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Quantum Electronics and Laser Science Conference

Preparation of a degenerate, two-component Fermi gas by evaporation in a single beam optical trap

S. R. Granade, Michael E. Gehm, Kenneth M. O’Hara, John E. Thomas

Published Jun 25, 2003• 1 citation
Publisher

Abstract

Summary form only given. We have recently produced a degenerate mixture of the two lowest hyperfine states of fermionic /sup 6/Li in a stable, single-beam CO/sub 2/ laser trap. The trap is loaded from a magneto-optical trap (MOT) at an initial temperature of 150 /spl mu/K. Degeneracy is obtained by forced evaporation, accomplished by continuously lowering the trap depth; the trap is then adiabatically recompressed to full depth. After 60 seconds of forced evaporation, more than 10/sup 5/ atoms remain at temperatures below half of the Fermi temperature. These results are consistent with scaling laws we have derived for the phase-space density as a function of trap depth. This degenerate system is predicted to exhibit a Feshbach resonance at 850 G.

DOI
10.1109/qels.2002.1031266

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Conference on Lasers and Electro-Optics•Aug 29, 2005•0 citations
Summary form only given. We have recently produced a degenerate mixture of the two lowest hyperfine states of fermionic /sup 6/Li in a stable, single-beam CO/sub 2/ laser trap. The trap is loaded from a magneto-optical trap (MOT) at an initial temperature of 150 /spl mu/K. Degeneracy is obtained by forced evaporation, accomplished by continuously lowering the trap depth; the trap is then adiabatically recompressed to full depth. After 60 seconds of forced evaporation, more than 10/sup 5/ atoms remain at temperatures below half of the Fermi temperature. These results are consistent with scaling laws we have derived for the phase-space density as a function of trap depth. This degenerate system is predicted to exhibit a Feshbach resonance at 850 G.
Summary form only given. Cold, dense gases of fermions offer exciting new opportunities for fundamental studies of quantum degeneracy, collective behavior, and superfluidity. Fermionic /sup 6/Li has been the subject of numerous theoretical treatments. This is due in part to its anomalously large and negative triplet scattering length, a/sub T/=-2160 a/sub O/, which arises from a near zero energy resonance. The large scattering length leads to two-state mixtures which are predicted to undergo a transition to a superfluid state at a relatively high temperature. To increase our well depth, we retroreflect the CO/sub 2/ beam which forms our optical trap, nearly doubling the intensity of the trapping field and thus the well depth. We will discuss this technique and its effect on evaporative cooling, and report our progress towards producing a degenerate gas of /sup 6/Li.
Summary form only given. Evaporative cooling of atoms in an optical trap has recently received renewed attention as a means for obtaining degeneracy in an optically trapped bosonic Cs vapor as well as a fermionic Li gas. We have developed a new model describing the evaporative cooling process for atoms confined in a time-dependent optical potential formed by a single focused Gaussian laser beam. We find that a substantial increase in the phase-space density can be obtained by adiabatically reducing the trap laser power and thereby the trap well depth as a function of time. Since we are specifically interested in the evaporative cooling of fermionic /sup 6/Li atoms, we have included the effect of Fermi statistics in the model. Although the collision rate is suppressed as the temperature T is reduced below the Fermi temperature T/sub F/, we find that values of T/T/sub F//spl Lt/1 can be achieved for suitable initial conditions.
We achieve degeneracy in a mixture of the two lowest hyperfine states of 6Li by direct evaporation in a CO2 laser trap, yielding the first all optically produced degenerate Fermi gas. More than 10(5) atoms are confined at temperatures below 4 microK at full trap depth, where the Fermi temperature for each state is 8 microK. This degenerate two-component mixture is ideal for exploring mechanisms of superconductivity ranging from Cooper pairing to Bose-Einstein condensation of strongly bound pairs.
Summary form only given. The last five years has seen an explosive growth in activity in the field of quantum degenerate gases, and there are now many realizations of Bose-Einstein condensation (BEC) of trapped atomic gasses. However, only one experiment has produced a quantum degenerate Fermi gas. The ultimate goal of the experiment discussed here is to achieve a BCS phase transition in a trapped gas of fermionic /sup 6/Li atoms. This requires a phase-space density that is much higher than that needed for BEC, as the temperature must be made much less than the Fermi temperature.BEC in trapped gases has only been achieved using forced evaporative cooling. Since this method depends on elastic collisions, however, it fails for an ultracold Fermi gas, as the Pauli principle forbids s-wave collisions between identical fermions. To circumvent this obstacle, we simultaneously trap a /sup 7/Li-/sup 6/Li mixture, and use rf-induced evaporation to cool /sup 7/Li. /sup 6/Li is then cooled via thermalizing elastic collisions with the /sup 7/Li. The clover-leaf-type magnetic trap is loaded from a two-species magneto-optical trap (MOT).
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.
Summary form only only given. We report on trapping of /sup 40/K atoms at low temperatures and high densities in a far-detuned optical trap. The trap is a vertical standing-wave created with about 550 mW of radiation tuned 10 nm on the red of the D/sub 2/ transition, and is loaded from a sample held in a magneto optical trap (MOT) at sub-Doppler temperatures, with the help of a temporal dark-spot phase and a magnetic compression phase. We are able to trap as many as 5/spl times/10/sup 6/ atoms with a lifetime of 200 ms in the F=9/2 ground state. From the temperature of the trapped atoms, which is around 30 /spl mu/K, and the measured trap frequencies (1 kHz radial, 500 kHz axial) we estimate a density of 5/spl times/10/sup 12/ cm/sup -3/.