We demonstrate an ultrastable CO 2 laser trap that provides tight confinement of neutral atoms with negligible optical scattering and minimal laser-noise-induced heating. Using this method, fermionic 6 Li atoms are stored in a 0.4 mK deep well with a 1͞e trap lifetime of 300 sec, consistent with a background pressure of 10 211 Torr. To our knowledge, this is the longest storage time ever achieved with an all-optical trap, comparable to the best reported magnetic traps. [S0031-9007(99)09193-0] PACS numbers: 32.80.PjOff-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][3][4][5][6]. For CO 2 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 the 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 which 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 CO 2 lasers are powerful and extremely stable in both frequency and intensity [13,14], resulting in laser-noiseinduced 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 CO 2 laser traps.In this Letter, we report storage of 6 Li fermions in an ultrastable CO 2 laser trap. Trap 1͞e lifetimes of 300 sec are obtained, consistent with a background pressure of 10 211 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 CO 2 laser trap is determined by the induced dipole potential U 2a gĒ 2 ͞2, where a g is, to a good approximation, the ground state static polarizability [7], andĒ 2 is the time average of the square of the laser field. In terms of the maximum laser intensity I for the Gaussian CO 2 laser beam, the ground state well depth U 0 in Hz isIn our experiments, a...

We derive a simple formula for the heating rate that arises from quantum-diffractive background gas collisions in atom traps. This result appears to explain the residual heating rates reported for recent experiments with a Cs vapor-loaded, far-detuned optical trap at Ӎ10 Ϫ9 Torr ͓Phys. Rev. Lett. 81, 5768 ͑1998͔͒. Diffractive collisions may determine the minimum heating rates achievable in shallow all-optical or magnetic atom traps operating at low temperature and high density. ͓S1050-2947͑99͒50307-7͔PACS number͑s͒: 32.80.Pj RAPID COMMUNICATIONS R32PRA 60 BALI, O'HARA, GEHM, GRANADE, AND THOMAS

We present measurements of the intensity dependence of excited-state collisions of laser-cooled %Rb atoms involving repulsive excited-state potentials. For high intensities, the collision rate decreases with increasing intensity, in accordance with a simple Landau-Zener treatment of the collision dynamics.

We demonstrate a first simultaneous measurement of the real and imaginary parts of the refractive index of a highly turbid medium by observing the real-time reflectance profile of a divergent laser beam made incident on the surface of the turbid medium. We find that the reflectance data are well described by Fresnel theory that correctly includes the effect on total internal reflection of angle-dependent penetration into the turbid medium.

We use an all-optical trap to confine a strongly attractive two-state mixture of lithium fermions. By measuring the rate of evaporation from the trap, we determine the effective elastic scattering cross section 4pia(2) to show that the magnitude of the scattering length |a| is very large, in agreement with predictions. We show that the mixture is stable against inelastic decay provided that a small bias magnetic field is applied. For this system, the s-wave interaction is widely tunable at low magnetic field, and can be turned on and off rapidly via a Raman pi pulse. Hence, this mixture is well suited for fundamental studies of an interacting Fermi gas.