Tech Briefs

A Bose-Einstein condensate is adiabatically compressed to drive coherent spin-mixing evolution.

An atom laser now undergoing development simultaneously generates two pulsed beams of correlated 87Rb atoms. (An atom laser is a source of atoms in beams characterized by coherent matter waves, analogous to a conventional laser, which is a source of coherent light waves.) The pumping mechanism of this atom laser is based on spinor dynamics in a Bose-Einstein beam running-wave dipole trap that has been formed by focusing of a CO2-laser beam. By a technique that is established in the art, the trap is loaded from an ultra-high-vacuum magneto-optical trap that is, itself, loaded via a cold atomic beam from an upstream two-dimensional magneto-optical trap that resides in a rubidium-vapor cell that is differentially pumped condensate. By virtue of the angular-momentum conserving collisions that generate the two beams, the number of atoms in one beam is correlated with the number of atoms in the other beam. Such correlations are intimately linked to entanglement and squeezing in atomic ensembles, and atom lasers like this one could be used in exploring related aspects of Bose-Einstein condensates, and as components of future sensors relying on atom interferometry.

A Bose-Einstein Condensate of 87Rb atoms is shown at the instant of turning off the optical trap (0 ms) and at an instant 20 ms later. The original field depicted in these images measures 1 by 0.25 mm. Gravitation was directed toward the lower right; the trapping laser beam was aimed toward the upper right.
In this atom-laser apparatus, a Bose-Einstein condensate of about 2 × 106 87Rb atoms at a temperature of about 120 μK is first formed through all-optical means in a relatively weak single- from an adjoining vacuum chamber, wherein are performed scientific observations of the beams ultimately generated by the atom laser.

In the condensate as thus prepared, the atoms are in the magnetic-field-insensitive mF = 0 sublevel of the F = 1 state [where F is the quantum number of total resultant angular momentum (electron spin plus nuclear spin plus electron orbital angular momentum) and mF is the quantum number of the component of total resultant angular momentum along a physically distinguishable coordinate axis (typically defined by a magnetic field)]. Then the intensity of the trapping laser beam is increased to drive coherent spin-mixing evolution: The increase in the intensity of the trapping laser beam adiabatically compresses the condensate to cause 87Rb atoms to collide and thereby to undergo the angular-momentum-conserving reaction

2(mF =0)↔(mF =+1)+(mF =–1).

As a result of this reaction, the original condensate becomes a superposition of (1) equal numbers of atoms in the mF =+1 and mF =–1 levels and (2) some other number of atoms in the initial mF = 0 level.

Unlike the mF = 0 level, the mF = +1 and mF = –1 levels are sensitive to an applied magnetic field. Therefore, several milliseconds before turning off the optical trap, a suitably oriented magnetic field having a gradient is turned on. By virtue of their different sensitivities to the magnetic field, atoms in the mF = +1 level can be coupled out of the trap region in one direction and atoms in the mF = –1 level in a different direction (see figure), thereby generating the desired two pulsed beams containing equal numbers of atoms. (The mF = 0 atoms are affected only by the same gravitational force that affects the mF = +1 and mF = –1 atoms.)

This work was done by Robert Thompson, Nathan Lundblad, Lute Maleki, and David Aveline of Caltech for NASA’s Jet Propulsion Laboratory. NPO-43741

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