Quantum crypto gear shrinks

By Eric Smalley, Technology Research News

Researchers around the world are closing in on realizing the centuries-old dream of being able to send secret messages that are perfectly secure against any possible code breaking attempt. Before the average computer user can protect messages using quantum cryptography, however, the bulky laboratory equipment involved must be redesigned to fit on a few computer chips.

The key to this miniaturization is figuring out how to produce pairs of entangled photons using small, low-power lasers.

Researchers from Ludwig Maximilians University in Germany have taken a step in this direction by producing entangled photons using a small laser diode. The advancement opens the way to building quantum cryptographic devices on circuit boards.

Quantum particles like photons can be entangled, or linked so that they have properties like polarization in common. Particles can remain entangled regardless of the distance between them.

Entangled photons are the main ingredient of quantum cryptography. Two people who want to secretly communicate can, in theory, split a series of entangled photon pairs. They can then measure their photons and use the results as a key to encrypt a message that can be read by the person holding the other half of the entangled photons.

Because an eavesdropper cannot look at the photons without disturbing them, any intrusion can be detected immediately and the compromised key discarded.

Photons entangled using polarization work like this: the electric field of light vibrates in a plane perpendicular to the direction the light is traveling. When light is polarized, its electric field vibrates in one of four directions on that plane: horizontal, vertical or one of the two diagonals.

Entangled photons occur in some mix of the four possible polarization orientations, but when one of the particles is measured both particles snap into one polarization, regardless of the physical distance between them.

The preferred method for producing polarization-entangled photons is shining a laser through a special crystal that can turn a single ultraviolet or blue photon into a pair of entangled infrared photons. But this process is very inefficient, said Jürgen Volz, now a graduate student at the University of Munich.

"Usually this problem is overcome by the use of intense laser beams," he said. "Only large-frame ion lasers can be used... because only these lasers are able to create an adequate power output. These lasers are quite large and need enormous amounts of electrical energy... and possibly also water cooling. This makes standard entangled-photon-pair sources very expensive," said Volz.

The researchers got around the power problem by taking advantage of a basic principle of lasers. Lasers work by stimulating the atoms of a gas, which causes the atoms to emit photons. A pair of mirrors facing each other at opposite ends of the laser's gas chamber keeps these photons bouncing back and forth through the gas. As the photons bump into the gas atoms, they stimulate the emission of more photons.

"We placed an optical resonator around the nonlinear crystal," said Volz. In this case, the photons that bounce back and forth pass through the crystal rather than hitting the atoms of a gas. "With each pass, entangled photon pairs are created. So we can use much lower laser powers," he said.

Lower laser power means the lasers can be much smaller. "We use a simple laser diode, which could be operated from a simple battery," Volz said. "That makes our source much cheaper and quite compact in contrast to those based on ion lasers."

The researchers' entangled photon source generated about 10,000 pairs of entangled photons per second. Although this is only a tiny fraction of the astronomically large number of photons generated by even low-power lasers, it is sufficient for many quantum cryptography schemes, according to Volz.

The solid-state entangled photon source could be used for quantum cryptography in a few years, said Volz. "Two to five years seems possible," he said.

Volz's research colleagues were Christian Kurtsiefer and Harald Weinfurter of Ludwig Maximilians University. They published the research in the August 6, 2001 issue of the journal Applied Physics Letters. The research was funded by the German Research Foundation and the European Union.

Timeline:   2-5 years
Funding:   Government
TRN Categories:   Quantum Computing; Optical Computing, Optoelectronics and Photonics
Story Type:   News
Related Elements:  Technical paper, "Compact All-Solid-State Source of Polarization-Entangled Photon Pairs," Applied Physics Letters, August 6, 2001


October 3, 2001

Page One

Neurons battle to a draw

Quantum crypto gear shrinks

Toy shows bare bones of walking

Tiny jaws snatch cells

Plastic mix helps shrink circuits


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