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Just Submitted

Maximal refraction and superluminal propagation in a gaseous nanolayer
J. Keaveney, I. G. Hughes, A. Sargsyan, D. Sarkisyan, C. S. Adams

We present an experimental measurement of the refractive index of high density Rb vapor in a gaseous atomic nanolayer. We use heterodyne interferometry to measure the relative phase shift between two copropagating laser beams as a function of the laser detuning and infer a peak index n = 1.26 \pm 0.02, close to the theoretical maximum of 1.31. The large index has a concomitant large index gradient creating a region with steep anomalous dispersion where a sub-nanosecond optical pulse is advanced by >100 ps over a propagation distance of 390 nm, corresponding to a group index of -1x10^5, the largest negative group index measured to date.

Coherent control of strongly interacting Rydberg polaritons
D T Maxwell, D J Szwer, D Paredes Barato, H Busche, J D Pritchard, A Gauguet, K J Weatherill, M P A Jones, C S Adams

Advances in quantum technology increasingly rely on combining the functions of different systems. Strongly interacting systems, such as ions or superconductors, are ideal for processing; large ensembles for memory; and optical photons for communication. However, interfacing these components remains a challenge. For example, although cavity QED in the microwave domain, using Rydberg atoms or superconducting circuits, provides efficient coupling between photons and static qubits, microwave photons are not ideal for quantum communication due to the blackbody background. For this reason, quantum interfaces that combine different functions of a network are desirable. Here we demonstrate an interface between the microwave and optical domains that allows processing of optical photons using microwave fields. We store optical photons in highly excited collective states (Rydberg polaritons) of a cold atomic ensemble. Subsequently, we apply a microwave field that performs a rotation of the collective spin of the Rydberg polaritons, leading to coherent many-body Rabi oscillations in the amplitude of the retrieved photon pulse. We show that the microwave field modifies the long-range interactions between the stored photons, a key step toward the realisation of an all-optical analogue of neutral atom quantum gates based on dipole blockade. In addition, Rydberg polaritons provide a powerful platform for studying strongly coupled atom-light interactions without a cavity, quantum simulation of resonant energy transfer or spin liquids, and quantum metrology using Dicke states.

Elastic scattering of a quantum matter-wave bright soliton on a barrier
Christoph Weiss, Yvan Castin

We consider a one-dimensional matter-wave bright soliton, corresponding to the ground bound state of N particles of mass m having a binary attractive delta potential interaction on the open line. For a full N-body quantum treatment, we derive several results for the scattering of this quantum soliton on a short-range, bounded from below, external potential, restricting to the low energy, elastic regime where the centre-of-mass kinetic energy of the incoming soliton is lower than the internal energy gap of the soliton, that is the minimal energy required to extract particles from the soliton.

Effective time-reversal via periodic shaking
C Weiss

For a periodically shaken optical lattice, effective time-reversal is investigated numerically. For interacting ultra-cold atoms, the scheme of [J. Phys. B 45, 021002 (2012)] involves a quasi-instantaneous change of both the shaking-amplitude and the sign of the interaction. As the wave function returns to its initial state with high probability, time-reversal is ideal to distinguish pure quantum dynamics from the dynamics described by statistical mixtures.

Non-equilibrium dynamics of a driven Bose-Einstein condensate at finite temperature
TP Billam, P Mason, SA Gardiner

While the Gross-Pitaevskii equation is well-established as the canonical dynamical description of atomic Bose-Einstein condensates at zero-temperature, describing the dynamics of Bose-Einstein condensates at finite temperatures remains a difficult theoretical problem, particularly when considering low-temperature, non-equilibrium systems in which depletion of the condensate occurs dynamically as a result of external driving. The BEC analog of the quantum delta-kicked rotor is the prototypical example of such a system. In this paper, we describe a fully time-dependent numerical method to propagate the equations of motion of a second-order, number-conserving description; these equations describe the coupled dynamics of the condensate and non-condensate fractions in a self-consistent manner, and correctly capture the phonon-like nature of excitations at low temperature, making them ideal for the study of low-temperature, non-equilibrium, driven systems. We use this numerical method to systematically explore the finite-temperature dynamics of the delta-kicked-rotor-BEC. We demonstrate that several qualitative features of this system at zero temperature are well-preserved at finite temperatures, and predict a finite-temperature shift of resonance frequencies which could be verified by future experiments.

Tunneling of the 3rd kind: A test of the effective non-locality of quantum field theory
SA Gardiner, H Gies, J Jaeckel, CJ Wallace

Integrating out virtual quantum fluctuations in an originally local quantum field theory results in an effective theory which is non-local. In this Letter we argue that tunneling of the 3rd kind - where particles traverse a barrier by splitting into a pair of virtual particles which recombine only after a finite distance - provides a direct test of this non-locality. We sketch a quantum-optical setup to test this effect, and investigate observable effects in a simple toy model.

Just Published

Nonlocal quantum superpositions of bright matter-wave solitons and dimers
B Gertjerenken and C Weiss
J. Phys. B 45, 165301 (2012)

The scattering of bright quantum solitons at barrier potentials in one-dimensional geometries is investigated. Such protocols have been predicted to lead to the creation of nonlocal quantum superpositions. The centre-of-mass motion of these bright matter-wave solitons generated from attractive Bose–Einstein condensates can be analysed with the effective potential approach. An application to the case of two particles being scattered at a delta potential allows analytical calculations not possible for higher particle numbers as well as a comparison with numerical results. Both for the dimer and a soliton with particle numbers on the order of N = 100, we investigate the signatures of the coherent superposition states in an interferometric setup and argue that experimentally an interference pattern would be particularly well observable in the centre-of-mass density. Quantum superposition states of ultra-cold atoms are interesting as input states for matter-wave interferometry as they could improve signal-to-noise ratios.

Optical preparation and measurement of atomic coherence at gigahertz bandwidth
Paul Siddons, Charles S Adams, Ifan G Hughes
J Phys B 45 124009 (2012)

We detail a method for the preparation of atomic coherence in a high-density atomic vapour of 87Rb, utilizing a coherent preparation scheme of off-resonant gigahertz bandwidth pulses. The scheme is found to be faster and more effective than techniques based on resonant interaction, such as coherent population trapping and population inversion. A numerical simulation of the preparation scheme is developed, and its efficiency in preparing coherent states is found to be close to unity at the entrance to the medium. The medium is then probed non-invasively with a laser field, the polarization of which is dependent upon the relative phase of the atomic coherence produced by the preparation fields.

Long-range Rydberg-Rydberg interactions in calcium, strontium and ytterbium
C L Vaillant, M P A Jones and R M Potvliege
J Phys B 45, 135004 (2012)

Long-range dipole–dipole and quadrupole–quadrupole interactions between pairs of Rydberg atoms are calculated perturbatively for calcium, strontium and ytterbium within the Coulomb approximation. Quantum defects, obtained by fitting existing laser spectroscopic data, are provided for all S, P, D and F series of strontium and for the 3P2 series of calcium. The results show qualitative differences with the alkali metal atoms, including isotropically attractive interactions of the strontium 1S0 states and a greater rarity of Förster resonances. Only two such resonances are identified, both in triplet series of strontium. The angular dependence of the long-range interaction is briefly discussed.