Sturdy quantum crypto proposed

April 21/28, 2004

Today's rudimentary quantum cryptography systems send encryption key data over fiber-optic lines or through the air, and each bit of the key is -- ideally -- encoded in a single photon.

Quantum cryptography systems promise potentially perfect security because it is impossible to eavesdrop on bits encoded in single photons without revealing the security breach.

Many quantum cryptography schemes involve encoding quantum bits, or qubits, in the polarizations of photons and transmitting them over fiber-optic lines. Polarization is the orientation of a photon's electric field.

But the polarizations of photons tend to rotate as the photons travel through optical fibers. Researchers have devised several schemes for overcoming polarization rotation, including sending photons on round trips to reverse the effect and monitoring the lines to compensate for changes. These approaches introduce overhead and don't work if the rate of change is too fast, however.

Researchers from the University of Waterloo in Canada have come up with a way to make quantum signals that can better withstand this kind of noise.

The method takes advantage of decoherence-free subspaces, which encode a logical qubit in two or more physical qubits. Each qubit is encoded using the polarization states of three or four photons. A qubit is encoded in the relationship of three or four photons, so when a fiber optic line rotates the polarization it affects all of the qubit's photons equally, thus preserving the qubit.

The protocol could be implemented in today's technology, according to the researchers. The work appeared in the January 9, 2004 issue of Physical Review Letters.

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