Toshiba Research Europe has announced that it has developed two new technologies to realise ‘unconditionally secure’ quantum key distribution (QKD) by overcoming a potential security loophole.
In principle, quantum key distribution provides an absolutely secure means for transmitting secret keys between two parties on fibre optical networks. However, the QKD systems developed so far have a vulnerability which leaves them open to hacking. The weak laser diode used to generate single photon pulses which carry the quantum keys, will sometimes generate pulses with multiple photons.
As a result, an eavesdropper could split off one of these extra photons and measure it, while leaving the other photons in the pulse undisturbed, thus determining part of the key while remaining undetected. Furthermore, an eavesdropper could even determine the entire key, by blocking the single-photon pulses and allowing only the multi-photon pulses to travel through the fibre.
Now two solutions to this problem have been found, the first of which has already been implemented by Toshiba in their QKD system.
Toshiba has implemented a new method for QKD, in which the photon signal pulses are interspersed randomly with a number of ‘decoy pulses’. These decoy pulses are weaker on average and so very rarely contain two or more photons. If an eavesdropper attempts a pulse-splitting attack, she will transmit a lower fraction of these decoy pulses than signal pulses. Thus by monitoring the transmission of the decoy and signal pulses separately this type of intervention can be detected.
By introducing decoy pulses, stronger laser pulses may be used securely, increasing the rate at which keys may be sent. Toshiba has demonstrated a 100-fold increase in the rate that keys could be transmitted securely over a 25km fibre to an average bit rate of 5.5kbits/sec – the highest value to date for a full QKD system. This work is part of the EU initiative SECOQC to build a secure communication network based on QKD.
The second method, based on nanotechnology, will produce even higher bit rates in the future. Toshiba has created the first semiconductor diode that can be controlled with electrical signal input to emit only single photons at a wavelength compatible with optical fibres. This ‘single photon source’ method eliminates the problem of multi-photon pulses altogether. It was developed as part of a DTI funded programme involving the University of Cambridge, Imperial College London and Toshiba.