Atom laser
A device that generates an intense coherent beam of atoms through a stimulated process. It does for atoms what an optical laser does for light. The atom laser emits coherent matter waves, whereas the optical laser emits coherent electromagnetic waves. Coherence means, for instance, that atom laser beams can interfere with each other. See Coherence
Laser light is created by stimulated emission of photons, a light amplification process. Similarly, an atom laser beam is created by stimulated amplification of matter waves. The conservation of the number of atoms is not in conflict with matter-wave amplification: The atom laser takes atoms out of a reservoir and transforms them into a coherent matter wave similar to the optical laser, which converts energy into coherent electromagnetic radiation (but, in contrast, the number of photons need not be conserved). See Laser
Elements
A laser requires a cavity (resonator), an active medium, and an output coupler (see table).
| Atom laser* | Optical laser |
| Atoms | Photons |
| Matter waves | Electromagnetic waves |
| Atom trap | Laser cavity |
| Atoms in the Bose condensate | Photons in the lasing mode |
| Thermal atoms | Gain medium |
| Evaporative cooling | Excitation of the gain medium |
| Stimulated scattering of atoms | Stimulated emission of photons |
| Critical temperature for | Laser threshold |
| Bose-Einstein condensation | |
| * Based on evaporative cooling. | |
Cavity
Various analogs of laser cavities for atoms have been realized. The most important ones are magnetic traps (which use the force of an inhomogeneous magnetic field on the atomic magnetic dipole moment) and optical dipole traps (which use the force exerted on atoms by focused laser beams). See Particle trap
Active medium
The active medium is a reservoir of atoms which are transferred to one state of the confining potential, which is the analog of the lasing mode. The reservoir can be atoms confined in other quantum states of the atom cavity or an ultraslow atomic beam. The atoms are transferred to the lasing mode either by collisions or by optical pumping. The transfer of atoms is efficient only for an ultracold sample, which is prepared by laser cooling or evaporative cooling. This cooling ensures that the atoms in the reservoir occupy only a certain range of quantum states which can be efficiently coupled to the lasing mode.
Output coupler
The output coupler extracts atoms from the cavity, thus generating a pulsed or continuous beam of coherent atoms. A simple way to accomplish this step is to switch off the atom trap and release the atoms. This method is analogous to cavity dumping for an optical laser, and extracts all the stored atoms into a single pulse. A more controlled way to extract the atoms requires a coupling mechanism between confined quantum states and propagating modes.
Such a beam splitter for atoms can be realized by applying the Stern-Gerlach effect to atoms in a magnetic trap. Initially, all the atoms have their electron spin parallel to the magnetic field, say spin up, and in this state they are confined in the trap. A short radio-frequency pulse rotates (tilts) the spin of the atoms by a variable angle. Quantum-mechanically, a tilted spin is a superposition of spin up and spin down. Since the spin-down component experiences a repulsive magnetic force, the cloud of atoms is split into a trapped cloud and an out-coupled cloud. By using a series of radio-frequency pulses, a sequence of coherent atom pulses can be formed. These pulses are accelerated downward by gravity and spread out. See Quantum mechanics
The illustration shows such a sequence of coherent pulses. In this case, sodium atoms are coupled out from a magnetic trap by radio-frequency pulses every 5 ms. The atom pulses are observed by illuminating them with resonant laser light and imaging their shadows, which are caused by absorption of the light. Each pulse contains 105–106 sodium atoms.
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Potential applications
Although a basic atom laser has now been demonstrated, major improvements are necessary before it can be used for applications, especially in terms of increased output power and reduced overall complexity. The atom laser provides ultimate control over the position and motion of atoms at the quantum level, and might find use where such precise control is necessary, for example, for precision measurements of fundamental constants, tests of fundamental symmetries, atom optics (in particular, atom interferometry and atom holography), and precise deposition of atoms on surfaces. See Fundamental constants, Symmetry laws (physics)
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