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Quantum-Atom Optics Downunder (QAO)

Coherence Properties of a Continuously Pumped Atom Laser at Finite Temperature

Geoff M. Lee, Ashton S. Bradley, Matthew J. Davis

Published Jan 1, 2007• 0 citations
Publisher

Abstract

A continuous wave (cw) atom laser will most likely be realised by outcoupling from a reservoir containing a Bose-Einstein condensate at finite temperature. This will mean that the correlation properties and linewidth of the cw atom laser beam will be influenced by the thermal nature of the reservoir. Here we report on simulations using the stochastic Gross-Pitaevskii formalism for finite temperature dynamics to find the steady state properties of the outcoupled beam. We determine how the linewidth and correlation length depend on the condensate size, temperature, and outcoupling properties, and determine the optimum parameters for cw atom laser operation.

DOI
10.1364/qao.2007.qwe26

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A continuous wave (cw) atom laser will most likely be realised by outcoupling from a reservoir containing a Bose-Einstein condensate at finite temperature. This will mean that the correlation properties and linewidth of the cw atom laser beam will be influenced by the thermal nature of the reservoir. Here we report on simulations using the stochastic Gross-Pitaevskii formalism for finite temperature dynamics to find the steady state properties of the outcoupled beam. We determine how the linewidth and correlation length depend on the condensate size, temperature, and outcoupling properties, and determine the optimum parameters for cw atom laser operation.
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A continuous-wave atom laser formed by the outcoupling of atoms from a trapped Bose-Einstein condensate (BEC) potentially has a range of metrological applications. However, in order for the device to be truly continuous, a mechanism to replenish the atoms in the BEC is required. Here we calculate the temporal coherence properties of a continuously pumped atom laser beam outcoupled from a trapped Bose-Einstein condensate that is replenished from a reservoir at finite temperature. We find that the thermal fluctuations of the condensate can significantly decrease the temporal coherence of the output beam due to atomic interactions between the trapped BEC and the beam, and this can impact the metrological usefulness of the device. We demonstrate that a Raman outcoupling scheme imparting a sufficient momentum kick to the atom laser beam can lead to a significantly reduced linewidth.
We present a model for a continuous atom laser beam in a one-dimensional waveguide. The beam is formed by continuous Raman outcoupling of a trapped one-dimensional (quasi-)condensate, which is created by imposing a tightly confining transverse optical potential on a three-dimensional magnetically trapped ultracold thermal gas. The trapped (quasi-)condensate is modeled by a stochastic Langevin equation in which pumping arises naturally from the surrounding three-dimensional thermal cloud, which acts as its heat bath. When the outcoupling is turned on, such pumping leads to continuous replenishing of the (quasi-)condensate and thus to a steady-state operation for the atom laser, which is described by a nonlinear Schrodinger equation coupled to the Langevin equation for the (quasi-)condensate. This model is used to study the temporal and spatial evolution of the coherence length of the (quasi-)condensate and the atom laser over the entire system.
We present a model for a continuous atom laser in a one-dimensional waveguide. The beam is formed by continuous Raman outcoupling of a trapped one-dimensional (quasi-)condensate, which is created by imposing a tightly confining transverse optical potential on a three-dimensional magnetically trapped ultracold thermal gas. The trapped (quasi-)condensate is modelled by a stochastic Langevin equation, in which pumping arises naturally from the surrounding three-dimensional thermal cloud, which acts as its heat bath. When the outcoupling is turned on, such pumping leads to continuous replenishing of the (quasi-)condensate, and thus to a steady-state operation for the atom laser, which is described by a nonlinear Schroedinger equation, coupled to the Langevin equation for the (quasi-)condensate. This model is used to study the temporal and spatial evolution of the coherence length of the (quasi-)condensate and the atom laser over the entire system.
We study an output coupler for matter waves that could be used to develop a cw atom-laser, based on current Bose-Einstein-condensation experiments with alkali-metal atoms. Assuming a cw pump mechanism for the condensate and taking into account losses due to outgoing atoms allows a description of the output by a coherently driven and damped Schr\"odinger equation. By contrast to a pulsed atom-laser output the stationary outgoing matter wave is extremely monoenergetic. Phase fluctuations of the condensate and technical noise limit the coherence length of the output beam. Using a Monte Carlo simulation we obtain the spatial correlation function of the output wave function, also including the influence of gravity.
We develop a general theory of adiabatic output coupling from trapped, weakly interacting, atomic Bose-Einstein condensates at finite temperatures. For weak coupling, the output rate from the condensate and the excited levels in the trap settles in a time proportional to the inverse of the spectral width of the coupling to the output modes. We discuss the properties of the output atoms in the quasi-steady-state, where the population inside the trap is not appreciably depleted. We show how the composition of the output beam, containing the condensate and the thermal component, may be controlled by changing the frequency of the output coupling lasers. This composition determines the first- and second-order coherence of the output beam. We discuss the changes in the composition of the Bose gas left in the trap, and show how nonresonant output coupling can stimulate either the evaporation of the thermal excitations in the trap or the growth of the nonthermal excitations, when pairs of correlated atoms leave the condensate.