We report a likely observation of cw lasing without inversion in a gas of hot rubidium atoms in an optical cavity under conditions of electromagnetically induced transparency (EIT). The medium is pumped coherently and resonantly by a single “coupling” beam which also produces EIT in the lasing transition. The steady-state intensity exhibits thresholds as a function of the atomic density and the strength of the coupling beam. A theoretical model for an effective three-level lambda system indicates that gain without inversion is possible in this system if the two ground states are coupled by depolarizing collisions, and if the decay branching ratios meet certain conditions.

We show that the quantum Zeno effect (QZE) prevails even if the entanglement with the measuring probe is not complete. The dynamics toward the asymptotic regime as a function of $N$, the number of measurements, reveals surprising results: the transition probability, for some values of the coupling to the measuring probe, may decrease much faster than the normal QZE.

We study entanglement dynamics in a system consisting of a qubit dispersively coupled to a finite-temperature, dissipative, driven oscillator. The robustness against dissipation of two generic kinds of entanglement is studied: the qubit can be entangled with either the phase or amplitude of the oscillator’s motion. In the zero-temperature limit, an analytic expression is derived for the logarithmic negativity. We also discuss how the generated entanglement may be detected via dephasing revivals, being mindful that revivals can occur even in the absence of any useful entanglement.

Protecting quantum information from the detrimental effects of decoherence and lack of precise quantum control is a central challenge that must be overcome if a large robust quantum computer is to be constructed. The traditional approach to achieving this is via active quantum error correction using fault-tolerant techniques. An alternative to this approach is to engineer strongly interacting many-body quantum systems that enact the quantum error correction via the natural dynamics of these systems. Here we present a method for achieving this based on the concept of concatenated quantum error correcting codes. We define a class of Hamiltonians whose ground states are concatenated quantum codes and whose energy landscape naturally causes quantum error correction. We analyze these Hamiltonians for robustness and suggest methods for implementing these highly unnatural Hamiltonians.

We discuss how to significantly reduce leakage errors in topological quantum computation by introducing an irrelevant error in phase, using the construction of a controlled-NOT (CNOT) gate in the Fibonacci anyon model as a concrete example. To be specific, we construct a functional braid in a six-anyon Hilbert space that exchanges two neighboring anyons while conserving the encoded quantum information. The leakage error is $\sim {10}^{-10}$ for a braid of $\sim 100$ interchanges of anyons. Applying the braid greatly reduces the leakage error in the construction of generic controlled-rotation gates.

For itinerant fermionic and bosonic systems, we study “particle entanglement,” defined as the entanglement between two subsets of particles making up the system. We formulate the general structure of particle entanglement in many-fermion ground states, analogous to the “area law” for the more usually studied entanglement between spatial regions. Basic properties of particle entanglement are uncovered by considering relatively simple itinerant models.

This work reports measured Stark parameters of singly ionized krypton lines. A low-pressure pulsed arc with 92% of He and 8% of Kr was used as a plasma source. Measured electron densities and electron temperatures were in the range $(0.7–2.0)\times {10}^{23}{\mathrm{m}}^{-3}$ and $16000–20000\mathrm{K}$, respectively. All investigated spectral lines belong to the uv spectral region.

In this paper we address the behavior of the superfluid transition temperature $T_c$ in the attractive Hubbard model. The attractive Hubbard model can be regarded as the generalization of the BCS to Bose-Einstein condensation (BEC) crossover to a lattice and, as such, may have implications for future optical lattice studies. Nevertheless, the BEC limit of the Hubbard model is very different from that of the better-studied Fermi gases, owing to the strong intersite repulsion between pairs. Here we address systematically the effects of pairing fluctuations for all filling fractions over the entire range of attractive interaction strength. A central conclusion of our work is that in a lattice, around half filling, the smooth evolution from the BCS to the BEC limits is interrupted: $T_c$ vanishes when the system approaches the bosonic regime with increasing interaction strength. We suggest that this interruption of crossover may signal a quantum critical transition to another form of superfluid not continuously connected to a BCS-like phase. A simple variational ansatz for an alternate ground state in this more strongly coupled superfluid is presented. A generalization of the ($s$-wave) Hubbard model to $d$-wave pairing allows us to address issues of relevance to high-$T_c$ superconductivity. The phase diagram (representing the pairing or pseudogap onset temperature $T^{*}$ and $T_c$) shows that here too one observes a vanishing of $T_c$ when $T^{*}$ becomes sufficiently large. Given this predicted breakdown of the crossover, and given the striking similarity to features of the cuprate phase diagram, we suggest that future experiments on ultracold fermions in optical lattices should not be exclusively limited to the repulsive Hubbard model, but should address the attractive model in order to elucidate features of high-temperature superconductivity.

We use hydrodynamic equations to study the formation of Faraday waves in a superfluid Fermi gas at zero temperature confined in a strongly elongated cigar-shaped trap. First, we treat the role of the radial density profile in the limit of an infinite cylindrical geometry and analytically evaluate the wavelength of the Faraday pattern. The effect of the axial confinement is fully taken into account in the numerical solution of hydrodynamic equations, and shows that the infinite cylinder geometry provides a very good description of the phenomena.

Starting from Maxwell equations in curved space, we derive the field equations of light when constrainted to a curved surface via a possible nonlinear waveguide. We show that surfaces with constant extrinsic curvature, in particular minimal surfaces, allow solutions with a well-defined polarization. We derive a generalized nonlinear Schrödinger equation and solve the linear propagation for surfaces of revolution with constant positive and negative Gaussian curvature.

We report clear evidence of the role of the attractive dipole-dipole interaction in Penning ionization of Rydberg atoms, leading to the formation of an ultracold plasma. Penning ionization of $np$ Rydberg cesium atoms does not occur for states with $n<42$, which have repulsive potentials, but it does occur for states of $n$ larger than 42, which have attractive potentials. Blackbody radiation is mostly responsible for the background and initial ionization, although ion–Rydberg-atom collisions and population transfer due to limited superradiance may have to be considered.

We report interferometric schemes to prepare arbitrary states of four-dimensional qudits (ququarts) based on biphoton states of ultrafast-pumped frequency-nondegenerate spontaneous parametric down conversion. Preparation and tomographic characterization of a few examples of general single-ququart states, a pure state, a mixed state (fully diagonal), and a mixed state (partially coherent), are experimentally demonstrated.

We propose a strategy to implement the Grover search algorithm by adiabatic passage in a very efficient way. An adiabatic process can be characterized by the instantaneous eigenvalues of the pertaining Hamiltonian, some of which form a gap. The key to the efficiency is based on the use of parallel eigenvalues. This allows us to obtain nonadiabatic losses that are exponentially small, independently of the number of items in the database in which the search is performed.

The Casimir-Polder interaction of an atom with a metallic wall is investigated in the framework of the Lifshitz theory. It is demonstrated that in some temperature (separation) region the Casimir-Polder entropy takes negative values and goes to zero when the temperature vanishes. This result is obtained both for an ideal metal wall and for real metal walls. Simple analytical representations for the Casimir-Polder free energy and force are also obtained. These results are used to make a comparison between the phenomenological potential used in the theoretical description of quantum reflection and exact atom-wall interaction energy, as given by the Lifshitz theory. Computations are performed for the atom of metastable ${\mathrm{He}}^{*}$ interacting with metal (Au) and dielectric (Si) walls. It is shown that the relative differences between the exact and phenomenological interaction energies are smaller in the case of a metallic wall. This is explained by the effect of negative entropy which occurs only for a metal wall. More accurate atom-wall interaction energies computed here can be used for the interpretation of measurement data in the experiments on quantum reflection.

We have calculated $S$-wave resonances in the positron-lithium system using the stabilization method in the framework of hyperspherical coordinates. Resonances below the Li $(n=4)$ thresholds have been investigated. We have located the previously known $S$-wave resonances by Roy and Ho [J. Phys. B 35, 2149 (2002)] and observed relatively good agreement for the resonance positions. In addition, one Feshbach resonance associated with the Ps $(n=2)$ threshold is also identified.

We derive the criteria for the Thomas-Fermi regime of a dipolar Bose-Einstein condensate in cigar-shaped, pancake-shaped, and spherical geometries. These also naturally gives the criteria for the mean-field one- and two-dimensional regimes. Our predictions, including analytic forms for the density profiles, are shown to be in excellent agreement with numerical solutions. Importantly, the anisotropy of the interactions has a profound effect on the Thomas-Fermi and low-dimensional criteria.

Triple differential cross sections (TDCSs) for two-photon double ionization of helium are calculated using the method of exterior complex scaling both above and below the threshold for sequential ionization $\left(54.4\mathrm{eV}\right)$. It is found that sequential ionization produces characteristic behavior in the TDCSs that identifies that process when it is in competition with nonsequential ionization. Moreover, we see the signature in the TDCSs and nuclear-recoil cross sections of “virtual sequential ionization” below the threshold for the sequential process.

We establish a framework to study the classical-communication properties of primitive local operations assisted by classical communication which realize various redistributions of entanglement, like, e.g., entanglement swapping. On the one hand, we analyze what local operations and how much classical communication are needed to perform them. On the other hand, we investigate whether and to what extent such primitives can help to establish classical communication when they are used in the form of black boxes available to spatially separated users. In particular, we find that entanglement swapping costs more communication than it can signal; in this sense, entanglement swapping is a weaker primitive than quantum teleportation.

We analyze the interplay of chaos, entanglement, and decoherence in a system of qubits whose collective behavior is that of a quantum kicked top. The dynamical entanglement between a single qubit and the rest can be calculated from the mean of the collective spin operators. This allows the possibility of efficiently measuring entanglement dynamics in an experimental setting. We consider a deeply quantum regime and show that signatures of chaos are present in the dynamical entanglement for parameters accessible in an experiment that we propose using cold atoms. The evolution of the entanglement depends on the support of the initial state on regular versus chaotic Floquet eigenstates, whose phase-space distributions are concentrated on the corresponding regular or chaotic eigenstructures. We include the effect of decoherence via a realistic model and show that the signatures of chaos in the entanglement dynamics persist in the presence of decoherence. In addition, the classical chaos affects the decoherence rate itself.