Quantum mechanics may play a larger part in encrypted communications in the near future thanks to a successful test that has sent a quantum key along conventional broadband fibre.
Researchers at Toshiba’s Cambridge Research Laboratory have hit on a method of using normal fibre to send quantum keys, a method of information transmission that has previously relied on “dark fibres” which were dedicated to transmitting only the quantum key. This transmission of information, called “quantum key distribution” (QKD), is usually drowned out by crosstalk from normal data traffic on broadband lines but the team led by Dr Andrew Shields has hit on a solution.
Quantum key distribution is achieved by sending single photons of differing polarisations, created by laser pulses, along a fibre line. These polarisations constitute 1’s and 0’s and the photons make up a key that can encode larger amounts of information. Thanks to the quantum nature of the transmission anyone spying on the key being sent will alter the polarisation or alignment of the photon, giving an instant warning that a transmission is being watched.
The problem with QKD is how to receive the key at the other end of the line, something that Shields and his team have accomplished. It has been successfully attempted in the past, over 10 kilometers of fibre, but this latest attempt manages the feat over 90 kilometers.
The photons that are being sent over live traffic or “lit” fibres can be extracted from the stream of information flowing through the line thanks to detects that have been build that handle only one photon at a time. The creation of a gate that opens just long enough and at just the correct time to catch the relevant photon has allowed the team to transmit the quantum key at a rate of megabits-per-second while retaining normal gigabit-per-second data traffic.