30 November 2010, Cambridge, UK
The Cambridge Research Laboratory of Toshiba Research Europe announced today that
it has discovered a simple method to prevent detector blinding attacks on quantum
Quantum cryptography is a method to distribute digital encryption keys across an
optical fibre. The protocol has been proven to be perfectly secure from eavesdropping.
However, any differences between the theoretical protocol and its real-world implementation
can be exploited to compromise the security of specific systems.
A recent paper published in the September edition of Nature Photonics suggests a
method to blind the Indium Gallium Arsenide (InGaAs) avalanche photo-detectors that
are commonly used in quantum cryptography. If successful, this attack could allow
an eavesdropper to gain information about the secret key.
Now an investigation by the Cambridge team, to be published in the December edition
of Nature Photonics, demonstrates that the detector blinding attack is completely
ineffective, provided that the single photon detectors are operated correctly.
The new study shows that the attack is only successful if a redundant resistor is
included in series with the single photon detector, or if the discrimination levels
are set inappropriately. Furthermore, by monitoring the photocurrent generated by
the detector it is possible to prevent all bright light attacks on avalanche photodiodes.
Dr Andrew Shields, Assistant Managing Director, Toshiba Research Europe, comments,
“Quantum cryptography is now entering a new phase in which the security of
particular implementations is carefully analysed and tested. This is important to
uncover any security loopholes and to devise appropriate countermeasures. It will
allow real-world devices to approach the perfect security that can be proven for
Toshiba recently implemented its quantum key distribution (QKD) technology in the
quantum cryptography network set up in the Tokyo metropolitan area in October 2010.
In a series of trials Toshiba demonstrated record average secure bit rates on installed
fibre in the network. A secure bit rate of 304 kb/s was demonstrated, averaged
over a 24 hour period, on a 45 km fibre despite a relatively high loss
on the link of 14.5 dB. In April 2010 the same team announced an average
secure bit rate of 1 Mb/s for a laboratory based demonstration on a 50 km
For further information about the work of Toshiba's Cambridge Research Laboratory
in Quantum Information Technology, go to
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Z L Yuan
et al., Nature Photonics, 4, 800–801 (2010)