Phys. Rev. X 2, 041010 (2012)

It would be economically advantageous and incredibly convenient if both data transmission and quantum key distribution could take place over a single optical fibre. However, this has so far proved problematic because the Raman scattered noise arising from the intense data signal can easily contaminate the low-intensity quantum signal. Now, Ketaki Patel and co-workers from Toshiba Research Europe and Cambridge University in the UK have shown that a temporal filtering technique allows the coexistence of quantum key distribution and gigabit-per-second data communications over a single fibre at distances of up to 90 km, with secure bit rates three orders of magnitude higher than those of current quantum key distribution schemes. The researchers found that time gating can filter out the forwards and backwards Raman scattering because the Raman photons reach the detector at random times with respect to the regularly pulsed quantum signals. The researchers therefore operated their InGaAs avalanche photodiodes with synchronized temporal filtering in order to enhance the quantum-signal-to-Raman-noise ratio by a factor of ten. They then removed any residual Raman photons by employing a conventional narrow bandpass filter.