Photons can now be emitted one at a time, an advance towards
quantum computers and cryptography
A tiny disc that emits light in an orderly procession of single photons, and
with no background noise, has been created.
The "photon turnstile" could ultimately transmit the keys to secret codes
safely past prying eyes or help computer scientists exploit the strange
laws of quantum mechanics.
Researchers have long tried to develop devices that emit one photon at
a time. In September, researchers reported that a single molecule could
do the trick. But the material surrounding the molecule produced a lot of
noise - an unwanted second photon popped out about 20% of the time.
However, the new semiconductor disc, made by a team from the
University of California, Santa Barbara, avoids this problem. "Our
principle improvement is that we were able to remove this background,"
says applied physicist Atac Imamoglu.
Imamoglu and colleagues embedded a "quantum dot" inside a
200-nanometer-thick disc of gallium arsenide. The dot is a dab of indium
arsenide less than 50 nanometres across and 3 nanometres thick. They
cooled the device to about 25 Kelvin and shone pulses of laser light on it.
The light sent electrons cruising through the gallium arsenide. These left
behind vacancies or "holes" that acted like positive charges floating
through the disk. An electron-hole pair would quickly settle onto the
quantum dot, where they would recombine to give off one photon of a
In the meantime, all the other electron-hole pairs would annihilate in
ways that produce no light, or light of the wrong wavelength. In this way,
the researchers obtained precisely one of the desired photons per laser
Extraneous photons didn't crop up because the disk was so small,
Imamoglu says. "The microdisc helps by reducing the volume of material
that is excited and thereby reducing the background," he says.
Keys and Q-bits
A stream of single photons could safely transmit code keys because a
spy would have to measure each photon. According to the laws of
quantum mechanics, this would alter its state, says team member
Christoph Milcher, now at the University of Bremen, in Germany. "You
can send a key with single photons so that no-one else can detect it
without you knowing," he says.
Two photons might also be joined through a process called
entanglement, Imamoglu says, to form a logic bit that can be 0, 1, or
0-and-1 at the same time. Such "q-bits" are essential for quantum
But currently, Imamoglu and colleagues capture only one in every
10,000 single-photon pulses. They hope to adjust the shape of their tiny
device to quickly raise that to a more practical one in every 100.