when bad quantum physics invade military news

Quantum communication is a powerful tool, but it's not impossible to hack or very practical yet, especially for military purposes.

As said by Futurama's mad scientist at large, Professor Farnsworth, quantum mechanics mean that anything can happen for any reason or without one. Of course this was really a swipe at how so many of us tend to see the complex physics of quantum objects, and it's a very valid one since there's a seemingly endless stream of cranks and quacks more than happy to abuse quantum physics to justify whatever woo they're selling at the moment. But unfortunately, physics abuse also tends to invade the news, and this time, it lodged itself rather deep into a column about a recent Chinese experiment with quantum communication, which says that an elaborate system which tries to use quantum entanglement for communication is destined to be a brand new superweapon in China's military arsenal, its security guaranteed by the very laws of quantum mechanics. But as with all splashy headlines and sensational articles, the author lets his imagination get the better of him.

We've dealt with quantum entanglement before, noting that this effect travels at least 10,000 times the speed of light, and may actually be instantaneous as far as we know. According to the article, the Chinese military could easily harness quantum teleportation, in which entanglement is used to communicate different states virtually instantly across an entangled pair of photons, to completely revamp its military communications and use the Uncertainty Principle to ensure the security of their channels, picking up when someone interferes in the information stream to eavesdrop on their messages. But the reality of the matter is much more complex. It may be true that quantum teleportation instantly transports quantum states between two entangled photons, but when done outside of fiber channels, as the cited Chinese experiment was, you get a loss in fidelity since the photons have to travel through an atmosphere to a far-off receiver. Over 16 miles, the fidelity was 89%, an impressive result for an experiment, but certainly unacceptable for military communications since your call for a retreat, or a critical attack to sway the battle's momentum has an 11% chance of being lost or garbled.

Essentially, the setup works in the following way. A pair of photons is entangled, then the higher energy one is sent to a receiver as its counterpart's state is altered. When the traveling photon arrives, it could be measured to find out the state of its counterpart, and even though the entanglement itself is nearly instant, data exchange is still limited to the speed of light in a given medium as not to violate the laws of causality. But you can't drop in and try to eavesdrop on the information in the signal because, according to the article, trying to measure an active quantum system alters it, as per the Heisenberg Uncertainty Principle, right? Note how I said this was according to the article rather than the principle itself. That's because this is not what Heisenberg posited and if you were to check out a terrific explanation by physicist Chad Orzel, you'd see that the crux of the matter is the dual nature of quantum objects. Because they behave as particles and as waves, you can really only get a measurement of either their momentum or position in space, not both at the same time. That's the uncertain part here, not trying to put an obstacle in the way of the photons to measure their quantum states.

Yes, trying to interfere with a system will obviously disrupt it, but that part has nothing to do with Heisenberg's idea, and it also doesn't mean that you could never listen in on the data exchange because there's actually an elegant and effective way to eavesdrop on quantum communications. It involves mimicking the signals being sent, and presenting yourself as part of the exchange stream, reading the message as it comes in to then regurgitate it to the intended receiver without leaving either system aware that the data was just intercepted. It was reported in, of all places, Popular Science, and it seems that the author of the article in question decided to peruse the military angle and didn't bother to check if there were any ways of intercepting quantum signals, relying on a defense think tank consultant as his primary source for scientific information and not bothering to do a bit of researching of his own. The same applies to his claim that quantum communication would allow the Chinese navy to send messages to submerged submarines, a claim which ignores that photons travel so slowly in water, we break the speed of light in this medium all the time. This would severely affect a signal's fidelity. And besides, lasers fired from a satellite to the subs' receivers would quickly give away their position, something that no navy would think was a good idea to do.

So the moral of the story here is not to get a little too carried away when you come across something with the world quantum in its description and go off speculating about sci-fi warfare. Of course the U.S. military needs to stay competitive with other nations, but we really shouldn't feed politicians whose primary sources of hard scientific and information technology news tend to be these breezy, speculative columns, with red herrings to chase instead of simply leaving the research, development, and analysis to experts. Remember Liberman's quest to control the web? Something tells me that it might not be a good idea to expose military physics to a politician looking to brag about forcing a project that supposedly keeps the U.S. on the cutting edge of military technology, but in reality is simply trying to fulfill someone's sci-fi dreams…

References: Jin, X.-M., et al. (2010). Experimental free-space quantum teleportation Nature Photonics, 4 (6), 376–381 DOI: 10.1038/nphoton.2010.87

Lydersen, L., et al. (2010). Hacking commercial quantum cryptography systems by tailored bright illumination Nature Photonics DOI: 10.1038/nphoton.2010.214