Quantum Cryptography Delivers Secure Communication between Major Cities

Quantum Cryptography: the Ultimate in Communications Security

There is much information on the Internet that we wish to keep private, such as passwords and personal details. Generally this information is protected with public key encryption, which has been widely used since the 1980s. Public key encryption uses separate keys at the encryption and decryption stages to minimize the risk of unauthorized access to information.

However public key encryption is not totally secure. It can be deciphered, although doing so would take considerable time. Also, there are no mechanisms in place to prevent communications being hacked in transit. For this reason, an altogether different approach has been suggested: quantum encryption, which allows encryption keys to be shared safely between two parties by ascribing random information to individual photons. When an unauthorized third party attempts to access key data while it is in transit, the photons change state in accordance with quantum mechanics theory. As one can detect such changes and therefore detect whether there has been an interception, it is possible to make sure that communications remain fully protected.

Touted as the ultimate in security technology, quantum encryption was invented in response to rising demand for a more robust, physical law-based form of protection (i.e., based on quantum mechanics) against the hacking of communications.

Threat of Hacking of Keys

Although quantum encryption requires each photon to be transmitted separately, current quantum encryption key transmission technology* occasionally emits multiple photons simultaneously. When this happens, a third party could potentially gain access to some of these photons. Decoy signals are being introduced as a means of countering this threat, but this complicates device design and key extraction process, and in turn device management and operation.

Another challenge is transmission distance. At the optimum long-distance transmission wavelength of 1.5 µm, transmission distance is limited to around 50 km. Clearly, for quantum encryption key transmission to be viable, both the light source and the system require significant performance improvements.

* Secure communication based on the uncertainty principle in quantum mechanics, in which the sender and receiver share a private key that is verifiably safe. This is the ultimate form of key transmission, as it boasts guaranteed physical security since the uncertainty principle enables the detection of the signs of hacking.

Long-distance Communication between Tokyo and Utsunomiya

A three-way consortium involving Fujitsu Laboratories, the Institute for Nano Quantum Information Electronics (NanoQuine) of the University of Tokyo and NEC Corporation has recently completed a trial of a quantum encryption key transmission system that successfully achieved secure communication over a world record distance of 120 km. The system consisted of a high-purity 1.5 µm photon dot single photon generator and an optic fiber photon encryption key transmission module optimized for the single photon generator.

The transmission distance of 120 km in the trial is equivalent to the distance between the cities of Tokyo and Utsunomiya. The success of the trial will provide a major boost to the development of ultra-secure, impregnable communication routes between major cities.

Long-distance quantum key transmission system