Tightening photonic bonds strengthens security

By Eric Smalley, Technology Research News

The virtually perfect security afforded by quantum cryptography has been demonstrated in laboratories. But getting from the laboratory to practical applications is especially difficult for a technology that relies on the stranger aspects of quantum mechanics.

Quantum cryptography is based on manipulating and measuring the quantum states of individual photons, and keeping any particle in its quantum state means isolating it from its environment -- a difficult challenge in the real world of fiber-optic cables and communications devices.

A team of researchers in Austria has adapted a 5-year-old scheme for boosting quantum communications signals so they can be transmitted with relatively simple equipment, which should make it possible to build devices for transmitting secure information over long distances.

"For many protocols of quantum communication... it is necessary that two points which communicate with each other establish entanglement over long distances," said Anton Zeilinger, a professor of physics at the University of Vienna.

Two particles can become entangled, or linked, when they are in the quantum mechanical state of superposition, which is a mixture of all possible quantum states. Quantum states include particle attributes like the electron spin states spin up and spin down.

When one of the entangled particles is measured, the measurement knocks it out of superposition into a random state and the other particle immediately collapses into the same state, regardless of the physical distance between them.

Because it is impossible to observe a particle, such as a photon, in its quantum state without altering that state, anyone who eavesdrops on a message encoded in quantum particles will alter the message and reveal the security breach. If one person sends a key to another using quantum cryptography and they see that no one intercepted it, they can safely use the key to encrypt their communications.

"The problem is that if we create a pair of entangled particles in a source, the quality of the entanglement degrades with... distance," said Zeilinger. The greater the distance, the greater the amount of noise in the system.

"This noise is caused by the interaction of the particles -- in our case photons -- with the environment, in our case a glass fiber," he said.

Like other entanglement purification schemes, the University of Vienna proposal starts with some number of partially entangled pairs of photons and ends up with a smaller number of more highly entangled pairs. All of the pairs have the same level of entanglement to begin with and the various schemes eliminate some of the photons, which leaves the remaining ones more highly entangled.

The University of Vienna scheme sends the photons through polarization beam splitters, which filter photons based on their polarization. Photons are particles of electromagnetic energy and they have an electric field and a magnetic field. The electric field of a non polarized photon vibrates in a plane perpendicular to the direction the photon is traveling in. A polarized photon has an electric field that vibrates in only one direction of the perpendicular plane.

A sender transmits one photon from each of two entangled pairs of photons and keeps the other halves of the pairs. The sender and receiver then put their photons through a polarization beam splitter, which has two inputs and two outputs. If the photons have the same polarization, one will exit from each output.

If both sender and receiver have one photon come through each output, they both measure one photon from the same output channel of their polarization beam splitters. If the measured photons match each other, they keep the remaining photon pair, which is more highly entangled and therefore more suitable for quantum communications.

The key to the University of Vienna scheme is that it is simple enough to potentially be done outside a lab. Previous schemes have performed more complicated manipulations that require correspondingly more complicated equipment.

Combined with quantum repeaters, the method should allow for unconditionally secure quantum cryptographic key over arbitrary distances, said Zeilinger.

Quantum repeaters don't exist yet, but are theoretically possible. Classical repeaters copy a weakened signal and transmit the copy, effectively boosting the original signal. "But a classical repeater is worse than worthless for quantum states," said Daniel Gottesman, a fellow of the Clay Mathematics Institute working at the University of California at Berkeley. "Because it looks at the state, it actually creates more noise," he said.

A quantum repeater would need to be able to regenerate the state without looking at it. "That's where entanglement purification comes in," said Gottesman. "It's a way to take a number of noisy quantum states and distill out a few accurate ones. It's much harder than building a classical repeater, but perhaps this [University of Vienna] work will bring it within reach. In the context of quantum cryptography, that means you're back in business. Less noise means no foothold for an eavesdropper," he said.

Developing quantum cryptographic systems is still a difficult task; the researchers also have not addressed the key problem of how to store photons, said Seth Loyd, an associate professor of mechanical engineering at the Massachusetts Institute of Technology. "I think it's an excellent idea, but I'm not sure that it really improves the prospects for implementing quantum cryptography as much as [the researchers] claim," he said.

Practical quantum communication systems could be developed in 10 years, said Zeilinger.

Zeilinger's research colleagues were Jian-Wei Pan, Christoph Simon and Caslav Brukner of the University of Vienna. They published the research in the April 26, 2001 issue of the journal Nature. The research was funded by the Austrian Science Foundation, the Austrian academy of sciences and the European Union.

Timeline:   10 years
Funding:   Government
TRN Categories:   Quantum Computing
Story Type:   News
Related Elements:  Technical paper, "Entanglement purification for quantum communication," Nature, April 26, 2001


May 16, 2001

Page One

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Tightening photonic bonds strengthens security

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