Researchers from Toshiba have discovered a method to build quantum cryptography communication networks with a far greater scale than ever before. It will allow quantum cryptography to be used beyond its current niche applications, for example as part of the Smart Community Networks that will manage and control energy generation and consumption in the future. The breakthrough will be reported in the scientific journal Nature.

Quantum cryptography shows great potential to revolutionize the way sensitive data is protected. It can be used to distribute secret digital keys with a security that is not vulnerable to advances in computing, mathematics or engineering, and means any hacker that “taps’ an optical fiber will be detected. At the same time, it could become the first prevailing technology to harness the peculiar laws of quantum physics.

However, major obstacles still have to be overcome in order to make quantum cryptography viable for widespread use, particularly regarding the number of users than can be connected to a single network. Up until now, implementing a quantum cryptography network has required an elaborate photon detector for each additional user.

The Toshiba team has discovered a technique to allow many users to share a single detector and thereby greatly reduce the complexity of the network. The breakthrough means that with current technology, it would be possible for 64 users to connect to a single detector in a Quantum Access Network.

Toshiba’s Quantum Access Network uses standard fiber components that allow the signals from multiple users to be combined and transmitted on a single fiber. As the photon detector is the most complex part of the quantum cryptography system, this is placed at the common end of the Quantum Access Network, while each user has a photon transmitter comprising just standard components. This arrangement greatly reduces the hardware requirement and cost for each user added to the network.

This breakthrough is enabled by the fast detector developed by Toshiba, which can count up to one billion individual particles of light (photons) per second. Thanks to its very high detection rate, this receiver can be shared between multiple transmitters in a point-to-multipoint link.

Tests on an eight channel Quantum Access Network demonstrated that a user can transmit secret keys with a bit rate in excess of 250 kbit/sec or 80 GByte per month, enough for each user to encrypt one million emails.

Another major challenge has been to negate the effect of temperature fluctuations for multiple users sharing the same link. Small changes in the local temperature, by even a fraction of a degree, can significantly alter the length of the fibers in each transmitter.

If left unchecked, this would cause errors in the quantum cryptography system and stop its operation in a matter of seconds. However, the Toshiba team devised a method to compensate for the change in fiber length in each transmitter and thereby allow continuous operation. In the paper the team demonstrates operation over a 12 hour period.