The weird quantum world: Entanglement – 'spooky action at a distance' – causes the properties of two atoms to be linked even when they have no contact with one another.
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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.
Quantum networks
"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, 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.
Spooky action
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.
Entanglement is a bizarre phenomenon. It is a relationship between two particles such that an action undertaken on one particle – such as a measurement of its quantum state – affects the other particle, even if the two particles are far apart.
This is sometimes called 'spooky action at a distance' because there is nothing exchanged between the two particles, yet one can have an effect on the other.
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.
Quantum repeaters
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."
