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Quantum memory gets a reality check

Quantum memory The first blueprint for real-world quantum memory has been developed, report researchers.

Importantly, the theoretical protocols are independent of what technology is ultimately used to build quantum computers.

Dr Michael Biercuk, from the University of Sydney, and colleagues, report their findings today in Nature Communications.

"There's a lot of research on quantum computing but it is mainly 'proof of principle' small-scale demonstrations," says Biercuk.

"What we've achieved is identifying the path to building practical high-fidelity long-time quantum memory that is experimentally validated."

The same physics that makes quantum computers potentially powerful also makes them prone to errors, even when the quantum states are just being stored in memory.

To become useful, quantum memory needs to satisfy three criteria, says Biercuck. First, it must preserve the quantum state with high fidelity.

Second the quantum state needs to be preserved for a very long time, compared to, for example, how long it takes to do a calculation.

Third, the stored quantum information must be able to be retrieved quickly on demand.

These three criteria are at odds because, for example, the longer the data is stored, the more chance errors will creep in and counter the high fidelity storage.

This makes designing real-world large-scale quantum memory very difficult.

"The challenges in building something big are very different from just a lab bench demo," says Biercuk.

"There are all these mathematical tricks that people put in the theory to make everything work out perfectly but none of that is physical reality. Our work incorporates the physical reality."

Real-world conditions

Biercuk and colleagues now say they have a theoretical plan to build quantum memory suitable for large-scale "noisy" real-world conditions.

"Our approach is much more realistic," he says.

"It's theory that is very strongly supported by a whole variety of experiments that my group and others have done over the past five years or so."

Biercuk says the new approach satisfies the three competing needs of high-fidelity, long-time memory and quick on-demand retrieval. They say their calculations show it's possible for quantum states to last hours, long beyond the fractions of a second they last today.

"We've figured out from an engineering perspective how to put it all together to make these effective memories," he says.

Importantly, Biercuk and colleagues have tweaked a conventional error-suppression strategy, called "dynamic decoupling".

"If you do this error suppression in a very particular way, you can preserve the quantum states for very long times with very low error rates," says Biercuk.

"The beautiful thing is it can actually work for any technology," says Biercuk.

He says one of the next steps is to build experiments based on the blueprint.

"If we can do that then we have a very clear path to building a quantum memory," says Biercuk.

'Robust' protocol

Quantum computing expert Professor Gerard Milburn, from the University of Queensland, describes the protocol developed by Biercuk and colleagues as "robust".

"The importance of this paper is it shows a protocol, that's not technology specific, to enable one to build a good quantum memory even in the presence of significant noise and significant error," says Milburn.

"It relaxes some of the engineering requirements that we need to satisfy in building systems out of real stuff. You can make quantum memory in an easy environment and still get it to function."

Milburn says in the long run every quantum computer will need a quantum memory, which provides the simplest form of quantum computation.

But he says quantum computers are still a long way off and the most immediate impact of the latest findings will be on the building of quantum repeaters for use in quantum communication, including cryptography.

"This is a big step for quantum memory and quantum repeaters indeed," says Milburn.