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  Researchers perfect quantum memory
A quantum memory system that could improve encryption or act as random access memory in a quantum computer has been developed by Australian researchers.

Dr Ben Buchler of the ARC Centre of Excellence in Quantum-Atom Optics at the Australian National University in Canberra and colleagues report their prototype today in the journal Nature.

"We have developed a complete system for manipulation of quantum information once you've stored it," says Buchler.

The memory system could be used to transfer information to and from random access memory in a quantum computer

It could also be used to improve the range of quantum cryptography, which enables two parties to share a random secret "key" to encrypt and decrypt messages.

This encryption is considered ultra secure because measuring a quantum system affects it, which means any eavesdroppers could be detected.

Buchler says current quantum cryptography can generally only be used over 100 kilometres because photons are lost in the optical fibres.

He says a quantum memory device would extend this range by enabling information-carrying light pulses to be stored and re-released on demand at various "repeater" stations.
Storing light information

Scientists favour the idea of using photons to transfer information because light doesn't interact with the material around them and information is not lost.

But these properties also make photons very hard to contain and store.

One idea for storing light information is to couple photons with atoms, transferring the information from the photon to the atom for storage, then transferring the information back to the photon to retrieve it.

The challenge here is getting the same information out that you put in.

In the case of quantum computing and cryptography, the quantum state of the light retrieved must be the same as that originally stored.

A quantum state describes all the possible measurements of an object and their probability. When you measure the object you select one particular combination of those possibilities.
Photon echo

Buchler and colleagues tweaked a technique called "photon echo", which has been around since the 1960s, to solve this problem.

"The light goes in and at some point you can create an echo and the same light will come out," says Buchler.

They stored light pulses from a laser in a cloud of atoms, and used a second "control" laser to help retrieve the photons.

"We can now store a string of pulses and then recall them in any order that you like so it becomes a random access memory," says Buchler.

At this stage the quantum memory system is only 50% efficient. This efficiency is equal to other quantum memory systems, which don't have the advantage of random access, says Buchler.

The next step is to increase the efficiency of the memory by cooling the system down, which currently runs at 70 to 80s04;C.

In the near term Buchler says the quantum memory device is more likely to be used in quantum cryptography rather than quantum computing, which he describes as "very much an idea rather than a concrete fact."

He points to a recent report that an optical quantum computing chip has performed its first calculation by finding the prime factors of 15.

"It gives you an idea of how far away quantum computing is," says Buchler.

"But on the other hand if a quantum computer is developed, a system for random access memory will be essential."
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