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Laser Trapping of Lithium Fermions

Off-resonance optical traps have been explored for many years as an attractive means of tightly confining neutral atoms [1]. Far off resonance optical traps (FORTs) employ large detunings from resonance to achieve low optical heating rates and high density, as well as to enable trapping of multiple atomic spin states in nearly identical potentials [2–6]. For CO2 laser traps [7], the extremely large detuning from resonance and the very low optical frequency lead to optical scattering rates that are measured in photons per atom per hour. Hence, optical heating is negligible. Such traps are potentially important for development of new standards and sensors based on spectroscopic methods, for precision measurements such as determination of electric dipole moments in atoms [8], and for fundamental studies of cold, weakly interacting atomic or molecular vapors. However, all-optical atom traps have suffered from unexplained heating mechanisms that limit the minimum attainable temperatures and the maximum storage times in an ultrahigh vacuum [4,9,10]. Recently, we have shown that to achieve long storage times in all-optical traps that are not limited by optical heating rates, heating arising from laser intensity noise and beam pointing noise must be stringently controlled [11,12]. Properly designed CO2 lasers are powerful and extremely stable in both frequency and intensity [13,14], resulting in laser-noise-induced heating times that are measured in hours. Hence, in an ultrahigh vacuum (UHV) environment, where loss and heating arising from background gas collisions are minimized [15,16], extremely long storage times should be obtainable using ultrastable CO2 laser traps. In this Letter, we report storage of Li fermions in an ultrastable CO2 laser trap. Trap 1/e lifetimes of 300 seconds are obtained, consistent with a background pressure of 10−11 Torr. This constitutes the first experimental proof of principle that extremely long storage times can be achieved in all-optical traps. Since arbitrary hyperfine states can be trapped, this system will enable exploration of s-wave scattering in a weakly interacting fermi gas. The well-depth for a focused CO2 laser trap is determined by the induced dipole potential U = −αg Ē2/2, where αg is, to a good approximation, the ground state static polarizability [7], and Ē2 is the time average of the square of the laser field. In terms of the maximum laser intensity I for the gaussian CO2 laser beam, the ground state well-depth U0 in Hz is