Breaking Quantum Cryptography's 150-Kilometer Limit

Scientists want to put an unbreakable-code generator on the International Space Station

Illustration: European Space Agency, General Studies Programme

Secrets by SATELLITE

International Space Station that would generate an unbreakable code from entangled pairs of photons.

Researchers in Europe in the field of quantum cryptography have demonstrated for the first time that it should be possible--with the help of satellites--to communicate across thousands of kilometers using unbreakable codes whose security is guaranteed by the laws of quantum physics. For many business or government uses, the codes must be usable between cities and continents, but quantum cryptography machines today are limited to about 150 kilometers by the length of individual optical fibers and the loss of photons within them.

The team that performed the experiment, made up of researchers from Italy and Austria, did not actually encrypt a message, but they demonstrated a key principle: the detection of single photons sent from a satellite. This month they will present a plan to the European Space Agency to install a quantum cryptography system on the International Space Station (ISS) and use it to perform the first satellite-based quantum communication.

The researchers, led by Paolo Villoresi at the University of Padua, used the Matera Laser Ranging Observatory, in Italy, to bounce weak laser pulses off the Ajisai satellite, a mirrored orbiter 1485 km up. What returned to the observatory were single photons.

Single-photon exchange is important because it provides part of the security of quantum cryptography. In a common scheme, single photons are converted into ”entangled pairs”--pairs of photons that are mutually dependent, even when they travel far apart. Quantum theory says that if you measure one of the photons in an entangled pair, the properties of the other are also instantly revealed. For the purposes of quantum cryptography, the pairs are split up and sent to the parties wishing to communicate secretly. Through a series of steps involving polarization filters and other measures, the photons produce the same random series of bits, or quantum key, for each party. In theory, no one can intercept individual photons and steal the key, because any attempt to do so would alter the key in an easily detectable way.

The difficulty lies in preserving the entanglement over long distances. The success of the Ajisai experiment demonstrates that a source of single photons on a satellite can indeed be detected at a ground station many hundreds of kilometers away, against very high background noise.

The success with Ajisai ”is an important step, as many of the potential problems, such as timing and tracking, have been proven to be in a manageable regime,” says Norbert Lütkenhaus, professor at the Institute for Quantum Computing at the University of Waterloo, in Canada, who was not involved in the research.

For the proposed ISS experiment, the entangled photons would be beamed from orbit to two distant ground stations, allowing them to communicate using the quantum key.

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