A Bose-Einstein condensate of a trapped gas of atoms is similar to the photons stored in the optical cavity of a laser: both are systems of many quantum particles occupying a single quantum state. We can develop a rather complete analogy between the so-called "atom laser" and the traditional photon laser. The magnetic trap used in BEC research is analogous to the optical cavity of the laser. The bath of thermal, uncondensed atoms is analogous to the gain medium of a laser, and evaporative cooling serves the role of the pump. The final component to make a laser useful is the output coupler, usually one of the mirrors forming the cavity which is made partially transmitting. This output coupler "leaks" out a small portion of the stored photons, forming the familiar laser beam.

In what is regarded as the first demonstration of an atom laser (M.-O. Mewes, et al., Phys. Rev. Lett. 78, 582 (1997)), the MIT group of Wolfgang Ketterle used pulses of radio frequency radiation to flip the spins of the trapped atoms, so that they fell out of the trap. This led to a series of spreading output pulses. We have developed an output coupler for the atom laser which uses laser pulses to stimulate Raman transitions that change the internal state (flip the spin) of the atom to one which no longer feels the trapping force. This also imparts 6 cm/s of velocity to the outgoing atoms. (In related work (in PDF Format), we developed Bragg diffraction techniques to impart controlled velocities to BECs or portions of them.) This technique produces a well-collimated beam of sodium atoms with a deBroglie wavelength of 295 nm, which should have coherence properties similar to those of an optical laser. Because our trap uses a rotating magnetic field we had to pulse our atom laser synchronously, but the atom pulses were almost completely overlapping forming an essentially continuous stream of atoms. The duration of this stream is limited only by the finite number in the condensate.

Here is an image of the atom laser beam, with the BEC source the bright region on the left-hand side. The beam was formed with 140 overlapping pulses, each separated by 50 µs.

Download a PDF version of our atom laser paper [655 KB], E.W. Hagley, et al., Science 283, 1706 (1999).

Download a PDF version of our Bragg diffraction paper [318 KB], M. Kozuma, et al., Phys. Rev. Lett. 82, 871 (1999).

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