'One and the Same' Atom Laser Displays Unique Behavior

Daniel S. Burgess

Scientists investigating one-atom at California Institute of Technology in Pasadena lasers have developed a device that displays no threshold for lasing and uniquely regular emissions. Unlike previous one-atom lasers and masers, which operate by exciting single atoms in a stream that passes through an optical cavity, the new setup employs only one atom. They call the arrangement a "one and the same" atom laser.

H. Jeff Kimble, the William L. Valentine professor and professor of physics at the institute, explained that the setup uses transitions around the 852.4-nm D₂ line in cesium and an external laser to pump the cesium atom to an excited level. Because the lasing transition is resonant with and strongly coupled to a mode of the cavity, the atom emits predominantly into the high-finesse (4.2 X 10⁵ at 852 nm), 42.2 X 23.6-µm cavity, leading to lasing, rather than decaying into free space. The researchers confine the atom using a standing-wave far-off-resonance optical trap with a wavelength of 935.6 nm.

A new device called a "one and the same" atom laser displays no threshold for lasing and "manifestly quantum" emissions. In the setup, a standing-wave far-off-resonance optical trap confines a single cesium atom in a high-finesse cavity, and an external laser pumps the atom to an excited level. Courtesy of H. Jeff Kimble.

The behavior of the one-and-the-same atom laser differs from conventional lasers in two respects. Firstly, it is thresholdless, so lasing occurs as soon as the pump intensity is nonzero. At higher pump intensities, however, the output flux saturates because a single atom can be recycled back to the upper laser level for successive emissions at some maximum rate. Secondly, Kimble said, the emitted radiation is "manifestly quantum," yielding photons that are spread more regularly in time, one from another, compared with the Poisson stream from a conventional laser.

The new device primarily is a curiosity that reveals how a laser behaves when it is miniaturized to the degree that intrinsic quantum fluctuations in the system dominate its behavior. "First and foremost, this is a scientific advance that presses laser operation to its conceptual limit, namely, one atom strongly coupled to an optical cavity," Kimble said. It is hoped, however, that such emitters will find a place in applications such as quantum cryptography in the near term and, eventually, quantum computing.

"Ultimately," he said, "we are attempting to develop tools for the implementation of quantum circuits and for quantum networks -- the 'quantum Internet.'