Re: For the smart guys/girls/what-have-you here: "Towards a European quantum internet"

Well, I understood the first sentence of the abstract, but the rest sort of got away from me...

Perhaps this sums it up

If photons at a distance are entangled, what is the optical fibre needed for? I must be missing something.

The optical fibre gets one of the two entangled photons to the remote location where it's state gets inspected.

All this talk of quantum entanglement just serves to confuse investors and public money grantors. The more confused the stories, the more money they get.

'Quantum' just means that interactions between waves and matter can only occur in fixed-size chunks. There are plenty of macroscopic examples of that, so it shouldn't come as too much of a surprise.

'Entanglement' means that if you execute the same measurement in two places of wave phenomena with the same origin, the results are correlated. Again, that's what you'd expect.

Entanglement is kind of hard to do in the optical domain if the setup extends over kilometers and quantum-limited signal detection is kind of hard at frequencies much below optical, I'll grant them that.

Jeroen Belleman

No, it's real.

The idea here, I understand, is to use a system of entangled photons, carried over a fiber-optic cable, to create a secure system for doing encryption-key sharing/distribution between cooperating endpoints. Two data centers (for example) can use such a link to create encryption-session keys for doing bulk data transmission.

Doing secure key distribution is one of the big difficulties in setting up data encryption. The algorithms which perform the actual encryption of large amounts of data (for example, AES-256) can handle a lot of data (gigabits per second), and they're very strong and are considered infeasible to break anytime soon. However, they are "secret key" algorithms - the sender and receiver each need to know the same secret key (e.g. a 256-bit number). You need a way of enabling the sender and receiver to share a key (either one creates it and sends it to the other, or they mutually derive it through some sort of shared process) *without* anyone else being able to intercept the key.

In recent decades this sharing has usually been done through "public-key" cryptography - for example, the RSA or ElGamal algorithms. Unfortunately, these algorithms are known to be potentially open to attack by a "quantum computer" with a sufficiently large number of qubits. No such exist today, but it's suspected that they may be developed within the next couple of decades. Once that happens, RSA-based keys will be insecure (and any communications which were based on them today could be recorded, and eventually "broken" by a quantum computer).

There are new key-sharing/encapsulation algorithms being developed for software use, which are _believed_ to be resistant to attack by quantum computers, but they're still new, and there's always the risk that improvements in quantum computing might render these algorithms vulnerable.

So, researchers are trying to create ways of key-sharing which are fundamentally unbreakable/untappable, using _physical_ techniques based on quantum entanglement. You create entangled pairs of particles, send one in each pair to your friend in the next city, and then do measurements on your particles which enable you to (eventually) create a key known to both parties.

The advantage to this sort of approach is that (by current theory) it should be impossible for any attacker to be able to "wire-tap" the entangled photons being transmitted, and copy them (and thus be able to reverse-engineer the keys being created). Doing so would require interacting with the entangled photon being transmitted, and break the entanglement, and this would cause the secure key distribution algorithm to fail. In short, if you "peek" at the signal in the cable, the key-sharing system immediately stops working and an alarm goes off, and the people who "own" the cable will know immediately.

The amount of data one can send over an entangled-photon link isn't very high, but it doesn't need to be, since you'd be using it only for secure key creation.