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SYDNEY: Australian researchers have demonstrated a way of storing quantum pulses of light that could be used in an optical memory device, similar to the way computers store and retrieve digital information.
The technology, developed by a team at the Australian National University (ANU), in Canberra, also has applications in quantum cryptography, a secure means of communication that utilises properties of light to encrypt information.
Cryptography works if the information sender and receiver share a secret key.
Quantum repeater
"At the moment this is done by people travelling the globe carrying secret keys in briefcases," said team member and quantum physicist Ben Buchler.
"This works providing the guy with the briefcase isn't kidnapped. What quantum cryptography does is distribute the key, and you do that by using light," he said.
Currently quantum cryptography can only work along fibres less than 50 to 100 km long, because beyond this distance photons are scattered and absorbed by imperfections in the fibres. The newly developed device could be used as a part of a 'quantum repeater' that would extend the range of quantum communication networks.
The experiment, built by graduate student Mahdi Hosseini, consists of a cloud of hot rubidium atoms in a magnetic field and controlled by lasers. Small pulses of light are sent into the cloud and trapped in energy gaps within the atoms.
Photon echo
By reversing the magnetic field, the researchers can reverse the absorption and later release light. "When the magnetic field is flipped, the time evolution of the atoms is reversed. This makes the absorption process run backwards so light comes out," said Buchler.
The process produces a photon echo: a photon (particle of light) goes in and a photon echo comes out. The novel thing about the research – reported today in the British journal Nature – is the ability to control which packets come out, and also when they come out, using a second control laser.
"In a regular photon echo system, once the atoms all realign the stored light just comes out - you can't stop it. In our system, the combination of control beam and magnetic field switching makes it possible to choose exactly when to recall any one of the stored pulses, how much of it to recall and how fast to recall it," Buchler said.
