Creating a laser beam of atoms is accomplished with an experimental configuration termed an "rf output coupler" by the MIT researchers who developed it. In this illustration, the disk-shaped coils generate magnetic fields which trap a group of millions of atoms (yellow circles) in a cigar-shaped cloud. Cooled to temperatures just billionths of a degree above absolute zero, and packed to sufficiently high density, the atoms form a special state of matter known as a Bose-Einstein condensate, in which the atoms collectively enter a single quantum state. Such Bose-condensed atoms are an ideal source of laserlike atoms.
(a) Each of the atoms possesses a property known as spin (indicated by the arrows); the value of spin describes how it will respond to a magnetic field. At first, the atoms have the same spin value, corresponding to a state in which they are pushed towards the center of the trap.
(b) However, when the MIT researchers apply short pulses of radiofrequency (rf) radiation (red squiggle), which contains magnetic fields of its own, it "tilts" the spins of the atoms by an adjustable angle.
(c) Quantum mechanically, "tilting" the spins means that the atoms have a probability of flipping their spins. A certain fraction of the atoms flips their spins, reversing the magnetic forces on them. This allows them to escape from the trap. The Bose-Einstein condensate is therefore split into a trapped cloud and an untrapped cloud.
(d) By applying several pulses of rf radiation, the researchers can create several pulses of atom-laser beams falling in succession. These clouds spread out and are accelerated by gravity.(Figure courtesy of Massachusetts Institute of Technology.)
This research is described in Physical Review Letters 78, 582 (27 January 1997).
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