Laser emits linked photons

By Eric Smalley, 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 computers.

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 between them.

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
Funding:   Government
TRN Categories:   Quantum Computing
Story Type:   News
Related Elements:  Technical paper, "Stimulated Emission of Polarization-Entangled Photons," Nature, August 30, 2001


November 7, 2001

Page One

Hubs key to Net viruses

Electrified water spins gold into wire

Virtual reality gets easier

Laser emits linked photons

Dye brightens micromachines


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