Here in the Ars science section, we cover a lot of interesting research that may eventually lead to the sort of technology discussed in other areas of the site. In many cases, that sort of deployment will be years away (assuming it ever happens). But in a couple of fields, the rapid pace of proof-of-principle demonstrations hints that commercialization isn't too far beyond the horizon.
One of these areas is quantum key distribution between places that aren't in close proximity. Quantum keys hold the promise of creating a unique, disposable key on demand in such a way that any attempts to eavesdrop will quickly become obvious. We know how to do this over relatively short distances using fiber optic cables, so the basic technique is well-established. Throughout the past couple of years, researchers have been getting rid of the cables: first by sending quantum information across a lake, then by exchanging it between two islands.
The latter feat involved a distance of 144km, which is getting closer to the sorts of altitudes occupied by satellites. But exchanging keys with satellites would seem to add a significant challenge—they move. Over the weekend, Nature Photonics published a paper that indicates we shouldn't necessarily view that as an obstacle. The paper describes a team of German researchers who managed to obtain quantum keys transmitted from a moving aircraft.
The aircraft in question was a Dornier 228 turboprop in which the authors set up a shock-protected optical bench to generate the photons they needed for the experiment. Those were sent via a fiber optic cable to a transmitter on the underside of the aircraft. This included tracking equipment that allowed it to keep the transmissions pointed at a specific ground station.
That ground station was a 40cm telescope operated by the German Aerospace Center. It was kept pointed at the aircraft by using GPS coordinates transmitted by the aircraft over classical communications channels. Once it had a fix, a beacon laser was used to illuminate the aircraft, confirming that a directional link had been established. At that point, the plane's hardware could start transmitting bits using the polarization of photons.
Since this was a proof of principle, the authors simply rotated through four potential polarizations in order to ensure that they could tell when the ground station was picking up the appropriate bit. One of the big problems was noise. Each second, the ground station's detectors were picking up background noise at a rate of about 1,000 events per second, while the aircraft was only transmitting 800 bits per second (so there was a lot of noise to filter out). Some of this was actually from the aircraft's blinking anticollision light, which the detector picked up nicely.
By filtering out the noise (and discarding anything from when the anticollision light flashed), the authors were able to achieve a rate of about 145 bits a second. Adding the extra information needed to detect eavesdropping would drop that to eight bits a second. That would be a horrific rate for transmitting data, but remember, these are just the bits of a key. Once the key is established, encrypted communications can take place on much faster channels. If they were willing to gather keys for a while, they could get as many as 80 kilobits in a single passage of the plane.
In the end, the authors say that the hard part was developing the pointing system and developing a system that could account for the rotation of the hardware as it tracked, which can otherwise skew the measurements. But with those developed, it seems that exchanging keys with a free-moving object is relatively straightforward. We may not be ready to put this in orbit yet, but it certainly seems like we're getting very close to being ready to try.
Nature Photonics, 2013. DOI: 10.1038/NPHOTON.2013.46 (About DOIs).
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