Quantum Information Group, World Record QKD Bit Rates
Toshiba have succeeded in achieving continuous operation of QKD with a secure bit
rate exceeding 1 Megabit/sec over a fibre spool with a length of 50 km.
Averaged over a 36-hour period, the bit rate is 100–1000 times higher than
anything reported previously for a 50-km link.
Toshiba's high bit rate QKD system has been implemented in the Tokyo QKD network
launched in October 2010. Operating over a 45-km link (with 14.5 dB of optical
loss) the system demonstrated stable operation with an average secure bit rate of
304 kb/s. This is the world-best key rate for installed fibre.
The advance derives from two innovations: a novel light detector and a feedback
system. Using a ‘self-differencing’ circuit for single photon detection,
Toshiba have increased the operating frequency of InGaAs avalanche photodiodes by
a factor of 100, to beyond 2 GHz, with a maximum count rate of 500 MHz.
Active feedback maintains a high bit rate at all times and requires no manual set-up
This dramatic enhancement of the bit rate is significant for two reasons. It will
allow QKD to be implemented on networks that connect many users. While the previous
bit rate has been sufficient for simple point-to-point links, it was not enough
to allow frequent key refresh on multi-user networks, in which the bit rate must
These higher bit rates will allow one-time-pad encryption to be used for video conferencing
and other high bandwidth applications. The one-time-pad is the only encryption algorithm
that allows unconditionally secure communication and is often regarded as the Holy
Grail of information security. However, its implementation has been hampered in
the past by the requirement for a secret key of the same length as the data. Toshiba's
advance with the QKD bit rate allows the one-time pad to be implemented for data
streams of up to 1 Mb/s over 50 km fibre.