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arXiv:cond-mat/0201400 · Physical Review Letters

Microscopic Dynamics in a Strongly Interacting Bose-Einstein Condensate

N. R. Claussen, Elizabeth A. Donley, Sarah T. Thompson, Carl Wieman

Published Jun 14, 2002• 99 citations
arXivHTMLPDF

Abstract

An initially stable 85Rb Bose-Einstein condensate (BEC) was subjected to a carefully controlled magnetic field pulse near a Feshbach resonance. This pulse probed the strongly interacting regime for the BEC, with the diluteness parameter (na(3)) ranging from 0.01 to 0.5. Condensate number loss resulted from the pulse, and for triangular pulses shorter than 1 ms, decreasing the pulse length actually increased the loss, until very short time scales (approximately 10 micros) were reached. The observed time dependence is very different from that expected in traditional inelastic loss processes, suggesting the presence of new microscopic BEC physics.

DOI
10.1103/physrevlett.89.010401

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An initially stable 85Rb Bose-Einstein condensate (BEC) was subjected to a carefully controlled magnetic field pulse near a Feshbach resonance. This pulse probed the strongly interacting regime for the BEC, with the diluteness parameter (na(3)) ranging from 0.01 to 0.5. Condensate number loss resulted from the pulse, and for triangular pulses shorter than 1 ms, decreasing the pulse length actually increased the loss, until very short time scales (approximately 10 micros) were reached. The observed time dependence is very different from that expected in traditional inelastic loss processes, suggesting the presence of new microscopic BEC physics.
An initially stable 85 Rb Bose-Einstein condensate (BEC) was subjected to a carefully controlled magnetic field pulse in the vicinity of a Feshbach resonance.This pulse probed the strongly interacting regime for the condensate, with calculated values for the diluteness parameter (na 3 ) ranging from 0.01 to 0.5.The field pulse was observed to cause loss of atoms from the condensate on remarkably short time scales (≥10 µs).The dependence of this loss on magnetic field pulse shape and amplitude was measured.For triangular pulses shorter than 1 ms, decreasing the pulse length actually increased the loss, until extremely short time scales (a few tens of microseconds) were reached.Such time scales and dependencies are very different from those expected in traditional condensate inelastic loss processes, suggesting the presence of new microscopic BEC physics.
A dilute Bose-Einstein condensate (BEC) near a Feshbach resonance provides experimental physics with a clean and controllable system to investigate strongly interacting many-body systems. The ability to tune the scattering length allows BECs to be projected onto strong interactions from an initial weakly interacting state. However, historically, studying a bulk 3D strongly interacting BEC has been diffcult, as these systems are inherently unstable due to three-body inelastic collisions. Thus, 2D, 1D, lattice confined, and two component Fermi gases were used to explore the strong interactions in an ultracold gas. In this thesis, I present the first measurement of a strongly interacting 3D 85Rb BEC. I introduce our experimental system and the techniques we used to probe the BEC. I first report on probes of a BEC in the weakly interacting regime. To probe a spherical 85Rb BEC with strong interactions we implemented a novel experimental technique that allowed us to quickly change the interactions, bypassing inelastic losses. We projected the BEC onto unitarity, where the scattering length diverges and the interactions are infinite, in order to observe dynamics and the unexpectedly long lifetime of the gas. Additionally, we observe a universality of the gas with respect to the length scale set by the interparticle spacing.
We discuss the response of a Bose-Einstein condensate to a change in the scattering length, which is experimentally realized by tuning the magnetic field near a Feshbach resonance. In particular, we consider the collapse of the condensate induced by a sudden change in the scattering length from a large positive to a small negative value. We also consider the condensate dynamics that results from a single pulse in the magnetic field, due to which the scattering length is rapidly increased from zero to a large value and then after some time rapidly decreased again to its initial value. We focus primarily on the consequences of the quantum evaporation process on the dynamics of the Bose-Einstein condensate, but also discuss the effects of atom-molecule coherence.
Bose-Einstein condensation has been achieved in a magnetically trapped sample of 85Rb atoms. Stable condensates of up to 104 atoms have been created by using a magnetic-field-induced Feshbach resonance to reverse the sign of the zero-field scattering length. These condensates provide unique opportunities for the study of BEC physics. The variation of the scattering length near the resonance has been used to magnetically tune the condensate self-interaction energy over a very wide range. This range extended from very strong repulsive self-interactions to large attractive ones. The effect of moving the condensate through the Feshbach resonance has been studied and compared with theory. Long-lived metastable condensates with attractive interactions have been produced near the zero of the Feshbach resonance. The transition from repulsive to attractive interactions can lead to a “collapse” of the condensate in which the cloud shrinks below our resolution limit, loses a significant number of atoms due to inelastic losses, and emits a burst of high-energy atoms.
Bose-Einstein condensation has been achieved in a magnetically trapped sample of 85Rb atoms. Long-lived condensates of up to 10(4) atoms have been produced by using a magnetic-field-induced Feshbach resonance to reverse the sign of the scattering length. This system provides new opportunities for the study of condensate physics. The variation of the scattering length near the resonance has been used to magnetically tune the condensate self-interaction energy over a wide range, extending from strong repulsive to large attractive interactions. When the interactions were switched from repulsive to attractive, the condensate shrank to below our resolution limit, and after approximately 5 ms emitted a burst of high-energy atoms.
The properties of Bose-Einstein condensed gases can be strongly altered by tuning the external magnetic field near a Feshbach resonance. Feshbach resonances affect elastic collisions and lead to the observed modification of the scattering length. However, as we report here, the observed rate of inelastic collisions was strongly enhanced in a sodium Bose-Einstein condensate when the scattering length was tuned to both larger or smaller values than the off-resonant value. These strong losses impose severe limitations for using Feshbach resonances to tune the properties of Bose-Einstein condensates.