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arXiv:quant-ph/0011107 · Europhysics Letters (EPL)

Loading of a Bose-Einstein condensate in the boson accumulation regime

Filip Floegel, Luis Santos, Maciej Lewenstein

Published May 1, 2001• 3 citations
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Abstract

We study the optical loading of a trapped Bose-Einstein condensate by spontaneous emission of atoms in excited electronic state in the boson accumulation regime. We generalize the previous simplified analysis of Cirac and Lewenstein ( Phys. Rev. A , 53 (1996) 2466), to a 3D case in which more than one trap level of the excited-state trap is considered. By solving the corresponding quantum many-body master equation, we demonstrate that also for this general situation the photon reabsorption can help to increase the condensate fraction. Such effect could be employed to realize a continuous atom laser, and to overcome condensate losses.

DOI
10.1209/epl/i2001-00238-x

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We study the optical loading of a trapped Bose-Einstein condensate by spontaneous emission of atoms in excited electronic state in the boson accumulation regime. We generalize the previous simplified analysis of Cirac and Lewenstein ( Phys. Rev. A , 53 (1996) 2466), to a 3D case in which more than one trap level of the excited-state trap is considered. By solving the corresponding quantum many-body master equation, we demonstrate that also for this general situation the photon reabsorption can help to increase the condensate fraction. Such effect could be employed to realize a continuous atom laser, and to overcome condensate losses.
We consider the behavior of an atomic Bose-Einstein condensate in the presence of an atom in an (internal) excited electronic level. We analyze the boson-accumulation regime, defined by the relation N0{\mathit{N}}_{0}N0​\ensuremath{\gg}a,N-N0{\mathit{N}}_{0}N0​, where N is the total number of atoms in the ground electronic state, N0{\mathit{N}}_{0}N0​ is the number of atoms in the condensate, and a is the number of levels to which the excited atom can decay. In this regime, quantum statistical effects related to the boson nature of the atoms are predominant in the process of spontaneous emission. Simple arguments suggest that the proportion of atoms in the condensate decreases after the spontaneous decay. A more elaborated model based on a master equation description shows that, in general, these arguments may lead to erroneous conclusions. Under certain conditions it predicts that the proportion of atoms in the condensate increases. We give an interpretation of this phenomenon in terms of quantum interferences between processes that include reabsorption of the emitted photons, and dipole-dipole interactions between the atoms. The physical effect considered here may help to pump atoms into a condensate and to compensate for atomic losses in the atomic trap. \textcopyright{} 1996 The American Physical Society.
We discuss the optical loading of a Bose-Einstein condensate in the Thomas-Fermi regime. The condensate is loaded via spontaneous emission from a reservoir of excited-state atoms. By means of a master equation formalism, we discuss the modification of the condensate temperature during the loading. We identify the threshold temperature, T th , above (below) which the loading process leads to cooling (heating), respectively. The consequences of our analysis for the continuous loading of an atom laser are discussed.
We analyze the continuous pumping of atoms into a Bose-Einstein condensate via spontaneous emission from a thermal reservoir of atoms. We consider the case of atoms with a three-level $\ensuremath{\Lambda}$ scheme, in which one of the atomic transitions has a very much shorter lifetime than the other one. We found that in such a scenario the photon reabsorption in dense clouds at the frequency of the narrow transition can be considered negligible. If, in addition, inelastic processes can be kept negligible, we find that optical pumping can be used to continuously load and refill Bose-Einstein condensates, i.e., it provides a possible way to achieve a continuous atom laser.
Demonstrating that, despite loss processes, Bose-Einstein condensates can be formed in steady state is a prerequisite for obtaining a coherent beam of atoms in a continuous-wave atom laser. In this paper we propose a method for loading atoms into the thermal component of a Bose condensed cloud confined in a magnetic trap. This method is aimed at allowing steady-state dynamics to be achieved. The proposed scheme involves loading atoms into the conservative magnetic potential using the spontaneous emission of photons. We show that the probability for the reabsorption of these photons may be small.
It has been proposed that the adiabatic loading of a Bose-Einstein condensate (BEC) into an optical lattice via the Mott-insulator transition can be used to initialize a quantum computer [D. Jaksch et al., Phys. Rev. Lett. 81, 3108 (1998)]. The loading of a BEC into the lattice without causing band excitation is readily achievable; however, unless one switches on an optical lattice very slowly, the optical lattice causes a phase to accumulate across the condensate. We show analytically and numerically that a cancellation of this effect is possible by adjusting the harmonic trap force constant of the magnetic trap appropriately, thereby facilitating quick loading of an optical lattice for quantum computing purposes. A simple analytical theory is developed for a nonstationary BEC in a harmonic trap.
We show that the phase difference of two overlapping ground-state Bose-Einstein condensates can affect the optical spontaneous emission rate of excited atoms. Depending on the phase difference the atom-stimulated spontaneous emission rate can vary between zero and the rate corresponding to all the ground-state atoms in a single condensate. Besides giving control over spontaneous emission this provides an optical method for detecting the condensate phase difference. It differs from previous methods in that no light fields are applied. Instead the light is spontaneously emitted when excited atoms make a transition into either condensate.