A magnetic trampoline for cold atoms.

It's just a strip of ordinary magnetic audio tape, recorded not with the sounds of Bruce Springsteen or Hootie & the Blowfish but with a pure, high-frequency tone. Yet researchers can use it as a mirror for reflecting superchilled atoms.

This represents "the first demonstration of atomic retroreflection from a surface with microscopic magnetic structure, opening the way to a simple new technique for atom manipulation without the use of laser light," Edward A. Hinds and his coworkers at Yale University report in the July 24 Physical Review Letters.

To observe these atomic bounces, the researchers cool rubidium atoms in a trap created by a combination of magnetic fields and intersecting laser beams. After the atoms reach 30 microkelvins (a temperature approaching absolute zero), the trap is briefly turned off, releasing some of the atoms, which then begin to fall because of gravity.

These atoms behave like little bar magnets. Passing through a weak magnetic field, they align themselves in a particular direction. When the falling atoms reach the specially prepared audio tape, the intense magnetic field created by oriented domains at its surface repels the atoms, forcing them back upward to be recaptured by the trap.

Using strips of audio tape, the researchers can keep atoms bouncing up and down for about half a second before the atoms spread out and start missing the mirror. But by using a computer floppy disk, deformed to create a concave surface, they can focus the atoms to keep them rebounding longer.

Suitably magnetized surfaces may prove useful in the storage and manipulation of cold atoms, Hinds and his colleagues conclude. For example, magnetic mirrors may eventually serve to bounce moving atoms into narrow beams or to keep them safely in cold storage.