[ExI] pop quantum news

[Damien Broderick]

Friday, 23 January 2009

<http://www.cosmosmagazine.com/node/2491/full>

by Jacqui Hayes
Cosmos Online


SYDNEY: Quantum information has been successfully 
teleported between two single atoms a metre apart 
– a significant step towards long distance 
quantum communication and quantum computing, researchers say.

Quantum information is information about the 
physical state of a particle: its energy or spin, 
for example. Physicists in the U.S. have now 
managed to faithfully transfer this information 
between two atoms separated by a short distance, 
according to their study published today in the journal Science.

"The teleportation of quantum information in this 
way could form the basis of a new type of quantum 
internet that could outperform any conventional 
type of classical network for certain tasks," 
said Christopher Monroe, study author with the 
Joint Quantum Institute at the University of Maryland in College Park.

Before now, quantum teleportation had been 
achieved over very long distances with groups of 
atoms or with photons (see, 
<http://www.cosmosmagazine.com/news/1361/quantum-communication-breaks-distance-record>Quantum 
communication breaks distance record), but this 
is the first time it has demonstrated with single atoms.

Only quantum teleportation between single atoms 
is a feasible way to hold and manage quantum 
information over long distances. "Photons are 
ideal for transferring information fast over long 
distances, whereas atoms offer a valuable medium 
for long-lived quantum memory," said Monroe.

In the study, the researchers put two ytterbium 
ions (A and B) in separate vacuum traps separated 
by one metre. Ion A was irradiated by microwaves, 
putting it into an unknown quantum state – this 
state was the information to be transported.

The ions were excited by a laser pulse, which 
caused them to emit a single photon and returned 
to their initial state. The photons were measured 
in such a way that it was impossible to tell 
which ion emitted which photon. In the curious 
world of quantum mechanics, this projects an 
'entangled' state onto the ions....

Once the two ions were entangled, the researchers 
took a measurement of ion A. Because ion A and 
ion B were entangled, this forced ion B to embody 
the initial unknown quantum state of ion A. The 
team then applied a microwave pulse to ion B to 
recover the original state of ion A.

The researchers report that atom-to-atom 
teleported information can be recovered 
accurately 90 per cent of the time – a figure they hope to improve.

Monroe said that their set-up could one day have 
a use as a 'quantum repeater' – these tackle the 
problem of generating a signal by temporarily 
storing the state of a photon. New photons with 
the same state are generated at each repeater propagating the signal.

"Our system has the potential to form the basis 
for a lage-scale 'quantum repeater' that can 
network quantum memories over vast distances," 
added Monroe. Without quantum repeaters, 
long-distance quantum communication would not be possible.

"This experiment is an important step toward the 
realisation of quantum repeaters," wrote 
physicists M. S. Kim and Jaeyoon Cho of Queen's 
University in Belfast in an accompanying 
commentary in the same issue of Science. "With 
the recent experimental advances, the 
theoretically presumed quantum paradoxes are 
slowly revolutionising information technology."