NEC, the Telecommunications Advancement Organization of Japan (TAO), and Japan Science and Technology, have tested a quantum cryptography system with a distance between transmitter and receiver of over 100 kilometres for the first time, the companies announced in a joint statement.

Scientists assert that the laws of physics relating to quantum states of matter make communications using quantum effects secure and able to detect if an attempt is made to eavesdrop. The problem is that maintaining the integrity of the quantum communication is technically very difficult, including the need to accurately detect individual photons of light.

The partners achieved the milestone of transmitting and detecting a single light photon over the distance through normal low-cost optical fibre. This would allow users to engage in secure communications using existing infrastructure. Using high-quality optical fibre, the transmission distance could be pushed to over 200 kilometres, the partners said.

These distances would be long enough for quantum cryptography to be used for intercity networks in many countries, the partners said in the statement.

In November last year, Mitsubishi Electric announced it had achieved quantum-based key exchange over a distance of 87 kilometres, a record at that time. But while Mitsubishi said it planned to commercialize its quantum cryptography technology, vendors have yet to set a launch date for such products.

To build a fully-secure integrated communication system, quantum transmission is only used for exchanging the encryption keys, which are typically 128 bits long. Due to the nature of quantum phenomena, this transaction can be made unbreakable. Once the sender and receiver have exchanged encryption keys in this way, the full message is sent using standard high-speed communication methods, with the messages encrypted by the exchanged keys.