Toshiba researchers have overcome a major barrier to the cost of creating large scale quantum cryptography networks

Large quantum cryptography networks possible

A breakthrough in quantum cryptography could allow large scale communication networks using the technology to be developed.

Quantum cryptography shows great potential to revolutionise the way sensitive data is protected, but adoption has been hindered by the fact that implementing a quantum cryptography network has required the installation of an elaborate photon detector for each additional user.

But now researchers from Toshiba Research Europe and Toshiba Corporation have discovered a new method that means, with current technology, it would be possible for 64 users to connect to a single detector in a network.

Quantum cryptography relies on the rules of quantum theory to generate uncrackable codes that are not vulnerable to advances in computing, mathematics or engineering and that encrypt data in a way that reveals if it has been eavesdropped or tampered with, making it hugely promising in applications such as distributing secret digital keys.

In research to be reported in the journal Nature, the researchers explain how Toshiba’s Quantum Access Network uses standard fibre components that allow the signals from multiple users to be combined and transmitted on a single fibre.

A statement released by Toshiba said: “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 for the researchers 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 fibres in each transmitter and if left unchecked, this would cause errors in the quantum cryptography system and stop its operation in a matter of seconds.

To overcome the problem the Toshiba team devised a method to compensate for the change in fibre length in each transmitter and thereby allow continuous operation. In the paper the team demonstrates operation over a 12-hour period.

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