World’s First Achievement: NICT Verified the Potential forQuantum Key Distribution (QKD) Using a Satellite

－ Feasibility of Laser Communications and QKD in Space Ensured －

November 23, 2009

Space Communications Group of National Institute of Information and Communications Technology (NICT, President: Hideo Miyahara) has succeeded in the world’s first measurements of polarization characteristics　of an highly polarized laser source through the space-to-ground atmosphere by using a low-earth -orbit (LEO) satellite. This actual measurement will contribute to further progress of space laser communications, as well as the precise system design of QKD via space in future. This achievement appeared in the “Optics Express”, the on-line journal of Optical Society of America on November 23, 2009.

Background

The classical cryptography widely used at present bears the risk to be decrypted if the higher ability of calculation in computers is realized, on the other hand, the quantum cryptography has a distinguished feature that it can be never broken no matter how highly the science technology progresses in future. QKD is said so far that it is transmittable only within 300km due to transmission losses and background noise in optical fibers; however, if using a satellite, QKD enables farther transmission distances over a world-wide extent. This is the world’s first measurement of the polarization characteristics through space-to-ground propagation paths using a highly polarized optical source.

This satellite experiment was executed under the collaborative research agreement with Japan Aerospace Exploration Agency (JAXA), and the analytical study was independently performed by NICT.

Achievements

NICT has been conducting R&D on space laser communications and QKD. NICT succeeded in the world’s first polarization measurements through the space-to-ground atmosphere at NICT optical ground station (Fig. 1) using a LEO satellite, the Optical Inter-Orbit Communications Engineering Test Satellite (OICETS), Japanese name “Kirari,” (Fig. 2) launched by JAXA in August 2005. The obtained result showed less than 2.8% of depolarization occurred (Fig. 3 & 4). The polarization of photons is commonly used for QKD because of simplicity and stability of its system. However, if a degree of polarization degrades, the safety of sharing keys cannot be ensured. The results of this research confirmed that atmospheric influences including those of upper atmosphere can be considered under the level that may not affect the satellite-to-ground QKD.

Future Prospects

This successful actual measurement of polarization characteristics through the space-to-ground atmosphere will enable the link budget analysis for QKD, and become a key to realize world-wide satellite QKD. It will also make it possible to develop the feasible design of a test satellite aiming for realization of space laser communications.

Reference

Terminology and Interpretations

Quantum Key Distribution (QKD)
QKD is a communication system in which two parties share a cryptographic key utilizing a quantum state (polarization for example) that is a physical nature of a particle of faint light (photon). This system is able to detect the presence of an eavesdropper as the quantum state is distorted if a third party is trying to measure the key for eavesdropping. Combined with the one-time pad that is the encryption method of one-time use of the key, QKD can provide communications with perfect secrecy.

URL of “Optics Express”, On-line Journal of Optical Society of America
http://www.opticsinfobase.org/oe/Issue.cfm

The title of the paper: “Polarization measurements through space-to-ground atmospheric propagation paths by using a highly polarized laser source in space”
Abstract:
http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-17-25-22333

PDF:
http://www.opticsinfobase.org/DirectPDFAccess/23221380-BDB9-137E-CE3EA24BB3405A31_190651.pdf?da=&id=190651&seq=0

Cryptography
The means of hiding information between two parties in order not to be eavesdropped by the third party.

Features of Quantum Cryptography Technology
Importance of info-communications security is rapidly increasing due to the coming of the network society by cloud computing and the growth of online administrative services (e-Government). Cryptography in widely use at present always have risks of decryption in case that computer technology may provide higher capability to decode. On the other hand, QKD has a distinguished feature that it is unable to be intercepted no matter how highly technology progresses in future. The experiment on QKD using optical fibers confirmed the transmission of over 200 km, however, it is said that 300km is the limit of QKD transmission due to inevitable transmission losses. Further technology for quantum data relay between cities and continents is awaited, however, it still remains unexplored. Expected as a possible near-future technology is QKD using a satellite around the earth; it will make possible the global QKD as well as higher security.

R&D on spacious optical communications and QKD
NICT has been conducting on R&D on space laser communications and QKD. Followings are the links about the related experiments NICT conducted so far:

Successful demonstration experiments on optical communications between the Optical Inter-Orbit Communications Engineering Test Satellite (OICETS) Kirari and NICT optical ground station on April 7, 2006
http://www3.nict.go.jp/w/w122/ogs/video_eng.html

Bidirectional Laser Transmission with a German Satellite on March 24, 2009
https://www2.nict.go.jp/pub/whatsnew/press/h21/090324/090324_e.html

Development of mobile free-space quantum cryptography terminal on September 15, 2008
http://spie.org/x27490.xml?highlight=x2414&ArticleID=x27490

NICT Optical Ground Station
The facility in NICT that is equipped with an optical telescope of 1.5m in diameter, where the world’s first experiment on optical communications between a geostationary earth orbit (GEO) satellite and the NICT optical ground station was successfully made using the Engineering Test Satellite VI (ETS-VI), Japanese name “KIKU No.6,” in 1994, and the laser communications experiment with a low-earth-orbit (LEO) satellite was succeeded in 2006. Laser ranging to measure the distance between a satellite and the station precisely is constantly conducted at NICT optical ground station.

Optical Inter-Orbit Communications Engineering Test Satellite (OICETS) “Kirari”
The Optical Inter-Orbit Communications Engineering Test Satellite (OICETS) was launched on August 24, 2005 by a Dnepr rocket from the Baikonur Cosmodrome and named "KIRARI", in order to conduct demonstrations with ARTEMIS, the latest geostationary satellite belonging to the European Space Agency (ESA). OICETS conducted optical inter-orbit communications tests with ARTEMIS at a distance of more than tens of thousands of kilometers apart. In 2006, the laser communication experiments between NICT optical ground station and OICETS were successfully conducted. Since 2008, the additional experiments were carried out with the new research objectives that NICT proposed the verification of the polarization characteristics and so on.

http://www.jaxa.jp/projects/sat/oicets/index_e.html

Fig.1： NICT optical ground station which contains the 1.5-m optical telescope for space laser communications.

Fig.2： An image toward the satellite taken by a camera which is equipped on the telescope. The bright spot corresponds to the laser beam from the satellite. This shows the laser communication link could be maintained even under cloudy condition.

Fig.3： Degree of polarization (DOP) and the received optical power measured at NICT optical ground station. The data was acquired under clear sky condition and the received power was increased at the middle of the experiment time because of higher elevation angles of the satellite.

Fig.4： Polarization characteristics over the Poincare sphere. The upper side indicates the right handed circular polarization and the lower side shows the left handed circular polarization. The result showed that the degradation of polarization was less than 2.8 %.