Pixels speed quantum crypto

By Eric Smalley, Technology Research News

Scientists working to develop ultra powerful quantum computers and ultra secure quantum cryptography systems generally use subtle aspects of particles like photons and atoms to represent the 1s and 0s of computer information.

When these systems use photons, for example, they tend to tap polarization, phase, or angular momentum -- aspects of light that have to do with the orientation of a lightwave or its electric field.

Researchers from the University of Rochester are using photons to represent data in a simpler way: a photon's position within an array of pixels. The approach also packs more information per photon than standard methods.

The researchers' pixel entanglement method could be used to increase the speed of quantum cryptography systems. Quantum cryptography promises potentially perfect security because the laws of quantum physics make it theoretically impossible for someone eavesdropping on information transmitted this way to go undetected. Today's systems are relatively slow, however.

The researchers method involves sending each photon of a quantum mechanically linked, or entangled, pair of photons into identical arrays of pixels and observing which pixels light up. Entangled photons have one or more properties that are linked regardless of the distance between them. Measuring one photon instantly causes the other to mirror it.

Standard ways of encoding data into photons use properties of a photon that can be set one of two ways to represent a 1 or a 0. The researchers' scheme packs more information per photon because the number of pixels is the number of possible states. "[Pixel entanglement] allows us to impress more information on the photon pairs, which... in communication schemes can translate into higher bit rates," said Malcolm O'Sullivan-Hale, a researcher at the University of Rochester.

The researchers scheme works by generating pairs of entangled photons using the standard parametric downconversion method. When ultraviolet photons are fired into a special crystal, some are split into a pair of entangled infrared photons. The researchers then channel the entangled photons separately through a series of lenses into identical arrays of pixels. The entangled pairs occupy the same positions in the two arrays, which, in turn, causes those positions, or pixels, to become entangled. The pixels that are entangled are determined at random, and the random numbers resulting from a series of entangled pixels makes up the secret key for encrypting information.

The researchers demonstrated their system using three-pixel arrays, and they also showed that the method works for six-pixel arrays, said O'Sullivan-Hale. A six-pixel array would allow a pair of entangled photons to represent three bits of information.

Pixel entanglement could theoretically be used with much higher numbers of pixels, and the researchers estimated that their system could be used in 16-pixel arrays, meaning each photon pair could represent eight bits of information. "With the possibility of using entangled states with more [than two] levels, we foresee pixel entanglement being useful for distributing quantum keys at high bit rates," said O'Sullivan-Hale.

Today's optical fiber does not preserve lightwaves well enough to allow the method to work over optical networks, said O'Sullivan-Hale. "The most readily imaginable application [of pixel entanglement] is free-space quantum key distribution for the secure transmission of information," he said.

Another important advantage of pixel entanglement for quantum cryptography is that the higher number of possible states for each photon pair makes it harder for an eavesdropper to fool the system, said O'Sullivan-Hale.

Using the technique for practical quantum cryptography will require preserving the entanglement over long distances, minimizing losses and detecting photon positions with adequate resolution, said O'Sullivan-Hale.

Practical applications of pixel entanglement could be realized in five to ten years, said O'Sullivan-Hale.

O'Sullivan-Hale's research colleagues were Irfan Ali Khan, Robert W. Boyd and John C. Howell. They published the research in the June 7, 2005 issue of Physical Review Letters. The research was funded by the National Science Foundation (NSF), the Army Research Office (ARO), the Office of Naval Research (ONR), the Research Corporation, and the University of Rochester.

Timeline:   5-10 years
Funding:   Government; Private; University
TRN Categories:  Quantum Computing and Communications; Optical Computing, Optoelectronics and Photonics; Physics
Story Type:   News
Related Elements:  Technical paper, "Pixel Entanglement: Experimental Realization of Optically Entangled d=3 and d=6 Qudits," Physical Review Letters, June 7, 2005


August 10/17, 2005

Page One

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Letter: a short history of TRN
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Pixels speed quantum crypto

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