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Feshbach Resonance Cooling of Atomic Gases

Josh W. Dunn, Chris H. Greene, D. Blume, Brian Granger, Murray Holland

Published Aug 7, 2002• 0 citations
OpenAlex

Abstract

A method is proposed that utilizes a Feshbach resonance to cool an atomic gas to temperatures of quantum degeneracy. Through application of a sequence of alternating adiabatic and nonadiabatic magnetic field ramps in the region of a Feshbach resonance, a large number of atom pairs in a trapped gas lose energy at the expense of a relatively small number of pairs which gain energy. After removal from the trap of the atom pairs which gain energy, the remaining reduced energy atoms will equilibrate to a lower temperature.

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A method is proposed that utilizes a Feshbach resonance to cool an atomic gas to temperatures of quantum degeneracy. Through application of a sequence of alternating adiabatic and nonadiabatic magnetic field ramps in the region of a Feshbach resonance, a large number of atom pairs in a trapped gas lose energy at the expense of a relatively small number of pairs which gain energy. After removal from the trap of the atom pairs which gain energy, the remaining reduced energy atoms will equilibrate to a lower temperature.
Spectroscopic studies of few-body systems at ultracold temperatures provide valuable information that often cannot be extracted in a hot environment. Considering a pair of atoms, we propose a cooling mechanism that makes use of a scattering Feshbach resonance. Application of a series of time-dependent magnetic field ramps results in either zero, one, or two atoms remaining trapped. If two atoms remain in the trap after the field ramps are completed, then they have been cooled. Application of the proposed cooling mechanism to optical traps or lattices is considered.
The entropy and kinetic, potential, and interaction energies of an atomic Fermi gas in a trap are studied under the assumption of thermal equilibrium for finite temperature. A Feshbach resonance can cause the fermions to pair into diatomic molecules. The entropy and energies of mixtures of such molecules with unpaired atoms are calculated, in relation to recent experiments on molecular Bose-Einstein condensates produced in this manner. It is shown that, starting with a Fermi gas of temperature T=0.1TF0T=0.1{T}_{F}^{0}T=0.1TF0​, where TF0{T}_{F}^{0}TF0​ is the noninteracting Fermi temperature, an extremely cold degenerate Fermi gas of temperature $T\ensuremath{\lesssim}0.01{T}_{F}^{0}$ may be produced without further evaporative cooling. This requires adiabatic passage of the resonance, subsequent sudden removal of unpaired atoms, and adiabatic return. We also calculate the ratio of the interaction energy to the kinetic energy, a straightforward experimental signal which may be used to determine the temperature of the atoms and indicate condensation of the molecules.
We have observed condensation of fermionic atom pairs in the BCS-BEC crossover regime. A trapped gas of fermionic 40K atoms is evaporatively cooled to quantum degeneracy and then a magnetic-field Feshbach resonance is used to control the atom-atom interactions. The location of this resonance is precisely determined from low-density measurements of molecule dissociation. In order to search for condensation on either side of the resonance, we introduce a technique that pairwise projects fermionic atoms onto molecules; this enables us to measure the momentum distribution of fermionic atom pairs. The transition to condensation of fermionic atom pairs is mapped out as a function of the initial atom gas temperature T compared to the Fermi temperature T(F) for magnetic-field detunings on both the BCS and BEC sides of the resonance.
We present a cooling method for a strongly-interacting trapped quantum gas. By applying a magnetic field modulation with frequencies close to the binding energy of a molecular bound state we selectively remove dimers with high kinetic energy from the sample. We demonstrate cooling of the sample over a wide range of interaction strengths and measure a high cooling efficiency of γ=4γ=4γ=4 that exceeds all previous cooling near Feshbach resonances.
We measure the scaling laws for the number of atoms and the cloud size as a function of trap depth for evaporative cooling of a unitary Fermi gas in an optical trap. A unitary Fermi gas comprises a trapped mixture of atoms in two hyperfine states which is tuned to a collisional (Feshbach) resonance using a bias magnetic field. Near resonance, the zero energy s-wave scattering length diverges, and the s-wave scattering cross-section is limited by unitarity to be 4π/ k 2 , where k is the relative wavevector of the colliding particles. In this case, the collision cross-section for evaporation scales inversely with the trap depth, enabling runaway evaporation under certain conditions. We demonstrate high evaporation efficiency, which is achieved by maintaining a high ratio η of trap depth to thermal energy as the trap depth is lowered. We derive and demonstrate a trap lowering curve which maintains η constant for a unitary gas. This evaporation curve yields a quantum degenerate sample from a classical gas in a fraction of a second, with only a factor of three loss in atom number.
We optimize a collision-induced cooling process for ultracold atoms in the nondegenerate regime. It makes use of a Feshbach resonance, instead of rf radiation in evaporative cooling, to selectively expel hot atoms from a trap. Using functional minimization we analytically show that for the optimal cooling process the resonance energy must be tuned such that it linearly follows the temperature. Here, optimal cooling is defined as maximizing the phase-space density after a fixed cooling duration. The analytical results are confirmed by numerical Monte Carlo simulations. In order to simulate more realistic experimental conditions, we show that background losses do not change our conclusions, while additional nonresonant two-body losses make a lower initial resonance energy with nonlinear dependence on temperature preferable.