We show that vortices, induced in cold atom superfluids in optical lattices, may order in a novel vortex-Peierls ground state. In such a state vortices do not form a simple lattice but arrange themselves in clusters, within which the vortices are partially delocalized, tunneling between classically degenerate configurations. We demonstrate that this exotic quantum many-body state is selected by an order-from-disorder mechanism for a special combination of the vortex filling and lattice geometry that has a macroscopic number of classically degenerate ground states.

We present a new nonlocal updating scheme for quantum Monte Carlo simulations, which conserves particle number and other symmetries. It allows exact symmetry projection and direct evaluation of the equal-time Green’s function and other observables in the canonical ensemble. The method is applicable to a wide variety of systems. We show results for bosonic atoms in optical lattices, neutron pairs in atomic nuclei, and electron pairs in ultrasmall superconducting grains.

New scaling behavior has been both predicted and observed in the spontaneous production of fluxons in quenched $\mathrm{Nb}\mathrm{\text{−}}\mathrm{Al}/{\mathrm{Al}}_{\mathrm{ox}}/\mathrm{Nb}$ annular Josephson tunnel junctions (JTJs) as a function of the quench time, ${\tau }_Q$. The probability $f_1$ to trap a single defect during the normal-metal–superconductor phase transition clearly follows an allometric dependence on ${\tau }_Q$ with a scaling exponent $\sigma =0.5$, as predicted from the Zurek-Kibble mechanism for realistic JTJs formed by strongly coupled superconductors. This definitive experiment replaces one reported by us earlier, in which an idealized model was used that predicted $\sigma =0.25$, commensurate with the then much poorer data. Our experiment remains the only condensed matter experiment to date to have measured a scaling exponent with any reliability.

We have studied mixtures of fermionic ${}^{40}\mathrm{K}$ and bosonic ${}^{87}\mathrm{Rb}$ quantum gases in a three-dimensional optical lattice. We observe that an increasing admixture of the fermionic species diminishes the phase coherence of the bosonic atoms as measured by studying both the visibility of the matter wave interference pattern and the coherence length of the bosons. Moreover, we find that the attractive interactions between bosons and fermions lead to an increase of the boson density in the lattice which we measure by studying three-body recombination in the lattice. In our data, we do not observe three-body loss of the fermionic atoms. An analysis of the thermodynamics of a noninteracting Bose-Fermi mixture in the lattice suggests a mechanism for sympathetic cooling of the fermions in the lattice.

We observe a localized phase of ultracold bosonic quantum gases in a 3-dimensional optical lattice induced by a small contribution of fermionic atoms acting as impurities in a Fermi-Bose quantum gas mixture. In particular, we study the dependence of this transition on the fermionic ${}^{40}\mathrm{K}$ impurity concentration by a comparison to the corresponding superfluid to Mott-insulator transition in a pure bosonic ${}^{87}\mathrm{Rb}$ gas and find a significant shift in the transition parameter. The observed shift is larger than expected based on a simple mean-field argument, which indicates that disorder-related effects play a significant role.

The diffusive motion of Brownian particles near irregular interfaces plays a crucial role in various transport phenomena in nature and industry. Most diffusion-reaction processes in confining interfacial systems involve a sequence of Brownian flights in the bulk, connecting successive hits with the interface (Brownian bridges). The statistics of times and displacements separating two interface encounters are then determinant in the overall transport. We present a theoretical and numerical analysis of this complex first-passage problem. We show that the bridge statistics is directly related to the Minkowski content of the surface within the usual diffusion length. In the case of self-similar or self-affine interfaces, we show and check numerically that the bridge statistics follows power laws with exponents depending directly on the surface fractal dimension.

We propose a simple microscopic model for active nematic particles similar in spirit to the Vicsek model for self-propelled polar particles. In two dimensions, we show that this model exhibits a Kosterlitz-Thouless-like transition to quasi-long-range orientational order and that in this nonequilibrium context, the ordered phase is characterized by giant density fluctuations, in agreement with the predictions of Ramaswamy et al. [Europhys. Lett. 62, 196 (2003)].

A maximum value for the magnetic field is determined, which provides the full compensation of the positronium rest mass by the binding energy in the maximum symmetry state and disappearance of the energy gap separating the electron-positron system from the vacuum. The compensation becomes possible owing to the falling to the center phenomenon. The maximum magnetic field may be related to the vacuum and describe its structure.

We observe bright matter-wave solitons form during the collapse of ${}^{85}\mathrm{Rb}$ condensates in a three-dimensional (3D) magnetic trap. The collapse is induced by using a Feshbach resonance to suddenly switch the atomic interactions from repulsive to attractive. Remnant condensates containing several times the critical number of atoms for the onset of instability are observed to survive the collapse. Under these conditions a highly robust configuration of 3D solitons forms such that each soliton satisfies the condition for stability and neighboring solitons exhibit repulsive interactions.

In this Letter, we present an experimental benchmark of operational control methods in quantum information processors extended up to 12 qubits. We implement universal control of this large Hilbert space using two complementary approaches and discuss their accuracy and scalability. Despite decoherence, we were able to reach a 12-coherence state (or a 12-qubit pseudopure cat state) and decode it into an 11 qubit plus one qutrit pseudopure state using liquid state nuclear magnetic resonance quantum information processors.

Entanglement detection typically relies on linear inequalities for mean values of certain observables (entanglement witnesses), where violation indicates entanglement. We provide a general method to improve any of these inequalities for bipartite systems via nonlinear expressions. The nonlinearities are of different orders and can be directly measured in experiments, often without any extra effort.

We present a classical protocol, using the matrix product-state representation, to simulate cluster-state quantum computation at a cost polynomial in the number of qubits in the cluster and exponential in $d$—the width of the cluster. We use this result to show that any log-depth quantum computation in the gate array model, with gates linking only nearby qubits, can be simulated efficiently on a classical computer.

We have observed high-order quantum resonances in a realization of the quantum -kicked rotor, using Bose-condensed Na atoms subjected to a pulsed standing wave of laser light. These resonances occur for pulse intervals that are rational fractions of the Talbot time, and are characterized by ballistic momentum transfer to the atoms. The condensate’s narrow momentum distribution not only permits the observation of the quantum resonances at and of the Talbot time, but also allows us to study scaling laws for the resonance width in quasimomentum and pulse interval.

The bunching of two single photons on a beam splitter is a fundamental quantum effect, first observed by Hong, Ou, and Mandel. It is a unique interference effect that relies only on the photons’ indistinguishability and not on their relative phase. We generalize this effect by demonstrating the bunching of two Bell states, created in two passes of a nonlinear crystal, each composed of two photons. When the two Bell states are indistinguishable, phase-insensitive destructive interference prevents the outcome of fourfold coincidence between the four spatial-polarization modes. For certain combinations of the two Bell states, we demonstrate the opposite effect of antibunching. We relate this result to the number of distinguishable modes in parametric down-conversion.

A new general expression is derived for the fluctuating electromagnetic field outside a metal surface in terms of its surface impedance. It provides a generalization to real metals of Lifshitz theory of molecular interactions between dielectric solids. The theory is used to compute the radiative heat transfer between two parallel metal surfaces at different temperatures. It is shown that a measurement of this quantity may provide an experimental resolution of a long-standing controversy about the effect of thermal corrections on the Casimir force between real metal plates.

The unitarity regime of the BCS-BEC crossover can be realized by diluting a system of two-component lattice fermions with an on-site attractive interaction. We perform a systematic-error-free finite-temperature simulation of this system by diagrammatic determinant Monte Carlo method. The critical temperature in units of Fermi energy is found to be $T_c/{\epsilon }_F=0.152(7)$. We also report the behavior of the thermodynamic functions, and discuss the issues of thermometry of ultracold Fermi gases.

We propose a new way to generate an observable geometric phase by means of a completely incoherent phenomenon. We show how to imprint a geometric phase to a system by adiabatically manipulating the environment with which it interacts. As a specific scheme, we analyze a multilevel atom interacting with a broadband squeezed vacuum bosonic bath. As the squeezing parameters are smoothly changed in time along a closed loop, the ground state of the system acquires a geometric phase. We also propose a scheme to measure such a geometric phase by means of a suitable polarization detection.

We analyze the effects of imbalancing the populations of two-component trapped fermions, in the Bose-Einstein condensate limit of the attractive interaction between different fermions. Starting from the gap equation with two fermionic chemical potentials, we derive a set of coupled equations that describe composite bosons and excess fermions. We include in these equations the processes leading to the correct dimer-dimer and dimer-fermion scattering lengths. The coupled equations are then solved in the Thomas-Fermi approximation to obtain the density profiles for composite bosons and excess fermions, which are relevant to the recent experiments with trapped fermionic atoms.

We present an experimental realization of a robust quantum communication scheme [Phys. Rev. Lett. 93, 220501 (2004)] using pairs of photons entangled in polarization and time. Our method overcomes errors due to collective rotation of the polarization modes (e.g., birefringence in optical fiber or misalignment), is insensitive to the phase’s fluctuation of the interferometer, and does not require any shared reference frame including time reference, except the need to label different photons. The practical robustness of the scheme is further shown by implementing a variation of the Bennett-Brassard 1984 quantum key distribution protocol over 1 km optical fiber.

In an example of Bose-Einstein condensates embedded in two-dimensional optical lattices, we show that in nonlinear periodic systems modulational instability and interband tunneling are intrinsically related phenomena. By direct numerical simulations we find that tunneling results in attenuation or enhancement of instability. On the other hand, instability results in asymmetric nonlinear tunneling. The effect strongly depends on the band gap structure and it is especially significant in the case of the resonant tunneling. The symmetry of the coherent structures emerging from the instability reflects the symmetry of both the stable and the unstable states between which the tunneling occurs. Our results provide evidence of the profound effect of the band structure on the superfluid-insulator transition.