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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
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
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
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
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