Entanglement of remote atomic qubits

We report observations of entanglement of two remote atomic qubits, achieved by generating an entangled state of an atomic qubit and a single photon at site A, transmitting the photon to site B in an adjacent laboratory through an optical fiber, and converting the photon into an atomic qubit. Entanglement of the two remote atomic qubits is inferred by performing, locally, quantum state transfer of each of the atomic qubits onto a photonic qubit and subsequent measurement of polarization correlations in violation of the Bell inequality |S|?2. We experimentally determine Sexp?=2.16±0.03. Entanglement of two remote atomic qubits, each qubit consisting of two independent spin wave excitations, and reversible, coherent transfer of entanglement between matter and light represent important advances in quantum information scienc

Variational Approach to the Modulational Instability

We study the modulational stability of the nonlinear Schr\"odinger equation (NLS) using a time-dependent variational approach. Within this framework, we derive ordinary differential equations (ODEs) for the time evolution of the amplitude and phase of modulational perturbations. Analyzing the ensuing ODEs, we re-derive the classical modulational instability criterion. The case (relevant to applications in optics and Bose-Einstein condensation) where the coefficients of the equation are time-dependent, is also examined

Cavity QED and quantum information processing with "hot" trapped atoms

We propose a method to implement cavity QED and quantum information processing in high-Q cavities with a single trapped but non-localized atom. The system is beyond the Lamb-Dick limit due to the atomic thermal motion. Our method is based on adiabatic passages, which make the relevant dynamics insensitive to the randomness of the atom position with an appropriate interaction configuration. The validity of this method is demonstrated from both approximate analytical calculations and exact numerical simulations. We also discuss various applications of this method based on the current experimental technology.Comment: 14 pages, 8 figures, Revte

Heating and decoherence suppression using decoupling techniques

We study the application of decoupling techniques to the case of a damped vibrational mode of a chain of trapped ions, which can be used as a quantum bus in linear ion trap quantum computers. We show that vibrational heating could be efficiently suppressed using appropriate ``parity kicks''. We also show that vibrational decoherence can be suppressed by this decoupling procedure, even though this is generally more difficult because the rate at which the parity kicks have to applied increases with the effective bath temperature.Comment: 13 pages, 5 figures. Typos corrected, references adde

Gaussian quantum operator representation for bosons

We introduce a Gaussian quantum operator representation, using the most general possible multimode Gaussian operator basis. The representation unifies and substantially extends existing phase-space representations of density matrices for Bose systems and also includes generalized squeezed-state and thermal bases. It enables first-principles dynamical or equilibrium calculations in quantum many-body systems, with quantum uncertainties appearing as dynamical objects. Any quadratic Liouville equation for the density operator results in a purely deterministic time evolution. Any cubic or quartic master equation can be treated using stochastic methods

Theory of fluctuations and collisions in quantum optics

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Dynamic stability of dressed condensate mixtures

We analyze the dynamical stability of common spatial, or breathe-together, modes for a system of two trapped interacting Bose-Einstein condensates, when the condensates are resonantly coupled by a strong driving field. We show analytically that the dynamical stability is governed by a set of dressed atom collision parameters, and that the driving field typically destabilizes breathe-together modes which are stable in the absence of driving and vice versa. By means of numerical solutions we illustrate the consequences of this phenomenon for a number of interacting condensates consisting of pairs of ground hyperfine states of both sodium and rubidiu

Entanglement of a photon and an optical lattice spin wave

We propose and implement a scheme to produce long-lived entanglement between a signal field and a magnetically insensitive collective excitation in an atomic cloud cooled in a one-dimensional optical lattice. After a programmable storage time, we convert the spin-wave excitation into an idler field, and demonstrate violation of Bell’s inequality for storage times in excess of 3 ms.<br/

Quantum-noise-initiated symmetry breaking of spatial solitons

The spectra of ?(2) spatial solitons are measured close to the soliton-formation threshold and show the presence of sidebands, shifted by 39 THz from the laser line. By comparing with the predictions of a quantum optical field model, solved numerically in the full (3+1)-dimensional space, it is claimed that the observed temporal instability of the spatial soliton is seeded by vacuum state fluctuations of the electromagnetic fiel