crypto gear shrinks
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
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
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
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
"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
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
TRN Categories: Quantum Computing; Optical Computing, Optoelectronics
Story Type: News
Related Elements: Technical paper, "Compact All-Solid-State
Source of Polarization-Entangled Photon Pairs," Applied Physics Letters,
August 6, 2001
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