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Quantum-Atom Optics Downunder (QAO)

Atom entanglement from light scattering on BEC’s

K.M.R. van der Stam, R. Meppelink, J. M. M. J. Vogels, P. van der Straten

Published Jan 1, 2007• 0 citations
Publisher

Abstract

Using light scattering on Bose-Einstein condensates we study the entanglement of atoms. In the light interaction with the condensate, atoms are scattered in both the forward and the 'counterintuitive' backward direction, which results in entangled atom pairs. We have enhanced the backscattering by driving the process using two frequencies and observe an equal balance between the forward and backward scattered atoms. We detected the so-called end-fire modes of the light scattering and observe under certain conditions only one end-fire mode, which enhances the entanglement. Finally, we have applied noise correlations spectroscopy to directly observe the entangled pairs.

DOI
10.1364/qao.2007.qwc3

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Using light scattering on Bose-Einstein condensates we study the entanglement of atoms. In the light interaction with the condensate, atoms are scattered in both the forward and the 'counterintuitive' backward direction, which results in entangled atom pairs. We have enhanced the backscattering by driving the process using two frequencies and observe an equal balance between the forward and backward scattered atoms. We detected the so-called end-fire modes of the light scattering and observe under certain conditions only one end-fire mode, which enhances the entanglement. Finally, we have applied noise correlations spectroscopy to directly observe the entangled pairs.
A semiclassical theory of light scattering from a Bose-Einstein condensate of a dilute gas is developed. It is shown that the results of theoretical calculations are in qualitative agreement with the data obtained from recent experiments on the observation of atoms scattered in the backward direction with respect to the direction of a pump laser beam. It is demonstrated that there can arise a field that propagates in the backward direction and whose intensity exceeds the intensity of conventional Rayleigh scattering.
Due to its coherence properties and high optical depth, a Bose–Einstein condensate [BEC] provides an ideal setting to investigate collective atom-light interactions. Superradiant light scattering [SLS] in a BEC is a fascinating example of such an interaction. It is an analogous process to Dicke superradiance, in which an electronically inverted sample decays collectively, leading to the emission of one or more light pulses in a well-defined direction. Through time-resolved measurements of the superradiant light pulses emitted by an end-pumped BEC, we study the close connection of SLS with Dicke superradiance. A 1D model of the system yields good agreement with the experimental data and shows that the dynamics result from the structures that build up in the light and matter-wave fields along the BEC. This paves the way for exploiting the atom–photon correlations generated by the superradiance.
This thesis investigates thermalisation, correlations and entanglement in Bose-Einstein condensates. Bose-Einstein condensates are ultra-cold collections of identical bosonic atoms which accumulate in a single quantum state, forming a mesoscopic quantum object. They are clean and controllable quantum many-body systems that permit an unprecedented degree of experimental flexibility compared to other physical systems. Further, a tractable microscopic theory exists which allows a direct and powerful comparison between theory and experiment, propelling the field of quantum atom optics forward at an incredible pace. Here we explore some of the fundamental frontiers of the field, examining how non-classical correlations and entanglement can be created and measured, as well as how non-classical effects can lead to the rapid heating of atom clouds. We first investigate correlations between two weakly coupled condensates, a system analogous to a superconducting Josephson junction. The ground state of this system contains non-classical number correlation arising from the repulsion between the atoms. Such states are of interest because they may lead to more precise measurement devices such as atomic gyroscopes. Unfortunately thermal fluctuations can destroy these correlations, and great care is needed to experimentally observe non-classical effects. We show that adiabatic evolution can drive the isolated quantum system out of thermal equilibrium and decrease thermal noise, in agreement with a recent experiment [Esteve et al. Nature 455, 1216 (2008)]. This technique may be valuable for observing and using quantum correlated states in the future. Next, we analyse the rapid heating that occurs when a condensate is placed in a moving periodic potential. The dynamical instability responsible for the heating was the subject of much uncertainty, which we suggest was due to the inability of the mean-field approximation to account for important spontaneous scattering processes. We show that a model including non-classical spontaneous scattering can describe dynamical instabilities correctly in each of the regimes where they have been observed, and in particular we compare our simulations to an experiment performed at the University of Otago deep inside the spontaneous scattering regime. Finally, we proposed a method to create and detect entangled atomic wave-packets. Entangled atoms are interesting from a fundamental perspective, and may prove useful in future quantum information and precision measurement technologies. Entanglement is generated by interactions, such as atomic collisions in Bose-Einstein condensates. We analyse the type of entanglement generated via atomic collisions and introduce an abstract scheme for detecting entanglement and demonstrating the Einstein-Podolsky-Rosen paradox with ultra-cold atoms. We further this result by proposing an experiment where entangled wave-packets are created and detected. The entanglement is generated by the pairwise scattering that causes the instabilities in moving periodic potentials mentioned above. By careful arrangement, the instability process can be controlled to to produce two well-defined atomic wave-packets. The presence of entanglement can be proven by applying a series of laser pulses to interfere the wave-packets and then measuring the output populations. Realising this experiment is feasible with current technology.
We investigate the atom pair production by superradiant backward-scattering from a Bose-Einstein condensate. By driving the superradiant process with two frequencies we can extend both the range of pulse duration and intensity by two orders of magnitude and obtain full control over the number of scattered atoms in forward and backward direction. We show that the atoms scattered in forward direction are strongly correlated with the atoms scattered in backward direction, which makes resonant superradiant backward-scattering a promising candidate for many-particle entanglement.
Summary form only given.We consider two overlapping and optically thin dilute gas Bose-Einstein condensates occupying two different Zeeman sublevels. We propose an optical method for increasing the number of condensate atoms. Two laser beams drive Raman transitions between the Zeeman states and induce a coherent quantum tunneling between the Bose-Einstein condensates. The scattering of photons is dominantly coherent and in the forward direction due to the Bose enhancement. This corresponds to the scattering of atoms between the two condensates. Some laser-driven Raman transitions also scatter atoms between the condensate and noncondensate atom fractions. For a range of condensate phase differences there is destructive quantum interference of the amplitudes for scattering atoms out of the condensates. Because the total atom scattering rate into the condensates is unaffected the condensates grow. This mechanism is analogous to that responsible for optical lasing without inversion, and it may find application as a pump for an atom laser.
The non-equilibrium dynamics of the superradiant scattering of light from a BEC is experimentally investigated by monitoring the time evolution of the population in the condensate. In the experiment, a cigar-shaped Bose-Einstein condensate of /sup 87/Rb atoms is produced and is exposed to a single off-resonant laser pulse directed along the condensate symmetry axis. The superradiant back-scattering of light creates a matter wave pulse moving in the same direction of the laser beam at twice the photon recoil velocity with respect to the original condensate. The efficiency of the process is limited by decoherence between the two atomic spatially overlapped wavepackets. The temporal evolution of the condensate observed in the experiment as well as the results of a simple semiclassical model in which the condensate is described by a density operator for the motional states. The population of the two matter wavepackets as a function of the light pulse duration is reported, together with the experimental points with the analytical formula resulting from the theory of collective atomic recoil laser (CARL).