Device would boost quantum messages

By Eric Smalley, Technology Research News

Quantum physics makes it possible to send perfectly secure messages, and researchers have already achieved quantum cryptography in the laboratory.

The main stumbling block to using quantum cryptography in practical systems, however, is figuring out how to send the fragile quantum states of light used in the schemes over long distances. "At the moment, quantum cryptography is restricted to several tens of kilometers," said Ignacio Cirac, a professor of physics at the University of Innsbruck in Austria.

Cirac and several colleagues have found a way to boost quantum signals that could help make quantum cryptography practical within a decade.

Signals, whether optical or electrical, fade as they travel down communications lines. Messages wouldn't get very far if it weren't for repeaters, which are simple devices that receive a weakening optical or electrical pulse and send out a stronger pulse.

Ordinary repeaters, however, don't work with quantum communications. This is because quantum signals contain photons that are in the weird quantum mechanical condition of superposition. This means the photons are in some unknown mix of all possible states. For example, a photon is both vertically and horizontally polarized when it is in superposition, and so could come out of superposition horizontally or vertically polarized.

When a photon is observed or otherwise comes into contact with its environment, it is knocked out of superposition and can no longer be used for quantum communications. The trouble with ordinary optical repeaters is they have to observe photons in order to copy them.

To get around this problem, the researchers have proposed a way of storing quantum information in small clouds of atoms and forwarding the information from one atom cloud to another using photons. The device would transfer the weakened quantum information carried by inbound photons to the atoms, correct any errors in it, and then transfer it to outbound photons to produce a stronger signal. This would take place without disturbing the quantum state of the information.

"We have found a way of building quantum repeaters using [sets of atoms]," said Cirac. "A set of several thousands or millions of atoms are used to store quantum information in a given location, correct it, and send it to the next set of atoms."

Other proposals for building quantum repeaters call for transferring quantum information between individual atoms and photons, which is difficult to do, said Cirac. The researchers' scheme has several advantages over these proposals because "we do not have to isolate atoms, no low temperature is required, and quantum gates are not required either," he said. Quantum logic gates take the quantum states of particles through a series of changes in order to perform simple mathematical calculations. This is difficult to do even in carefully controlled laboratory environments.

The researchers' proposal quantum-mechanically links, or entangles, two distant containers of gas atoms. When two or more photons are entangled, one or more of their properties stay in lockstep while the particles are in superposition. For example, researchers can entangle two photons so that when one of the photons is knocked out of superposition and becomes, for instance, horizontally polarized, the other photon also leaves superposition and becomes horizontally polarized at the same instant, regardless of the physical distance between them.

The work is an improvement over other schemes because it uses large numbers of atoms to store the information light carries in quantum communications, said Emanuel Knill, a mathematician at Los Alamos National Laboratory. Other researchers are beginning to conduct experiments that demonstrate the advantages of using these groups of atoms in quantum information processing, he said.

One advantage of the researchers' proposal is that most of the errors this scheme is likely to generate yield no photons, said Knill. In quantum communications, there are two types of errors: photons appearing when none are called for and an absence of photons when they are expected. "Some of their suggested applications intrinsically reject errors, which only results in a relatively mild -- though not negligible -- loss in efficiency over distance," he said.

The experimental setup needed to implement the proposal is similar to the one recently used by researchers at the University of Aarhus in Denmark to demonstrate entanglement between two samples of gas atoms, said Cirac.

"As soon as quantum cryptography is used in practical applications -- this may happen in five to ten years -- quantum repeaters will be needed to extend the distances," said Cirac. "Our proposal can then play a... practical role."

Cirac's research colleagues were Lu-Ming Duan of the University of Innsbruck and the University of Science and Technology of China, Mikhail D. Lukin of Harvard University and Peter Zoller of the University of Innsbruck. They published the research in the November 22, 2001 issue of the journal Nature. The research was funded by the Austrian Science Foundation, the European Union (EU), the European Science Foundation, the National Science Foundation (NSF) and the Chinese Science Foundation.

Timeline:   5-10 years
Funding:   Government
TRN Categories:   Quantum Computing; Cryptography and Security
Story Type:   News
Related Elements:  Technical paper, "Long-distance quantum communication with atomic ensembles and linear optics," Nature, November 22, 2001


November 28, 2001

Page One

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Device would boost quantum messages

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Software sorts video soundtracks

Bigger disks won't hit quantum barrier


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