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The Atom Laser

A brief commentary by Wolfgang Ketterle
Dept. of Physics and Research Laboratory of Electronics, MIT

Recent work at MIT has realized an atom laser. In this note, the concept and properties of an atom laser are discussed, and also the techniques which were necessary to demonstrate the atom laser.
What is an atom laser?

An atom laser is analogous to an optical laser, but it emits matter waves instead of electromagnetic waves. Its output is a coherent matter wave, a beam of atoms which can be focused to a pinpoint or can be collimated to travel large distances without spreading. The beam is coherent, which means, for instance, that atom laser beams can interfere with each other. Compared to an ordinary beam of atoms, the beam of an atom laser is extremely bright. One can describe laser-like atoms as atoms "marching in lockstep". Although there is no rigorous definition for the atom laser (or, for that matter, an optical laser), all people agree that brightness and coherence are the essential features.
The parts of an atom laser

A laser requires a cavity (resonator), an active medium, and an output coupler. In the MIT atom laser, the "resonator" is a magnetic trap in which the atoms are confined by "magnetic mirrors". The active medium is a thermal cloud of ultracold atoms, and the output coupler is an rf pulse which controls the "reflectivity" of the magnetic mirrors.
The gain process in an atom laser

The analogy to spontaneous emission in the optical laser is elastic scattering of atoms (collisions similar to those between billiard balls). In a laser, stimulated emission of photons causes the radiation field to build up in a single mode. In an atom laser, the presence of a Bose-Einstein condensate (atoms that occupy a "single mode" of the system, the lowest energy state) causes stimulated scattering by atoms into that mode. More precisely, the presence of a condensate with N atoms enhances the probability that an atom will be scattered into the condensate by N+1.

In a normal gas, atoms scatter among the many modes of the system. But when the critical temperature for Bose-Einstein condensation is reached, they scatter predominantly into the lowest energy state of the system, a single one of the myriad of possible quantum states. This abrupt process is closely analogous to the threshold for operating a laser, when the laser suddenly switches on as the supply of radiating atoms is increased.

In an atom laser, the "excitation" of the "active medium" is done by evaporative cooling - the evaporation process creates a cloud which is not in thermal equilibrium and relaxes towards colder temperatures. This results in growth of the condensate. After equilibration, the net "gain" of the atom laser is zero, i.e., the condensate fraction remains constant until further cooling is applied.

Unlike optical lasers, which sometimes radiate in several modes (i.e. at several nearby frequencies) the matter wave laser always operates in a single mode. The formation of the Bose condensate actually involves "mode competition": the first excited state cannot be macroscopically populated because the ground state "eats up all the pie".
The output of an atom laser

The output of an optical laser is a collimated beam of light. For an atom laser, it is a beam of atoms. Either laser can be continuous or pulsed - but so far, the atom laser has only been realized in the pulsed mode. Both light and atoms propagate according to a wave equation. Light is governed by Maxwell's equations, and matter is described by the Schroedinger equation. The diffraction limit in optics corresponds to the Heisenberg uncertainty limit for atoms. In an ideal case, the atom laser emits a Heisenberg uncertainty limited beam.
Differences between an atom laser and an optical laser

* Photons can be created, but not atoms. The number of atoms in an atom laser is not amplified. What is amplified is the number of atoms in the ground state, while the number of atoms in other states decreases.
* Atoms interact with each other - that creates additional spreading of the output beam. Unlike light, a matter wave cannot travel far through air.
* Atoms are massive particles. They are therefore accelerated by gravity. A matter wave beam will fall like a beam of ordinary atoms.
* A Bose condensates occupies the lowest mode (ground state) of the system, whereas lasers usually operate on very high modes of the laser resonator.
* A Bose condensed system is in thermal equilibrium and characterized by extremely low temperature. In contrast, the optical laser operates in a non-equilibrium situation which can be characterized by a negative temperature (which means "hotter" than infinite temperature!). There is never any population inversion in evaporative cooling or Bose condensation.

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