War on the Rocks, or a quantum computing text book slowly being built as an online spaced-repetition website: Quantum Computing for the Very Curious.
Nonetheless, no specialized knowledge of quantum mechanics is required to see that the Quantum Communications field is full of activity. There are multiple commercial ventures selling quantum key distribution (QKD) equipment. The Chinese ‘quantum’ satellite, Micius, made a big splash several years ago by distributing quantum keys between ground stations 1200 km apart. Additionally, every major player in quantum technologies has some sort of quantum communications network. The Chinese network spans many nodes in several cities, Vienna, Tokyo, and Los Alamos are also running more limited QKD networks as well2. In general, the United States focuses more on quantum computing, but still has a very active quantum comms academic research community. China seems to be most advanced, as they not only have demonstrated ground-space QKD, but have also kicked off a flurry of research activity on sub-marine QKD links.
A quick glance through the list and you can see that quantum communications is currently limited pretty much entirely to QKD implementations. Further, what implementations exist are suboptimal in multiple ways: security proofs often rely on strong assumptions3 about many parts of the system, key rates are comically low, and many protocols are stymied by lossy transmission channels. There are entire research labs whose activities are entirely dedicated to exposing security flaws in laboratory and commercial QKD implementations. In the near term, there is not much use to be had from QKD, but as researchers in the field continue to iterate on their protocols, this may change.
Why? Why do all of this? Why launch what amounts to a full quantum optics lab into space? Why set up quantum communications nodes in major cities?
Pretty much everyone will tell you that quantum communications offer unhackable, provably secure communications channels which, as a bonus, can notify you when a third party is attempting to eavesdrop on your conversation.
Let’s leave aside for now the gaping holes in the phrase “provably secure” and consider the case of ideal quantum communications. We can assume the laws of physics as we know them are ironclad, and the bitrates of quantum communication protocols approach those of classical communications.
Unhackable messages sound great in principle, but do you need a fully quantum communications infrastructure to realize it? I could encrypt my emails and texts such that even the mighty US government would have no hope of cracking the encryption by brute force. The entirety of our society is built on being pretty sure that no-one can efficiently factor prime numbers. There’s no mathematical proof that it can’t be done, but clearly such proof is not required.
What circumstances would warrant such ironclad security guarantees?
The time horizon for this scenario is thought to be somewhere between 5-100 years. My personal guess is at least 50 years.
I expect that classical information encrypted with whichever post-quantum standard NIST chooses will be totally sufficient for the every needs of private citizens, businesses, and most local, state, and federal government entities. Predictably, extraordinarily sensitive government and military communications might benefit from what quantum comms have to offer. I could see something like the nuclear command and control system being run preferentially on quantum nodes, with the classical communications infrastructure as a redundant back-up. Perhaps time-sensitive and extremely secret diplomatic cables would also be a plausible use case. Even so, it’s worth pointing out that quantum comms only solve the signals intelligence side of security problem. Information will still be read and known by humans, some of whom will always be vulnerable to overtures from adversarial intelligence organizations.
The state of the art of quantum communications is still far, far from realistically satisfying the use cases above. Bitrates are low (< kbps over relevant distances), the protocols are largely limited to quantum key distribution, and security proofs have only recently begun to grapple with realistic empirical conditions just to name a few. You still have to do a lot of work crawling through the literature to properly compare various protocols and implementations. Still, here’s a list of a few important enabling advances:
The quantum business landscape is currently crowded with all kinds of companies, most of which are peddling some sort of software solutions for some hypothetical future quantum computer. None that I know of are offering anything like what I’ve described in the list above. In the coming weeks and months I will attempt to survey the business landscape to suss out what exactly is going on and determine exactly how high the bullshit has been piled up.