APS News

Quantum Leap Reported for Entangled Photons

General purpose quantum computers may still be a long way in the future, but a number of advances in quantum computing and quantum information were presented at the March Meeting.

For instance, Anton Zeilinger of the University of Vienna has transmitted a quantum key wirelessly over a distance of 144 km, between two of the Canary Islands. This is the longest distance quantum information has been transported through the air.

Zeilinger reported the record-breaking feat at the March Meeting. He and his research group produced entangled photons on the island La Palma, then sent one of the photons through the air to a receiving telescope on Tenerife, 144 km away.

The European Space Agency operates telescopes on those islands, located off the coast of Africa. The telescopes were ideal for the application because they are sensitive enough to detect single photons.

The photons’ polarization states, representing 0s and 1s, form a quantum key, a string of bits used to decode a message in a quantum cryptography scheme. A quantum encrypted message would be essentially unbreakable, since any attempt at eavesdropping would destroy the message, making the eavesdroppers presence known.

In a press conference, Zeilinger likened the entangled photons to a pair of “quantum dice,” that would always show the same number no matter how far they are separated.

Earlier this year the group reported having transmitted a quantum key using pulses containing more than one photon. At the March Meeting, they reported for the first time the transmission of single photons, which are more secure.

The rate of data transmission through the air was slow, at just 178 photons in 75 seconds.

Zeilinger says this experiment demonstrates the possibility of sending messages over much longer distances. He is now proposing a more ambitious scheme of using satellites or the International Space Station to relay quantum communications between two locations on Earth.

In addition to sending quantum codes over longer distances, Zeilinger is planning to set up a real quantum cryptography network in Vienna. Five participating groups would each build the hardware for their own nodes, and they would then be able to send each other quantum encrypted messages across the city. Zeilinger hopes to launch the network in 2008.

Quantum cryptography systems are commercially available, Zeilinger pointed out, but as far as he knows they have only been used for research, not for encrypting data.

Several groups reported at the March Meeting on progress towards quantum computers.

David Wineland of NIST leads one of several groups working on ion trap quantum computing, currently one of the most advanced quantum computation technologies.

In these systems, ions are trapped with electric fields, and then manipulated with lasers to act as qubits.  Wineland and other researchers have been working to reduce the size of these traps, because smaller traps would enable faster computers, Wineland said. However, the ions tend to overheat in small traps. Wineland’s design, with all electrodes on a single layer, resembles a computer chip. It could potentially reduce the overheating problem and serve as a building block for a larger quantum computer.

Also at the meeting, Jian-Wei Pan of the University of Heidelberg and Hefei National Laboratory in China, described his 6-photon quantum computer. He said his goal is to have a ten-photon quantum computer in five years.

Researchers generally agreed that a practical quantum computer is a distant goal. “It’s far too early to say what a future quantum computer will look like,” commented Zeilinger during the press conference. 

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Editor: Alan Chodos
Contributing Editor: Jennifer Ouellette
Staff Writer: Ernie Tretkoff