How useful is the quantum internet in practice? 

Admittedly, many applications of the quantum internet will depend on how much we will use quantum computers. If quantum computers become widespread in the future, then communication between them also seems to be extremely worthwhile. On the other hand, our current outlook of quantum computers focuses on special-purpose devices that are used to solve isolated problems. It is not immediately clear why they would benefit from a quantum internet.

There is a clear road map to build a reliable quantum internet in the future (involving fascinating tricks like entanglement distillation and teleportation), but this would require multiple error-corrected quantum computers by itself! For that reason, in this guide, we’re not yet looking ahead at applications like clustering computers, multi-party computations, private computing, or making sensors more effective.  Regarding clustered quantum computers, we frequently hear arguments that we can make a ‘bigger’ quantum computer by connecting individual ones. I’m always wondering why those computers are not just simply built next to each other. Connecting those is much, much easier than connecting them over a quantum internet. 

In the foreseeable future, the first interesting applications are those that work over a “noisy” connection, and transport just one qubit at a time (or perhaps a handful of them). For practical interest, Quantum Key Distribution (QKD) is by far the most interesting application.

The case for QKD

To fully understand QKD, we will need to have a bit more background about cryptography, especially the “key distribution problem”. For a full account, we recommend first reading the chapter on cryptography. In short: we’re wondering how Alice can agree on a secret key with her distant friend Bob, in a world where everyone can read plain data sent over the internet. Surely they can’t just generate a random key and send it over to each other, without having any encryption in the first place! This problem is commonly solved using public key cryptography (which we know will be revamped in the following years). If you really don’t trust public-key cryptography, the main alternative is to physically transport a usb stick by a trusted courier. 

Compared to conventional cryptography, the unique selling point of QKD is that it is fundamentally impossible for cybercriminals to obtain the secret key as it is being distributed. As long as our understanding of quantum mechanics is correct (arguably the most well-test theory in science), no amount of computational power or mathematical breakthroughs will let an attacker gain information about the key. Of course, this assumes that the protocol is executed precisely as prescribed, and there are no other vulnerabilities in the actual hardware or software used. 

This is fundamentally different from today’s approach of public key cryptography, which must rely on certain mathematical assumptions. We know for sure that, with sufficient computational power, these codes can be broken, but we argue that this takes such an incredibly long time that nobody will bother (except for bragging rights and prize money). Still, such statements about computation times are based on assumptions, and our trust is purely based on our experience that no brilliant cryptanalyst has broken it yet. In fact, well-regarded cryptosystems do get broken from time to time, such as SIKE, which was in the race to become a new NIST standard. 

That said, although QKD is “unhackable” in theory, the actual hardware and software are equally likely to contain vulnerabilities. Contrary to well-trusted public-key cryptography, no QKD system has received proper certification and accreditation, and a significant fraction of historical products have been hacked. 

QKD requires specialised hardware — although it is much less demanding than other quantum internet applications we mentioned. It can already prove useful on a basic point-to-point network with just two connected parties, of which one should send photons, and the other receive them. This is orders of magnitude less complex than building a fully-featured internet. Moreover, the qubits need only be sent and measured one at a time: no quantum memory or extensive computation are needed. There have already been several demonstrations that use standard telecom fiber (the stuff that’s already in the ground), or satellite-based systems that communicate through air. QKD hardware is fancy and expensive, but not completely out of reach. 

The major downside of QKD is that it has no way to confirm who the person on the other end of the line is. Some form of authentication is still needed – which is mostly done with secret keys that should already be present in the first place! This makes QKD just a partial solution to the key distribution problem. 

Further reading

• A video explanation of QKD for laymen or experts.
• Companies like Toshiba and ID Quantique offer commercial QKD systems for distances of around 100 km. 
• Chinese scientists achieve QKD through satellites over 1000 km. 
• Nature commentary why practical long-range QKD is still out of reach.