17 Jun 2002
Quantum cryptography, secure optical communications based on single photons of polarised light, is feasible up to 40 km with commercial equipment at wavelengths where fibres are most efficient at transmitting light, says a Swedish-Romanian research team.
Quantum cryptography is potentially a fundamentally secure means of optical communications. Eavesdroppers destroy the polarisation of the photons thus erasing the message and giving themselves away.
The team, based at the Royal Institute of Technology, write in Optics Express that there has been a lot of work to demonstrate the feasibility of quantum cryptography at 1300 nm. However losses at 1500 nm are 0.2 dB/km compared with 0.3 dB/km at 1300 nm which translates into a 50% increase in transmission distance for quantum cryptography at the same bit rate.
The researchers worked with a liquid-nitrogen cooled InGaAs avalanche photodiode (APD) which detects single photons by amplifying their presence. Their APD worked well at 210 Kelvin (minus 60 Celsius) which suggests that they could have used a Peltier cooler instead. Germanium APDs at 1300 nm must be cooled to 77 Kelvin.
They add that a faster key rate, better electronics and a Peltier cooler should make quantum cryptography possible at 1550 nm over 100 km.