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Quantum networks closer with Alberta discovery

Last Updated: Wednesday, January 12, 2011 | 11:00 PM ET

By Emily Chung, CBC News

'The
technology we use here is very close to what telecommunications use already,'
said University of Calgary quantum physicist Wolfgang Tittel. (Riley Brandt/University of Calgary)Information
encrypted using quantum physics could soon be transmitted over long-distance
networks, thanks to a discovery by Alberta physicists.

For the first time, information coded in light using a quantum physics
phenomenon called entanglement was successfully stored and retrieved from a
crystal, researchers at the University of Calgary and the University of
Paderborn in Germany reported Wednesday in the science journal Nature.

The experiment

Tittel and his colleagues created a memory device consisting of a crystal
embedded with particles called rare-earth ions. They then used a laser to
produce two entangled photons, one of which is absorbed by the crystal. When the
crystal is cooled to very low temperatures — around –270 C — the photons can
also be pulled back out of the crystal. After being put in and pulled out of the
crystal, the researchers tested the degree to which the two photons were still
entangled.

"What we found is that … entanglement has not been degraded after storage,"
Tittel said.

"The technology we use here is very close to what telecommunications use
already," said Wolfgang Tittel, a University of Calgary professor who co-wrote
the paper.

It was published online alongside a study from researchers at the University
of Geneva in Switzerland who reported a similar finding using a different
material.

Quantum mechanics is the branch of physics dealing with the strange behaviour
of very tiny things like elementary particles and atoms, and is extremely
different from the physics that humans experience every day.

Quantum entanglement, used in quantum encryption, is the linking of two
quantum objects so that any change to one is immediately experienced by another,
no matter how far apart they are.

The advantage of quantum encryption over traditional encryption is that it
has the built-in ability to allow people sending and receiving the encrypted
messages to detect whether someone has tried to "eavesdrop."
The
quantum memory device consists of a crystal embedded with small particles called
rare-earth ions. (Wolfgang Tittel/University of
Calgary)

Quantum encryption of messages is already available commercially, but current
technology only allows communication between two points less than 100 kilometres
apart, Tittel said.

That's because they're transmitted over a silicon fibre, and roughly 50 per
cent of the photons are lost for every 10 kilometres they travel.

Theoretically, the distance problem can be overcome by breaking the distance
into shorter segments and boosting the signal after each segment. To do that, a
device positioned between segments would have to take in the message as a weak
signal, repeat it as a strong signal and send it along the next segment without
damaging the information.

Such a theoretical device is called a quantum repeater. In order to work, it
would need to store the information until it has been completely uploaded to the
next segment.

"Our discovery is such a quantum storage device," said Tittel.

Physicists had previously shown it was possible to store entangled photons in
atomic gases, but that isn't very practical for everyday use, he said. The
advantage of the new storage method is that it uses solid-state crystals like
those used in regular computing.

The possibility of storage also allows for the possibility of a quantum
network, where information could be relayed to any of several different
locations via nodes between them.

It could also allow two pieces of information to be combined. Suppose you get
an email, Tittel says, then store it and wait for a second email.

"Once you have both emails, you can do something new — you can compare the
information in the two emails and derive something."

Tittel estimates that a proof-of-principle quantum repeater will be available
in five years, and quantum networks may be possible in 10. But in the meantime,
he says, engineers and digital physicists will need to get involved to modify
the device and make it more practical.

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Related

Internal Links

Quantum
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External Links

Abstract of the paper in Nature

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