People first started encrypting sensitive information some 4000 years ago. Since then, cryptography – the study and practice of secure communication ensuring data confidentiality and integrity – has penetrated virtually all spheres of human life. Governments, corporations and financial institutions transmit gigabytes of encrypted information over wire and air.
However, managing the encryption key that enables the recipient to read the message is often the Achilles' heel of most classical encryption systems – in fact, it is not the key per se which is vulnerable, but transmitting it to the other party in a secure manner so that it cannot be intercepted, hacked or tampered with. This is where quantum cryptography – the use of quantum physics to perform cryptographic tasks – comes to the rescue.
"We use single-photon transmission, where one photon equals one bit of information, while its polarization determines whether it is 1 or 0," explains Konstantin Smirnov, professor of the HSE MIEM Department of Quantum Optics and Communications, in conjunction with the Skontel Company. "The security of information transmitted using photons is guaranteed not just by mathematics, but also by the fundamental laws of physics."
One of them is the no-cloning theorem that forbids the creation of identical copies of an arbitrary unknown quantum state, and another is Heisenberg's uncertainty principle, where no measurement of a photon's parameter can be made without changing its other parameters. Together, these two principles are used to create a system capable of detecting any eavesdropping.
"Whenever an outside observer attempts to register the photon's state or to eavesdrop on the distributed key, the source signal will be destroyed," explains Smirnov. “Since cloning a photon's quantum state is impossible, any attempts at intercepting the message will be detected by the sender and the recipient." It means that a third party attempting to examine the communication will not be able to measure photons without distorting the message – and, most importantly, any such tampering will immediately be detected.
Stephen Wiesner, then a graduate student at Columbia University in New York, first proposed securing information using quantum cryptography in 1969. But his idea of quantum money was rejected as unscientific, and in addition, the technology available at the time was not advanced enough to support quantum cryptography. But after a decade, Charles H. Bennett of the IBM and Gilles Brassard of the Université de Montréal proposed a method for secure key distribution using quantum objects, and a few years later, they demonstrated the possibility of creating a secure transmission channel using quantum states. In 1989, Bennett and Brassard created the first quantum cryptography system capable of transmitting information over a distance of less than half a metre.
Today, fiber optics are used for data transmission in quantum cryptography systems, while the distance and quality of transmission depend on the type and properties of single-photon detectors.
The two main types of these devices – PMT (photomultiplier tubes) and APD (avalanche photodiodes) – enable data transmission over just a few dozen kilometers. This, according to Smirnov, limits the prospects for large-scale implementation of quantum cryptographic communication networks. But Smirnov and his colleagues have found a solution.
Back in 2001, Smirnov and another Russian scientist, Grigory Holtsman, discovered the effect of near-infrared single-photon detection using superconducting nanowire. "We realized that this discovery could lead to a new commercial product," Smirnov says, "and established a company, Scontel, to commercialize it. Our know-how in quantum cryptography is based on using a new type of single-photon detectors, namely the Superconducting Single Photon Detector (SSPD), a technology far more advanced than semiconductor avalanche diodes and photomultipliers."
Initial measurements of the SSPD parameters indicate that these devices may have superior characteristics compared to other existing solutions. In particular, SSPD enables quantum key distribution over longer distances.
A research team from the University of Geneva, led by professor Nicolas Gisin (back in 1989, his team created a fiber-optic quantum cryptography system capable of quantum key transmission over a distance of 23 kilometers), together with the ID Quantique company, have tested the Russian single-photon detector; according to their published findings, the quantum key was transmitted over a distance of 150 km at a speed of 2.5 bps, and by using fibers with extremely low optical loss in laboratory, messages could be transmitted over 250 km at a speed of 15 bps.
Later, in 2013, Scontel used enhanced superconducting single-photon detectors in laboratory and increased the maximum quantum key transmission distance to 300 kilometers. "Given a star-shaped pattern of quantum key distribution, we can cover an area of some 600 kilometers in diameter," Smirnov notes, "which is large enough to make quantum cryptography feasible. In fact, we can cover the entire city of Moscow and its metropolitan area with a quantum cryptography network."
According to Smirnov, many of the world's leading research institutes, universities and R&D departments of commercial companies are now developing quantum cryptography communication systems using SSPD. Soon, the Russian system will be field-tested, and if successful, may be used for implementing quantum cryptography networks in large cities, including the suburbs.
*Konstantin Smirnov, Galina Chulkova and Grigory Holtsman, Professors of the HSE MIEM Department of Quantum Optics and Telecommunications, and Sergei Ryabchun and Alexander Korneyev, Associate Professors of the same Department.