U. of Tokyo, Fujitsu advance towards quantum cryptography

[R. A. Hettinga]

<http://www.infoworld.com/article/04/07/23/HNquantumcrypto_1.html>

InfoWorld


  

U. of Tokyo, Fujitsu advance towards quantum cryptography
Project succeeds in generating single photo needed for securely sharing
keys across telecom networks
 
 

By Martyn Williams, IDG News Service
July 23, 2004 


TOKYO -- A joint research project of Fujitsu Ltd. and The University of
Tokyo has made progress towards realizing a viable quantum cryptography
system. Such a system allows parties to share encryption keys via
telecommunication networks with full confidence that they have not been
compromised en route.


The team has succeeded in generating and detecting a single photon at
wavelengths useful for telecommunications, said Yasuhiko Arakawa, director
of the Nanoelectronics Collaborative Research Center at The University of
Tokyo and leader of the research project, in an interview on Tuesday.

 The reliable generation and detection of single photons is vital if
quantum cryptography systems are to leave the laboratory and enter
practical use and the team has managed this through the development of a
new photon generator.

 Quantum cryptography is based on the physical properties of photons.

If two parties want to exchange encrypted data they need to share the
electronic key that will be used to encode the data. The data is encoded
with a corresponding private key, so using the genuine public key is vital.
Should a fake key be substituted for the real one the data could be read by
a third party rather than the intended recipient. Sharing of keys across
telecommunication networks can expose the key to tampering so many users
exchange keys offline via physical media, such as a floppy disk or CD-ROM.

 Under public key infrastructure (PKI) schemes, public keys are certified
as being genuine by a certificate authority.

Quantum cryptography systems allow users to exchange keys across networks
with the knowledge that they haven't been tampered with during transmission.

 This is because each data bit of the key is encoded onto individual
photons of light. A photon cannot be split so it can only end up in one
place: with the intended receiver or with an eavesdropper. Should a key be
completely received the recipient can be sure it hasn't been compromised
and should it be incorrectly received there's a chance that it has been
intercepted and so a new key can be issued.

 Thus, for a viable quantum cryptography system it must be possible to
reliably generate a single photon. If two or more photons are generated the
key's security is gone.

 "We have to avoid the key being received by other people," Arakawa said.
"It's not easy to avoid but if we use single photons it's possible. So its
very important to develop a single photon source."

 Until now most experiments involving quantum cryptography have used lasers
as their photon source and these haven't proven to be completely reliable
generators of single photons.

 "By reducing the output power of the laser we can create one photon
sometimes, however it is impossible to control accurately the number of
photons," Arakawa said. Reducing the laser power also means the overall
transmission speed is slowed.

 Arakawa's team has developed a new generator based on materials developed
by Fujitsu and Japan's National Institute for Materials Science. The
material is embedded with quantum dots, which are like tiny holes into
which individual electrons can enter and a photon be produced.

 "They are almost comparable to the wavelength of the electron so electron
motion is almost zero and the electron cannot move," Arakawa said. "The
energy state is fixed. So if we can control the energy of the electron, we
can control the number of photons that are emitted."

 The wavelength of the photons that are emitted can be controlled by
adjusting the size and shape of the quantum dots. Doing so very accurately
is difficult so additional filtering is employed to ensure that only those
with a wavelength suitable for transmission down commercial optical fiber
networks are let through, said Tatsuya Usuki, a researcher at Fujitsu
Laboratories Ltd., who also worked on the technology.

 Because the accurate generation of single photons is possible and there is
no need to throttle back the power, the transmission speed can be increased
from a few hundred bits per second to around 400 times that speed, Arakawa
said. He estimated a commercial system might be possible to transmit data
at up to 100k bps (bits per second).

 The group has also made progress on the detection end of the system. Light
coming out of the fiber is split into two and sent to two detectors. By
measuring the time at which photons arrive researchers can determine
whether one or two photons were generated. In the case photons arrive at
the same time at each detector, it means two were generated which was not
the case with the new system, Arakawa said.

 At present the team has succeeded in generating photons at both 1.3 micron
and 1.55 micron wavelengths and verified single photon transmission at the
former wavelength. Verification of the latter is one of the upcoming goals
for the team. The project hopes to develop a practical single photon
generator by 2007 and Arakawa predicts commercial systems based on the
technology could be available in 5 years.

 Details of the research are scheduled to be presented at the 27th
International Conference on the Physics of Semiconductors, which will begin
in Arizona, U.S., on July 26.

-- 
-----------------
R. A. Hettinga <mailto: rah at ibuc.com>
The Internet Bearer Underwriting Corporation <http://www.ibuc.com/>
44 Farquhar Street, Boston, MA 02131 USA
"... however it may deserve respect for its usefulness and antiquity,
[predicting the end of the world] has not been found agreeable to
experience." -- Edward Gibbon, 'Decline and Fall of the Roman Empire'

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