By Hamish Johnston
QKD is a popular quantum-cryptography technique that is already being used commercially. It allows two parties, usually called Alice and Bob, to exchange an encryption key, secure in the knowledge that the key will not have been read by an eavesdropper (Eve). This guarantee is possible because the key is transmitted in terms of quantum bits (qubits) of information, which if intercepted and read are changed irrevocably, thus revealing the actions of Eve.
QKD cannot be cracked if it is implemented using equipment that behaves exactly as expected. Qubits are normally transmitted as single photons, for example, and therefore Alice and Bob must be equipped with single-photon detectors. The problem is that these detectors are not perfect and by simply shining a bright laser at a detector, Eve can trick it into thinking that it has detected a single photon even though that photon has been read by her.
While physicists have come up with several ways of thwarting such attacks, these tend to complicate the QKD process so as to make it impractical. Now, two independent teams of physicists have demonstrated aspects of a new scheme called measurement device independent QKD (MDI-QKD) that seems to close the loophole.
The idea behind MDI-QKD seems crazy at first: instead of Alice and Bob making the measurement that reveals Eve in private, it is made in public and can be seen by all, including Eve. The public measurement is of correlations between pairs of qubits, rather than the values of the qubits themselves. Alice and Bob can then use the qubit values and the correlation information to create their secret key.
Now, Allison Rubenok and colleagues at the University of Calgary in Canada together with Yang Liu and co-workers at the Hefei National Laboratory for Sciences and the University of Science and Technology of China have revealed several important breakthroughs in getting MDI-QKD to work. Papers by both groups are discussed in the article “Foiling Quantum Hackers” in Physics.
So, problem solved. Well, not quite. As quantum-hacking expert Vadim Makarov pointed out when I spoke to him earlier this year at IQC in Waterloo, Eve will simply move on to finding ways of exploiting shortcomings in the equipment that Alice and Bob use to create the signals that they exchange.