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Physicists Blast a Quantum-Shielded Encryption Key 307 Kilometers

Optics researchers at the University of Geneva have successfully transmitted a quantum encryption key 307 kilometers. This is a new record—besting the nearest competition by about 50 km—and, what's more, they were able to implement the transmission scheme using detectors in non-cryogenic conditions, a "must" for IRL quantum encryption.

As with pretty much any potential practical quantum technology, quantum encryption is a great idea with a very difficult implementation. It's a victim of its own quantum-ness, mostly, which means that extending it to the large-scale, classical realm is precarious. Quantum states, whether being realized as qubit information multiplexes or encryption keys, are fragile, readily self-destructing given the slightest untoward glance.

In distributing encryption keys via quantum means—fiber-optically, in this case—the whole point is that the slightest interference will disrupt the system such that the interference will give away a potential eavesdropper immediately. The message will arrive obviously corrupted. The catch then is that it's hard to do anything with quantum information without disrupting it, even as a legitimate party.

The Geneva researchers, led by physicist Hugo Zbinden, were able to make this work by swapping out the delicate, cryogenic single-photon detectors used in most other long-distance quantum key distribution (QKD) schemes for a set-up based instead on a relatively off-the-shelf detection technology. Information here is encoded in time, "where the bit string is encoded in the time of arrival of weak coherent laser pulses," Korzh and co. write in  ​Nature Photonics. "Bob" receives and deciphers messages from "Alice" by measuring the timing of the pulses.

A shift forward or backward in time would reveal the presence of an eavesdropper

The general scheme for such a setup is called  ​coherent one-way (COW) QKD protocol. The security comes in the form of an extra pair of detectors at Bob's end, which are used not so much to extract information but to register whether or not decoy pulses sent by Alice arrive out of phase with proceeding test pulses. A shift forward or backward in time would reveal the presence of an eavesdropper, in which case Bob can torch the message and request a new key from Alice, starting the process over.


So far every other QKD scheme involving distances larger than 160 km has used superconducting nanowire single-photon detectors (SNSPD), which are a rather moody sort of technology requiring near-absolute zero temperatures to be fully effective. They have the advantage of being able to achieve low dark count rates. For example, they can register relatively large numbers of photons without environmental interference.

"The fundamental difference in our setup is the type of the detectors used," Boris Korzh, the lead author behind the new paper, told me. "For a QKD system to reach the long distance operation, the efficiency and the noise of the single photon detectors has to be very good. Previously, whenever researchers tried to reach the maximum distance with a QKD system, they would opt for using SPDs based on superconductivity, which meant operating them at cryogenic temperatures, typically lower than 3 K. In our setup, we made use of semiconductor SPDs, which do not require cryogenic temperatures."

"Such detectors have been around for a long time," Korzh said, "however their noise performance was always worse compared to superconducting detectors. We have been working with a new generation of semiconductor SPDs and by optimizing their operating conditions have demonstrated that they can perform just as well. For comparison, the operating temperature is now 150 K, which is easily achieved with a compact cooler."

The technology detector trade-off is in terms of response time: high-gain signals come at the expense of a slight wait. A 223 K operating temperature (about -60 °F) meant a delay of around eight microseconds.

Finally, Zibinden and his team conclude, "This work demonstrates that practical, robust and autonomous QKD is feasible over very long distances, even with standard telecom components and rack-mounted architectures."

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