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Microscopic Dynamics in a Strongly Interacting Bose-Einstein Condensate

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

Published Jan 1, 2008• 0 citations
Publisher

Abstract

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.

DOI
10.1142/9789812813787_0084

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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.
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.
In a recent experiment at JILA [E. A. Donley et al., Nature (London) 417, 529 (2002)] an initially pure condensate of 85Rb{}^{85}\mathrm{Rb}85Rb atoms was exposed to a specially designed time-dependent magnetic-field pulse in the vicinity of a Feshbach resonance. The production of additional components of the gas as well as their oscillatory behavior have been reported. We apply a microscopic theory of the gas to identify these components and determine their physical properties. Our time-dependent studies allow us to explain the observed dynamic evolution of all fractions, and to identify the physical relevance of the pulse shape. Based on ab initio predictions, our theory strongly supports the view that the experiments have produced a molecular condensate.
Summary form only given. For most values of magnetic field, very cold /sup 85/Rb atoms have a large negative scattering length and large loss rates from magnetic traps. The latter makes it difficult to cool the atoms low enough to achieve Bose-Einstein condensation (BBC), while the former constrains the size of a possible BEC to <100 atoms. However, we have found that near the Feshbach resonance at 154 G the loss rates decrease substantially, and the scattering length can be made positive. We have used these two features to create condensates of several thousand atoms in /sup 85/Rb. We are then able to study the dynamics and loss processes of these condensates as we vary the magnetic field. Small changes in magnetic field give rise to large changes in the magnitude and sign of the self-interaction in the condensate as well as possible couplings to molecular condensates. We will discuss the wide range of behaviors we have observed in this interesting system.
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.
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.
We experimentally studied the spin-dependent collision dynamics of 87Rb^{87}\mathrm{Rb}87Rb spin-2 Bose-Einstein condensates confined in an optical trap. The condensed atoms were initially populated in the $\ensuremath{\mid}F=2,{m}_{F}=0⟩$ state, and their time evolutions in the trap were measured in the presence of external magnetic field strengths ranging from 0.1 to 3.0G3.0\phantom{\rule{0.3em}{0ex}}\mathrm{G}3.0G. The atom loss rate due to inelastic two-body collisions was found to be $1.4(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.3em}{0ex}}{\text{cm}}^{3}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. Spin mixing in the F=2F=2F=2 manifold developed dramatically for the first few tens of milliseconds, and the oscillations in the population distribution between different magnetic components were observed over a limited range of magnetic field strengths. The antiferromagnetic property of this system was deduced from the magnetic field dependence on the evolution of relative populations for each mF{m}_{F}mF​ component.