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arXiv:cond-mat/9904007 · Journal of Physics B Atomic Molecular and Optical Physics

Output coupling of a Bose-Einstein condensate formed in a TOP trap

J L Martin, Callum R. McKenzie, N R Thomas, John C. Sharpe, D. M. Warrington, P. J. Manson, Weston J. Sandle, A. C. Wilson

Published Jan 1, 1999• 46 citations
arXivHTMLPDF

Abstract

Two distinct mechanisms are investigated for transferring a pure 87 Rb Bose-Einstein condensate in the | F = 2, m F = 2 state into a mixture of condensates in all the m F states within the F = 2 manifold. Some of these condensates remain trapped whilst others are output coupled in the form of an elementary pulsed atom laser. Here we present details of the condensate preparation and results of the two condensate output coupling schemes. The first scheme is a radio-frequency technique which allows controllable transfer into available m F states, and the second makes use of Majorana spin-flips to populate all the manifold sub-states equally.

DOI
10.1088/0953-4075/32/12/322

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Physical Review A•May 1, 1999•38 citations
Two distinct mechanisms are investigated for transferring a pure 87 Rb Bose-Einstein condensate in the | F = 2, m F = 2 state into a mixture of condensates in all the m F states within the F = 2 manifold. Some of these condensates remain trapped whilst others are output coupled in the form of an elementary pulsed atom laser. Here we present details of the condensate preparation and results of the two condensate output coupling schemes. The first scheme is a radio-frequency technique which allows controllable transfer into available m F states, and the second makes use of Majorana spin-flips to populate all the manifold sub-states equally.
A radio-frequency (RF) transition is used to convert a pure F = 2, m F = 2 87 Rb Bose-Einstein condensate confined in a TOP trap into a mixture of F = 2, m F = 2 and F = 2, m F = 1 states. We show that the nature of this coupling process is strongly influenced by the presence of the time-varying field of the TOP trap, and complicated by the presence of multiple Zeeman substates. In particular, the effective Rabi frequency associated with the applied RF field is not constant across the spatial extent of the cloud leading to a complex geometry for atom-laser output coupling and `averaging out' of Rabi oscillations. Further, a time-varying detuning can give rise to complex spatial structures.
We investigate atomic population dynamics of two Bose-Einstein condensate (BEC) states that are in different traps or in the same trap and are optically coupled through a common excited state by Raman laser pulses. For large interatomic interactions, almost all of the atoms that are initially in one of the traps (or one of the internal states) are either completely transferred into the other state or stay in the initially occupied state after the laser pulses, and the atoms (populations) are never distributed to both condensate states at the same time. Applications to the atomic logic gate and to output couplers of the atom laser are pointed out.
Summary form only given. We report results concerning both theoretical and experimental studies of collective excitations in connection with the atom laser operation. We use a radio-frequency field to out-couple a coherent matter-wave beam, either continuous or pulsed, from a /sup 87/Rb condensate. The latter scheme itself induces collective excitations in the atom laser. Starting from a condensate in the F=2, m/sub F/=2 state the radio-frequency field populates both trapped and untrapped Zeeman sublevels. A rich phenomenology arises including the production of multiple condensates exhibiting different dynamical behaviour, which make clear evidence of mutual interactions. The use of a static very elongated Ioffe trap allows a precise experimental investigation which can be compared with the predictions based on the solution of the time-dependent Gross-Pitaevskii equation. In the pulsed regime, the sudden change of the internal energy produces collective oscillations in the shape both for the m/sub F/=2 and the m/sub F/=1 trapped condensate. Preliminary results for the m/sub F/=2 atom laser show that the experimental behaviour is likely to be reproduced by the simulations only if mutual interactions are taken into account.
A pulsed atom laser is experimentally demonstrated by means of outcoupling coherent atoms from 87 Rb Bose–Einstein condensates in magnetic trap via radio-frequency pulses. To study the strong outcoupling dynamics of the atom laser, the original | F = 2, m F = 2⟩ condensate and the coupled | F = 2, m F = 1⟩ component, both of which overlap in space usually, are separated spatially by collective oscillations. The number of atoms in three of the five Zeeman states are measured and compared with the theoretical results.
Research on degenerate quantum gases of dilute atomic gases has become a well established field in modern physics and revealed many astonishing phenomena. The first experiments on Bose and Fermi gases at low temperatures aimed to prove the predictions of quantum mechanics on a macroscopic scale. Since then we make use of Bose and Fermi gases and mixtures of any combination to model other many body systems. A property of Bose Einstein condensates is superfluidity which we want to investigate further in toroidal potentials with a new apparatus which can produce an all-optical BEC. This means trapping thermal atoms in an optical dipole trap and cooling the sample by evaporation into the quantum degenerate regime. This new technique was first successfully demonstrated by M.D. Barrett et al. in 2001 and promises to be faster and simpler than the well established and common way of evaporation in magnetic traps. A new apparatus was built by the author and we report on the experimental details of the trapping of 87Rb atoms from a hot vapour and cooling them in a magneto-optical trap, and further evaporation to quantum degeneracy in a crossed optical dipole trap operating at a wavelength of l = 1064nm, with accurate control of the power in both beams via a feedback loop. The evaporation of neutral atoms of 87Rb in far red detuned optical dipole traps using linearly polarised laser light is spin independent, and mixed spinor condensates of the F=1 manifold can be formed. As an empirical technique we found by applying a magnetic gradient field during the final evaporation that we can selectively populate mF spin states or prepare mixtures. This intriguing mechanism was found earlier as well by M. S. Chang, but is yet not fully understood and subject of our future research. We can now routinely prepare an almost pure condensate containing up to 7000 atoms in the condensed phase purely in the mF = 0 spin state. This thesis contains the theoretical framework of Bose-Einstein condensation with in mean-field theory, the concepts of cooling of atoms in magneto-optical traps, the trapping and evaporation in optical dipole traps and the experimental realisation. In the future this experiment will be extended by a fast scanning dipole trap to apply arbitrary time averaged potentials of more complex geometries, such as toroidal traps to study the effects of superfluidity in a periodic potential. The production of toroidal traps and cold atoms in such a trap are outlined in this thesis.
We develop the quantum-mechanical description of output coupling of macroscopic coherent matter waves from a Bose-Einstein condensate (BEC) via a radio-frequency field in the pulsed and cw limits for both strong and weak field coupling. The theory is converted into a Fock state description to point out the stimulated (in Bose particle) nature of the output coupling. A useful analogy with the theory of molecular photodissociation is used to explain the various regimes of output coupling BECs using radiation sources. We present specific calculations of the rates of output coupling as a function of the number of Bose atoms in the condensate and the frequency, detuning, and intensity of the radio-frequency field.