the scattering matrix ($S$ matrix) used in quantum mechanics and quantum field theory. Focusing on electromagnetic scattering, we highlight an alternative formulation of the $T$-matrix approach, based on the use of the reactance matrix or $K$ matrix, which is more suited to formal studies of energy-conservation constraints (such as the optical theorem). We show in particular that electrostatics or quasistatic approximations can be corrected within this framework to satisfy the energy-conservation constraints associated with radiation. A general formula for such a radiative correction is explicitly obtained, and empirical expressions proposed in earlier studies are shown to be special cases of this general formula. This work therefore provides a justification of the empirical radiative correction to the dipolar polarizability and a generalization of this correction to any types of point or body scatterers of arbitrary shapes, including higher multipolar orders.

The $B$-spline $R$-matrix (BSR) with pseudostates method is employed to treat electron collisions with carbon atoms. Predictions for elastic scattering, excitation, and ionization are presented for incident energies between threshold and 60 eV. The structure description is further improved compared to a previous BSR calculation by Zatsarinny et al. [Phys. Rev. A 71, 042702 (2005)]. This change in the structure model, together with the inclusion of a large number of pseudostates in the close-coupling expansion, has a major influence on the theoretical predictions, especially at intermediate energies, where many of the excitation cross sections are reduced significantly. Estimates for ionization cross sections are also provided.

The interaction of moving discrete solitons with a linear Gaussian defect is investigated. Solitons with profiles varying from hyperbolic secant to exponentially localized are considered such that the mobility of soliton is maintained; the condition for which is obtained. Studies on scattering of the soliton by an attractive defect potential reveal the existence of total reflection and transmission windows which become very narrow with increasing initial soliton amplitude. Transmission regions disappear beyond the small-amplitude limit. The regions of complete reflection and partial capture correspond to the windows of the existence and nonexistence of solution of the stationary problem. Interaction of the discrete soliton with a barrier potential is also investigated. The critical amplitude of the defect at which splitting of the soliton into two parts occurs was estimated from a balance equation. The results were confirmed through direct numerical integration of the dynamical equation showing very good agreement with the analytical prediction.

We study the polarized vector dark-type solitons in nonlocal self-defocusing media with an arbitrary degree of nonlocality. The relations between the soliton parameters and the representation of the transverse velocity of vector dark solitons in the whole range of degree of nonlocality is analytically obtained. We also demonstrate the propagation properties of the polarized vector dark solitons numerically.