Using this decoy approach allows more-powerful laser pulses to be used, which in turn allows the bit rate to be increased and, likewise, the distance over which it can be sent, says Shields. Nondecoy signals can achieve about 43 bits per second over a distance of about 25 kilometers. But the decoy approach can achieve 5.5 kilobits per second, which is a 100-fold increase.
It is possible to get unconditional security already, but the challenge is to do this over longer distances, says Gregoire Ribordy, CEO and founder of id Quantique, the Swiss company that, in 2002, launched a commercial quantum-cryptography system. “This decoy allows you to increase the range or the bit rate for a given distance,” Ribordy says.
The decoy approach is a very useful defense against this sort of attack on quantum cryptography, and several groups are now working on similar approaches, says Franco Wong, of the quantum information science group at MIT. But the trouble with claims about unconditional security is that there are currently no means of testing it, except through simulation, Wong says.
The second advance the group has made has more long-term significance, says Shields. This is the development of a light-emitting diode capable of more reliably emitting single photons. “With quantum key distribution, the ideal is to send only single photons,” he says. If one can do this reliably, the transmission would be truly impervious to any attacks, and techniques like decoy pulses would be rendered redundant.
Toshiba’s approach is to create an array of quantum dots, each measuring 45 nanometers in diameter and capable of emitting only single photons. Although a light-emitting diode made using these quantum dots still occasionally emits more than one photon, the chances of this happening are five times less than they would be if using a laser. There are other ways to produce single photons, but one of the benefits of using quantum dots is that they can be easily integrated and controlled by electronics. “Control by a voltage rather than a laser is a great advantage because electrical devices are much more compact and robust,” says Shields.