Evolution of a spinor condensate: coherent dynamics, dephasing and revivals

We present measurements and a theoretical model for the interplay of spin dependent interactions and external magnetic fields in atomic spinor condensates. We highlight general features like quadratic Zeeman dephasing and its influence on coherent spin mixing processes by focusing on a specific coherent superposition state in a F=1 $^{87}$Rb Bose-Einstein condensate. In particular, we observe the transition from coherent spinor oscillations to thermal equilibration

Kinetic theory and dynamic structure factor of a condensate in the random phase approximation

We present the microscopic kinetic theory of a homogeneous dilute Bose condensed gas in the generalized random phase approximation (GRPA), which satisfies the following requirements: 1) the mass, momentum and energy conservation laws; 2) the H-theorem; 3) the superfluidity property and 4) the recovery of the Bogoliubov theory at zero temperature \cite{condenson}. In this approach, the condensate influences the binary collisional process between the two normal atoms, in the sense that their interaction force results from the mediation of a Bogoliubov collective excitation traveling throughout the condensate. Furthermore, as long as the Bose gas is stable, no collision happens between condensed and normal atoms. In this paper, we show how the kinetic theory in the GRPA allows to calculate the dynamic structure factor at finite temperature and when the normal and superfluid are in a relative motion. The obtained spectrum for this factor provides a prediction which, compared to the experimental results, allows to validate the GRPA. PACS numbers:03.75.Hh, 03.75.Kk, 05.30.-dComment: 6 pages, 1 figures, QFS2004 conferenc

Ghost imaging using homodyne detection

We present a theoretical study of ghost imaging based on correlated beams arising from parametric down-conversion, and which uses balanced homodyne detection to measure both the signal and idler fields. We analytically show that the signal-idler correlations contain the full amplitude and phase information about an object located in the signal path, both in the near-field and the far-field case. To this end we discuss how to optimize the optical setups in the two imaging paths, including the crucial point regarding how to engineer the phase of the idler local oscillator as to observe the desired orthogonal quadrature components of the image. We point out an inherent link between the far-field bandwidth and the near-field resolution of the reproduced image, determined by the bandwidth of the source of the correlated beams. However, we show how to circumvent this limitation by using a spatial averaging technique which dramatically improves the imaging bandwidth of the far-field correlations as well as speeds up the convergence rate. The results are backed up by numerical simulations taking into account the finite size and duration of the pump pulse.Comment: 17 pages, 10 figures, submitted to Phys. Rev.

Thermodynamics of a Bose-Einstein Condensate with Weak Disorder

We consider the thermodynamics of a homogeneous superfluid dilute Bose gas in the presence of weak quenched disorder. Following the zero-temperature approach of Huang and Meng, we diagonalize the Hamiltonian of a dilute Bose gas in an external random delta-correlated potential by means of a Bogoliubov transformation. We extend this approach to finite temperature by combining the Popov and the many-body T-matrix approximations. This approach permits us to include the quasi-particle interactions within this temperature range. We derive the disorder-induced shifts of the Bose-Einstein critical temperature and of the temperature for the onset of superfluidity by approaching the transition points from below, i.e., from the superfluid phase. Our results lead to a phase diagram consistent with that of the finite-temperature theory of Lopatin and Vinokur which was based on the replica method, and in which the transition points were approached from above.Comment: 11 pages, 5 figure

Order enhancement and coarsening of self-organized silicon nanodot patterns induced by ion-beam sputtering

3 pages, 3 figures.-- PACS nrs.: 81.05.Cy, 81.07.-b, 68.47.Fg, 81.16.Rf, 79.20.Rf, 68.35.Bs.The temporal evolution of the characteristic wavelength (λ) and ordering range (ξ) of self-organized nanodot patterns induced during Ar+ ion beam sputtering on Si(001) and Si(111) surfaces is studied by atomic force microscopy and grazing incidence x-ray diffraction. The patterns exhibit initial coarsening of λ (up to 54–60 nm) and increase in ξ (up to 400–500 nm) after which both features stabilize. The pattern formation is only weakly controlled by the crystallographic surface orientation, Si(111) surfaces showing a faster evolution into a proper stationary state. This trend is attributed to a higher sputtering rate at this orientation, as confirmed by theoretical simulations.This work has been supported by grants BFM2003- 07749-C05-01, BFM2003-07749-C05-02, and BFM2003- 07749-C05-05 from the Spanish Ministerio de Educación y Ciencia (MEC). Two of the authors (R.G. and J.M.-G.) acknowledge financial support from MEC through the “Ramón y Cajal” and FPU programs, respectively.Publicad

Stochastic effects at ripple formation processes in anisotropic systems with multiplicative noise

We study pattern formation processes in anisotropic system governed by the Kuramoto-Sivashinsky equation with multiplicative noise as a generalization of the Bradley-Harper model for ripple formation induced by ion bombardment. For both linear and nonlinear systems we study noise induced effects at ripple formation and discuss scaling behavior of the surface growth and roughness characteristics. It was found that the secondary parameters of the ion beam (beam profile and variations of an incidence angle) can crucially change the topology of patterns and the corresponding dynamics

Anomalous fluctuations of the condensate in interacting Bose gases

We find that the fluctuations of the condensate in a weakly interacting Bose gas confined in a box of volume $V$ follow the law $\sim V^{4/3}$. This anomalous behaviour arises from the occurrence of infrared divergencies due to phonon excitations and holds also for strongly correlated Bose superfluids. The analysis is extended to an interacting Bose gas confined in a harmonic trap where the fluctuations are found to exhibit a similar anomaly.Comment: 4 pages, RevTe

Persistent currents in a Bose-Einstein condensate in the presence of disorder

We examine bosonic atoms that are confined in a toroidal, quasi-one-dimensional trap, subjected to a random potential. The resulting inhomogeneous atomic density is smoothened for sufficiently strong, repulsive interatomic interactions. Statistical analysis of our simulations show that the gas supports persistent currents, which become more fragile due to the disorder.Comment: 5 pages, RevTex, 3 figures, revised version, to appear in JLT

Optimal quantum cloning of orbital angular momentum photon qubits via Hong-Ou-Mandel coalescence

The orbital angular momentum (OAM) of light, associated with a helical structure of the wavefunction, has a great potential for quantum photonics, as it allows attaching a higher dimensional quantum space to each photon. Hitherto, however, the use of OAM has been hindered by its difficult manipulation. Here, exploiting the recently demonstrated spin-OAM information transfer tools, we report the first observation of the Hong-Ou-Mandel coalescence of two incoming photons having nonzero OAM into the same outgoing mode of a beam-splitter. The coalescence can be switched on and off by varying the input OAM state of the photons. Such effect has been then exploited to carry out the 1 \rightarrow 2 universal optimal quantum cloning of OAM-encoded qubits, using the symmetrization technique already developed for polarization. These results are finally shown to be scalable to quantum spaces of arbitrary dimension, even combining different degrees of freedom of the photons.Comment: 5 pages, 3 figure