emits linked photons
Technology Research News
The way lasers work can only be explained
by quantum physics, the realm of atoms and subatomic particles. Lasers
stimulate already-energized atoms, causing them to emit energy in the
form of photons, the particles of light.
A team of researchers at the University of Oxford in England is taking
the technology deeper into the bizarre regions of quantum physics with
the development of a rudimentary laser that produces linked pairs of photons.
The work promises to make perfectly secure communications devices more
practical and advance long-term efforts to build ultra-powerful quantum
The device makes it easier to produce linked, or entangled, sets of two
or even four photons. The researchers have demonstrated "laser-like operation"
for entangled photons, said Antia Lamas-Linares, a graduate student at
the University of Oxford.
When two or more quantum particles become entangled, one or more of their
properties march in lockstep. For example, two photons can have their
polarizations, or electric field orientations, entangled.
But when photons are entangled they exist in an unmeasurable netherworld
of quantum mechanics where they are in some mixture of all possible polarizations
until one of the pair is observed or otherwise comes into contact with
the environment. When this happens, both photons are knocked out of entanglement
and into the same definite polarization, regardless of the physical distance
The usual way of producing pairs of entangled photons is shining ultraviolet
laser light into a crystal, which transforms a tiny percentage of the
ultraviolet photons into entangled pairs of infrared photons. The Oxford
device bounces the entangled photon pairs back into the crystal while
the laser is still shining on it. For each pair sent back into the crystal,
four new pairs are generated.
The laser action produces more pairs of entangled photons for the same
amount of power as non-lasing schemes, "and, perhaps more importantly,
higher-number entangled photon states," she said.
Ordinary conversion produces about 5,000 detectable photon pairs per second,
said Lamas-Linares. "Our source in its current form would produce four
times more pairs, and the number would grow exponentially with the number
of passes." In addition, the device entangles groups of four photons.
"Current sources produce about one 4-photon state per minute, while our
source will amplify this by a factor of 16, making it feasible to perform
experiments on them," she said.
The Oxford device currently passes the light through the crystal only
twice. Ordinary lasers use a reflective chamber, or cavity, to bounce
light back and forth through a gas hundreds of times, each pass causing
the gas atoms to emit more photons.
The researchers' next step is to add a reflective cavity to their device,
making it more like a true laser and multiplying further the number of
entangled photons it could produce. "We are working on building a cavity
system... to obtain a more conventional lasing action," said Lamas-Linares.
The goal is to produce a device that can generate useful numbers of pairs
of entangled photons. "Entanglements are the main resource in quantum
information," said Lamas-Linares. "One of the main problems in the field
currently is to produce entanglement in a controllable and reliable way."
Current sources of entangled photons are not bright enough for some proposed
quantum information processing experiments and a brighter source would
make them possible, said Paul Kwiat, a professor of physics at the University
of Illinois. A true entangled-photon laser "would be a very bright source
of entanglement," he said.
The Oxford source of entangled photons could be used for quantum cryptography
in five years and is currently being used as a tool by physicists
to explore the fundamentals of quantum mechanics, said Lamas-Linares.
"That is really our main interest," she said.
Lamas-Linares' research colleagues were John C. Howell and Dik Bouwmeester
of the University of Oxford. They published the research in the August
30, 2001 issue of the journal Nature. The research was funded by the UK
Engineering and Physical Sciences Research Council (EPSRC), the UK Defense
Evaluation and Research Agency and the European Union (EU).
Timeline: 5 years
TRN Categories: Quantum Computing
Story Type: News
Related Elements: Technical paper, "Stimulated Emission
of Polarization-Entangled Photons," Nature, August 30, 2001
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