Laser made from single atom

By Eric Smalley, Technology Research News

California Institute of Technology researchers have made a laser from a single atom.

The laser has the smallest active region possible and emits quantum light. Quantum light beams contain streams of individual photons rather than the usual, randomly timed bunches of photons.

The device could eventually be used as a source of single photons for quantum cryptography systems, which guarantee perfect security, and for quantum computers, which have the potential to crack secret codes and search large databases far faster than ordinary computers.

The photons produced by ordinary lasers come in randomly timed bunches because they are generated by the random interactions of multiple atoms. Lasers made from nanowires 1,000 times finer than a human hair still consist of millions of atoms. Even lasers formed from streams of individual atoms, which require only one atom to be present at any moment in the active part of the laser, employ multiple atoms.

The idea of making a laser from a single atom has been around for years, but producing one involved a particularly tricky technical challenge.

The difficult part was holding a single atom still enough for long enough to cause it to emit laser light, said H. Jeff Kimble, a professor of physics at Caltech. The atom produced a continuous beam of near-infrared light for the 50 to 100 milliseconds that the researchers managed to hold it in a trap. The advance presses a laser operation to its conceptual limit, he said.

Electrons orbit an atom's nucleus at specific energy levels. When atoms are pumped with energy the electrons at the lowest energy level, or ground state, move to a higher energy level, then move back down by giving off a photon -- the process of spontaneous emission. If an atom that is already at a higher energy level absorbs a photon, it releases two photons when it returns to the ground state -- the process of stimulated emission.

Lasers confine a set of atoms between two mirrors, which causes the emitted photons to bounce back into the atoms, pumping more atoms to the higher, excited, energy levels. As soon as there is more stimulated emission than spontaneous emission, the laser emits a beam of light.

The key to the single-atom laser was an optical trap that held the cesium atom nearly still between the two mirrors in the researchers' laser during the transitions between energy levels. The atom held nearly still because it was attracted to the high intensity of the trap beam's focal point.

Meanwhile, the space between the device's mirrors was small enough that a single photon was enough to produce stimulated emission. When the researchers used an ordinary laser to pump the lone atom to an excited state, enough emitted photons reflected back to the atom to set off stimulated emissions and produce a very weak quantum laser beam.

Rather than coming out in random bunches, the photons in the researchers' single-atom laser beam are generated one at a time and at more predictable moments, a quantum trait that is potentially very useful.

If the researchers can control when the photons are emitted, their device would provide the steady source of single photons required for quantum cryptography methods to reach their potential for perfectly secure communications. The ability to produce an orderly stream of photons is also potentially useful for quantum computing researchers, who are working to use the traits of particles like atoms and photons to represent the 1s and 0s of computer information.

The Caltech demonstration is a "technical tour de force," said Mark Saffman, a the assistant professor of physics at the University of Wisconsin. "Despite the conceptual simplicity, experimental demonstration [of a single-atom laser] is extremely difficult," he said.

The device has the potential to serve as a source of quantum light for quantum computing and communications applications, "but in its present incarnation the experiment requires very complex technical resources," said Saffman.

The work is part of an effort to develop tools for building quantum networks, said Kimble. "The immediate next step is to move from random emission times to gated, deterministic emission times for single photons," he said.

Any technological applications are many years away, said Kimble.

Kimble's research colleagues were Jason McKeever, Andreea Boca, David Boozer and Joseph Buck. They published the research in the September 18, 2003 issue of Nature. The research was funded by the National Science Foundation (NSF), the Army Research Office (ARO) and the Office of Naval Research (ONR).

Timeline:   Unknown
Funding:   Government
TRN Categories:   Optical Computing, Optoelectronics and Photonics; Quantum Computing and Communications; Physics
Story Type:   News
Related Elements:  Technical paper, "Experimental Realization of a One-Atom Laser in the Regime of Strong Coupling," Nature, September 18, 2003


September 24/October 1, 2003

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