Many upcoming quantum technologies will require a source of multiple lone photons with identical properties, and for the first time these researchers may have an efficient way to make them. With these quantum dots at their disposal, engineers might be able to start thinking about new, large-scale quantum communications networks.
The reason we need identical photons for quantum communication comes back to the non-quantum idea of key distribution. From a mathematics perspective, it’s trivially easy to encrypt any message so that nobody can read it, but very hard to encrypt a message so only some select individuals can read it, and nobody else.
The reason is key distribution: if everybody who needs to decrypt a message has the associated key needed for decryption, then no problem. So how do you get the key to everyone who needs to decrypt it?
This Stanford invention helps handle entangled photons, but does it introduce vulnerabilities in the process?
Quantum key distribution uses the ability of quantum physics to provide evidence of surveillance. Rather than making it impossible to intercept the key, and thus decrypt the message, quantum key distribution simply makes it impossible to secretly intercept the key, thus giving the sender of the message warning that they should try again with a new key until one gets through successfully. Once you’re sure that your intended recipient has the key, and just as importantly that nobody else has it, then you could send the actual encrypted file via smoke signal if you really wanted to — at that point, the security of the transmission itself really shouldn’t matter.
There has been some promising research in this field — it’s not to be confused with the much more preliminary work on using quantum entanglement to transfer information in such a way that it literally does not traverse the intervening space. That may come along someday, but not for a long, long time.
Regardless, one of the big problems with implementing quantum key distribution is that the optical technology necessary to get these surveillance-aware signals from sender to recipient just aren’t there. In particular, the wavelength of photons changes as they move down an optical fiber — not good, since creating photon with precise attributes is the whole source of quantum security.