Recently Verizon announced that they have begun to test quantum key distribution (QKD) with the goal to use it for securing communication links. That is great news. I believe that quantum key technology will become a significant element of security, going forward, to protect secret keys. It is closer to perfect than other security scheme, thus far. And not just for communications, but anywhere a secret key is used.
I think drilling down a bit on QKD is warranted. Why? Because most of the media outlets, including some of the wireless ones, do not have much of a clue as to what it really is. To wit, often when they talk about it, they put it in the wheelhouse of quantum cryptography, which it is not.
When they come across it, such as the recent Verizon release, the report simply puts some verbiage out there from the press release followed by some general data about QKD pulled from a quantum site that talks about it. Then they go on to discuss, in layman’s terms, what QKD will do.
This is because they just do not understand such technologies and cannot make much of a press release out of it without a lot of quotes and site data. So, they fill in with what is going on with the release topic and talk about other issues. This kind of reporting on QKD is well worn and it has been playing for years already.
Now, QKD is an awesome technology for securing data transmissions. And, just to reiterate, this is not the same as quantum cryptography, which many non-technical discussions seem to imply. So, let us expand this a bit and talk about what Verizon is doing and how it works.
First, Verizon’s experiment was between two points, using a fiber link, not an RF link. At the moment, QKD links can only be accomplished over a fiber link or from optical free-space links (telescopes) and point to point (although in 2017, a Chinese satellite named Micius sent entangled photons to three different ground stations, each separated by more than 745 miles, which broke the distance record for entangled particles). They have yet to be accomplished via any RF links, which make up most of the wireless communications in existence.
Second, they are relatively limited in real-world distance. In such experiments, the link has been limited to around 62 miles. In controlled lab experiments, that has fared better. However, the longest successful QKD transmission is just over 248 miles over special low-loss fiber and 745 miles via a free-space optical link. At present, the non-ideal condition is short because photon losses, for both fiber and free space, increase dramatically with distance.
However, 62 miles of distance may become acceptable for much of the future communications based on 5G technology – cell towers, small cell sites, various types of networks (Wi-Fi, particularly), and upcoming platforms such as mesh networks and autonomous vehicles. However, distance is not the major challenge. There are other factors that must be overcome, such as scattering and interference. However, eventually, QKD, just like quantum computing, will become a useable technology.
QKD is based on the quantum mechanics principles of entanglement (QE) and superposition. These were first proposed by Albert Einstein in the 1930s. QKD has been around since the 1970s (although it took the 1990s to give it traction).
The communications sector has been working with photons, but QE can be accomplished with a variety of particles – electrons, photons, molecules, etc. And it is not limited to individual particles. In practice, items like magnets, metals, even the human body have hundreds of entangled molecules, all of which act as a single object and can be used in the entanglement game.
The theory of entanglement, in short, is that multiple particles are linked together in a way such that the measurement of one particle’s quantum state determines the quantum states of the other particles, even separated by large distances. This is why QE is such a panacea for security. If you mess with one particle, it reflects on the others. Therefore, if the destination is not the exact replica of the source, one can assume the package and the key has been compromised.
A second required condition of quantum mechanics is superposition. It states that particles exist in multiple states, simultaneously. Photons, for example, can display simultaneously both horizontal and vertical states of polarization.
Superposition says that if the state of one of the entangled pair is disturbed, that disturbance will be reflected on the other particle. And, once the entangled state is compromised, even by observation, it will collapse or disappear altogether. Superposition also states that such particles can exist simultaneously, in separate places, hence any disturbance on one is immediately reflected on the other. In theory, QKD can alert whatever is monitoring it that a compromise has happened before the data actually arrives.
So, using quantum mechanics, in the form of QKD, to secure a key is where this is all heading. In a nutshell, if the quantum elements of the key have been compromised, the assumption is that the key may be as well, and the same for the data.
Simple enough, at least in theory. However, it will be years, if not decades, before QKD will see widespread use. There are just too many other environmental conditions, which must be controlled, that affect quantum transmissions. As well, practical applications of quantum mechanics are also years off.
Fiber is likely to become the first success story. Optical next, then wireless. But I would not hold my breath.