Refills make continuous-atom laser possible
In a so-called atom laser, a Bose-Einstein condensate (BEC) provides a coherent source of atoms that are formed into a beam.
by John Wallace
In a so-called atom laser, a Bose-Einstein condensate (BEC) provides a coherent source of atoms that are formed into a beam. Atom lasers will likely be used in very precise interferometers for basic physics researchhelping, for example, to fine-tune the experimental values of fundamental constants. Trapped as a cloud in an ultrahigh vacuum, the atoms in a BEC all lie in the same quantum state; such a cloud is formed in a batch process, severely limiting how long an atom laser operates. Now, a group at the Massachusetts Institute of technology (MIT; Cambridge, MA) has developed an optical-tweezers-based method of transporting sodium BEC clouds over meter-scale distances, then combining them. The result is a continuously replenishable BEC reservoir suitable for a continuous-wave (CW) atom laser.
A slowly leaking reservoir holding a sodium Bose-Einstein condensate (BEC) can be repeatedly replenished. Held by optical tweezers, a newly formed BEC refill (upper cigar-shaped cloud) is brought near the BEC-containing reservoir (top). The refill is lowered toward the reservoir (center) and finally merges with the BEC in the reservoir (bottom). Replenished every 18 s, the reservoir maintains at least 1 x 106 atoms at all times. Such a system can serve as the source for a continuous-wave atom laser.
In the MIT experiment, a BEC is created in a production chamber by a combination of laser and evaporative cooling, then transferred by a magnetic trap into optical tweezers at the focus of a 1064-nm Nd:YAG laser beam. The 500-mm-focal-length lens creating the tweezers is mounted to a translation stage that carries the BEC a distance of 0.3 m to a science chamber. After a pause to allow excitations to damp out, the BEC is brought close to a second optical trapa reservoir also created by a focused 1064-nm light beam. A glass slide placed in the converging transport beam is tilted to bring the tweezers focusand thus the BECin coincidence with the reservoir over a 0.5-s time period, allowing the reservoir to capture the atoms (see figure). The transport beam is then dimmed to zero power over another 0.5 s.
For the reservoir BEC to remain coherent enough for a CW atom laser, the incoming refill BEC must be phase-matched. A high radial-trap frequency of greater than 400 Hz ensures that, if the merger is done radially, radial excitations are damped out quickly. The refills have a random phase with respect to the reservoir BEC; to phase-match the two condensates, the refill can be "dressed" with a light field, says Ananth Chikkatur, one of the researchers. The dressed condensate can decay coherently into a stationary atom into the reservoir and a recoiling photon.
Atom lasers can be used to make better atomic clocks and gyroscopes, says Chikkatur. They might even someday deposit coherent atoms directly on integrated circuits to make finer lines for computer chips, he notes.
1. A. P. Chikkatur et al., Sciencexpress, 1 (May 16, 2002), 10.1126; www.sciencexpress.org.