Dynamic atom optics to produce ultraslow, ultracold helium atoms: Design study and possible applications

Atoms, unlike photons, travel at velocities readily attainable in the laboratory, making possible novel atom optical devices based on optical elements which move relative to the atomic beam. We report design studies of a device to accelerate or decelerate and simultaneously monochromatize a helium atomic beam. A helium supersonic free-jet (v equals 950 m/s, (Delta) v equals 5 m/s, FWHM) is reflected from a moving surface to produce an output beam is continuously tunable in velocity from epithermal (2000 m/s) to ultra-slow (ca. 1 m/s, De Broglie wavelength 1000 angstroms). The decelerator is highly dispersive in the low velocity regime, making the collimated ultra-slow output beam nearly monochromatic, i.e. ultra-cold. Measured properties of an actual supersonic helium free-jet expansion were used in Monte Carlo simulations of the decelerator to predict the attainable decelerated beam properties. Ultra- slow beams of usable intensity at T < 100 nK appear attainable. Finite element stress calculations for a magnetically suspended rotor verify that the necessary reflector velocities (475 m/s) can be safely achieved. Possible applications of an ultra-cold helium beam might include gas-surface scattering experiments to probe 'quantum reflection' of atoms from surfaces and to measure gas- surface bound states of exceptionally low binding energy, thereby testing the contributions of retardation to the gas- surface potential.