There is a great deal of buzz around the idea that with quantum cryptographic network links, we can shift to eavesdropping-proof communication that would be secure against every known form of attack. The catch? There is a serious risk of being tricked into believing that a totally insecure network link is a quantum cryptographic one. In fact, it may be much easier and cheaper to build and market a fake quantum link than to create a real one! Worse, the user probably wouldn't even be able to tell the difference. You could eavesdrop on a naive user or her easily if you built one of these fakes and managed to sell it. What's not to love, if you are hoping to steal secrets?
So, first things first. How does quantum cryptography actually work, and why is it secure? A good place to start is to think about a random pad: this is a source of random bits that is created in a paired read-once form. You and your friend each have one copy of the identical pad. For each message, you tear off one "sheet" of random bits, and use the random bits as the basis of a coding scheme.
For example, the current sheet could be used as a key for a fast stream cryptographic protocol. You would use it for a little while (perhaps even for just one message), then switch to the next sheet, which serves as the next key. Even if an attacker somehow was able to figure out what key was used for one message, that information wouldn't help for the next message.
This is basically how quantum cryptography works, too. We have some source of entangled photons, and a device that can measure polarization, or "spin". Say that up is 1 and down is 0. In principle, you'll see a completely random sequences of 0/1 bits, just like one sheet of a random pad.
Because the photons are entangled, even though the property itself is random, if we measure this same property for both of the entangled photons, we obtain the same bit sequence.
Thus if we generate entangle photons, sending one member of the pair to one endpoint and the other photon to the other endpoint, we've created a quantum one-time pad. Notice that no information is actually being communicated. In some sense, the photons do not carry information-per se, and can't be forced to do so. The actual bits will be random, but because the photons are entangled, we are able to leverage the correlation to read exactly two copies out, one copy at each endpoint. Then we can use this to obscure our messages (a classical method is used to authenticate the parties at each end, such as with RSA-based public and private keys).
Quantum cryptography of this form is suddenly being discussed very widely in the media, and there are more and more companies willing to sell you these cables, together with the hardware to generate entangled photons and to read out the binary bit strings using measurements on the entangled photon pairs. So why shouldn't everyone leap up this very moment and rush down to Home Depot to buy one?
To see the issue, think back to the VW emissions scandal from 2015. It turned out that from 2011 to 2015, the company was selling high-emission engines that had a way to sense when they were being tested. In those periods, they would switch to a less economical (but very clean) mode of operations This would fool the department of motor vehicles, after which the car could revert to its evil, dirty ways.
Suppose the same mindset was adopted by a quantum cable vendor. For the non-tested case, instead of entangling photons the company could generate a pseudo-random sequence of perfectly correlated unentangled ones. For example, it could just generate lots of photons and filter out the ones with an unwanted polarization. The two endpoint receivers measure polarization and see the same bits. This leads them to think they share a secret one-time pad... but in fact the vendor of the cable not only knows the bit sequence but selected it!
To understand why this would be viable, it helps to realize that today's optical communication hardware already encodes data using properties like the polarization or spin of photons. So the hardware actually exists, and it even runs at high data rates! Yet the quantum cable vendor will know exactly what a user will measure at the endpoints.
How does this compare to the quantum version? In a true quantum crytographic network link, the vendor hardware generates entanged data in a superposition state. Now, this is actually tricky to achieve (superpositions are hard to maintain). As a result, the vendor can predict that both endpoints will see correlated data, but because some photons will decorrelate in transmission, there will also be some quantum noise. (A careful fake could mimic this too, simply by computing the statistical properties of the hardware and then deliberately transmitting different data in each direction now and then).
So as a consumer, how would you test a device to unmash this sort of nefarious behavior?
The only way that a skeptic can test a quantum communication device is by running what is called a Bell's Inequality experiment. With Bell's, the skeptic runs the vendor's cable, but then makes a random measurement choice at the endpoints. For example, rather than always measuring polarization at some preagreed angle, it could be measured at a randomly selected multiple of 10 degrees. The idea is to pick an entangled superposition property and then to measure it in a way that simply cannot be predicted ahead of time.
Our fraudulent vendor can't know, when generating the original photons, what you will decide to measure, and hence can't spoof an entanglement behavior. In effect, because you are making random measurements, you'll measure random values. But if the cable is legitimate and the photons are genuinely entangled, now and then the two experiments will happen to measure the same property in the identical way -- for example, you will measure polarization at the identical angle at both endpoints. Now entanglement kicks in: both will see the same result. How often would this occur? Well, if you and I make random selections in a range of values (say, the value that a dice throw will yield), sometimes we'll bet on the same thing. The odds can be predicted very easily.
When we bet on the same thing, we almost always read the same value (as mentioned earlier, quantum noise prevents it from being a perfect match). This elevated correlation implies that you've purchased a genuine quantum cryptography device.
But now think back to VW again. The company didn't run with low emissions all the time -- they had a way to sense that the engine was being tested, and selected between emission modes based on the likelihood that someone might be watching. Our fraudulent vendor could try the same trick. When the cable is connected to the normal communication infrastructure (which the vendor supplies, and hence can probably detect quite easily), the cable uses fake entanglement and the fraudulent vendor can decode every message with ease. When the cable is disconnected from the normal endpoint hardware, again easy to detect, the vendor sends entangled photons, and a Bell's test would pass!
Clearly, a quantum communications device will only be trustworthy if the user can verify the entire device. But how plausible is this? A device of this kind is extremely complex.
My worry is that naïve operators of systems that really need very good security, like hospitals, could easily be fooled. The appeal of a quantum secure link could lure them to spend quite a lot of money, and yet most such devices may be black boxes, much like any other hardware we purchase. Even if a device somehow could be deconstructed, who would have the ability to validate the design and implementation? A skilled skeptical buyer might have no possible way to actually validate the design!
So, will quantum security of this form ever be a reality? They already are, in lab experiments where the full system is implemented from the ground up. But one cannot just purchase components and cobble such a solution together: the CIO of a hospital complex who wants a secure network would need to purchase an off-the-shelf solution. I can easily see how one might spend money and end up with a system that would look as if it was doing something. But I simply don't see a practical option for convincing a skeptical auditor that the solution actually works!