A team of researchers at the University of Science and Technology of China has successfully implemented a highly resilient form of quantum encryption over 404 kilometers of optical fiber, establishing a new distance record for the technique known as measurement-device-independent quantum key distribution (MDIQKD). The new record, a rough doubling of the prior longest distance, offers the promise of quantum communication between distant cities across relatively noisy networks.
Quantum key distribution (QKD) is, generally, a quantum cryptographic technique in which encryption keys—used to mathematically unlock protected data—are moved from place to place under the protection of quantum correlations. A quantum system has the handy property of fragility: The mere act of looking at a such a linkage of particles will have the effect of destroying it, even if the "looking" in question involves single particles of light. Consequently, the proper sender and receiver of a quantum key will know immediately if their key has been eavesdropped upon.
Because these quantum states are so fragile (and thus useful), there's an inherent challenge in getting them from place to place. Distance adds noise and no channel is perfectly pristine. A second, related complication is that quantum key distribution isn't actually perfectly secure—there's a loophole, of sorts. A potential eavesdropper can grab data from a quantum channel so long as they ensure that the intended recipient of the key still receives the key as expected. This is known as an "intercept-resend" attack and it means reproducing a seemingly unadulterated quantum state.
That's really just one example of several possible quantum hacks. The point is that QKD isn't actually perfect, though it may appear so in theory. This is what MDIQKD, a relatively recent flavor of QKD, aims to correct. It involves the usage of decoy states and an intermediary relay to parse them. Basically, a randomized set of four signals is sent across the channel, which are received by the relay and authenticated. (Sorry if that's a bit unsatisfying as an explanation, but things sail off a math cliff here.)
At "metropolitan" distance scales of around 100 kilometers, researchers have until now topped out at bit rates of around several bits per second. That doesn't cut it for practical applications, alas. To encrypt data using slowly-generated keys, we need to have data that exists in great big chunks, where one big chunk can be protected by one key. In, say, a telephone conversation, data is produced in quick units and encrypting each of those units means accumulating encrypting keys just as fast.
This is the second big advance offered by the Chinese team: speed. Compared to prior MDIQKD demonstrations, they were able to achieve a 500-fold increase in key generation rate. This is indeed fast enough to encrypt a telephone voice conversation. The catch is that the distance record and the key rate don't actually overlap. For metropolitan distances, the researchers were able to get rates of around 1.38 kbits per second, but this falls pretty dramatically as we get to maximal distances. Still, 1.38 kbits per second offers the possibility of metro-scale quantum-secured networking, which is a very real-world offering.
The group's work is described in the current Physical Review Letters.