Laser made from single atom
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
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
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
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,"
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,"
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
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
September 24/October 1, 2003
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