Entanglement on demand

One of the problems in quantum information processing is inefficiency. Photon entanglement is generally considered a leading candidate for quantum computing (it is used for teleportation and cryptography), but right now it is sort of a hit and miss proposition. “The only methods we have for generating entangled photons are random,” Yosi Avron tells PhysOrg.com. “You don’t always get an entangled pair, and when you do, after you test the photons, they are useless.”

Avron, a physicist at the Technion-Israel Institute of Technology, believes that he and his colleagues may have found a technique that could solve this problem of photon entanglement. Avron worked with Gershoni, Bisker, Lindner and Meirom at Technion-Israel, as well as Warburton at Heriot-Watt University in Edinburgh. The team’s scheme revolves around time reordering, and is explained in Physical Review Letters: “Entanglement on Demand through Time Reordering.”

“The key is something called switch path ambiguity,” explains Avron. He points out that in quantum mechanics, a particle can be in two places at once. “If you have two slits on a paper, and you send a photon toward them, the photon can go through both at once. It doesn’t have to choose. If you have a screen behind these two slits, the interference pattern that shows is the same behind each slit. The photon went through both.”

Avron goes on to explain that the key is in setting up different paths for the photons to follow, and in creating a situation in which two states give the same result. In that way, it would be possible to produced entangled photons on demand, rather than using hit and miss:

“If you have a system with an atom, you start it in an excited state. Then it relaxes and releases a photon at an intermediate state. Then it relaxes more and releases a photon at the ground state. The ambiguity comes in when we create a second intermediate state, identical to the first, so that you have a state where two photons are generated in alternative ways.”

It sounds nice, but Avron acknowledges that this where more problems arise. “It is difficult to get two states that are precisely identical like that,” he says. “The states have slightly different energies and this adversely affects entanglement.”

The solution the team came up with was to re-arrange the timing of the system. “This is not a new idea,” he qualifies, “but we were the first to create a believable theory of how it could be done.”

“All we have to do is reorder one of the paths,” Avron explains. “And if you do, then you could get a situation where you get entanglement because the states become identical…Instead of matching in each generation of photons, it is possible to match across generations and reorder them. Quantum mechanics allows for this without penalty.”

Avron is careful to point out that at this stage the results are theoretical. “But,” he continues, “we think that it is possible to design an experiment to test it. David Gershoni is working on such an experiment.”

The experiment may take one or two years to get done, but Avron is confident of the results. “It has been difficult to get high quality entanglement on demand,” he explains, “and what we have discovered is that it is possible and something that we can do in a few years’ time.”

Copyright 2007 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

Related stories:

Quantum computers take step toward practicality with demonstration of new device
Computers based on the powerful properties of quantum mechanics have the potential to revolutionize information technology and security, but for decades they have remained more theoretical than practical, and difficult to scale up. That is changing, however, as demonstrated in a report this week in the journal Science.
Measurement precision beats standard quantum limit
For physicists, measuring the precise magnitude of a physical quantity is a key to understanding quantum mechanics. However, there is a limit to how precise a measurement can be made, which is governed by quantum mechanical laws.
Can three-photon absorption lead to better bio-imaging?
One of the more interesting concepts being looked at in terms of quantum chemistry is that of three-photon absorption (3PA). 3PA works when three photons are simultaneously absorbed in one event. Because three photon absorption most commonly occurs in longer wavelengths (near infrared), some scientists see hope for it in terms of biomedical and photonic applications.
World's shortest single photon pulse created
The world’s shortest light pulse containing just one photon has been produced by Oxford University scientists.
T-REX is monster light source with multiple applications
When it comes to laser-based light sources, there are few brighter than T-REX, an LLNL project developed jointly by the NIF & Photon Science Principle Directorate and the Physical Sciences Directorate.
Bon MOT: Innovative atom trap catches highly magnetic atoms
A research team from the National Institute of Standards and Technology and the University of Maryland has succeeded in cooling atoms of a rare-earth element, erbium, to within two millionths of a degree of absolute zero using a novel trapping and laser cooling technique. Their recent report is a major step towards a capability to capture, cool and manipulate individual atoms of erbium, an element with unique optical properties that promises highly sensitive nanoscale force or magnetic sensors, as well as single-photon sources and amplifiers at telecommunications wavelengths. It also may have applications in quantum computing devices.
'Superdense' coding gets denser
The record for the most amount of information sent by a single photon has been broken by researchers at the University of Illinois. Using the direction of “wiggling” and “twisting” of a pair of hyper-entangled photons, they have beaten a fundamental limit on the channel capacity for dense coding with linear optics.
Loopy photons clarify 'spookiness' of quantum physics
Researchers at the National Institute of Standards and Technology and the Joint Quantum Institute (NIST/University of Maryland) have developed a new method for creating pairs of entangled photons, particles of light whose properties are interlinked in a very unusual way dictated by the rules of quantum physics. The researchers used the photons to test fundamental concepts in quantum theory.

News discussion:

Physics news