On 29/06/2020 20:09, jim bell wrote:
"/Scientists in China managed to exchange a crypto key at a distance of over 1,000 kilometers/"
Secure quantum communication is very tricky, and afaik the Chinese didn't do that, they just did some entangled photon measuring. First, the non-cloning theory doesn't say what most people think it does - it actually says that you can only clone a maximum of 5/6 of photons (technically 2/3 get cloned, plus half the remainder or 1/6 just happen to be the same orientation by chance). Second, you need a classically authenticated channel as well as the cloning channel to securely exchange keys - it doesn't have to be secret, but it does need to be authenticated. If Mallory can attack that authentication he can prevent key exchange, and in some cases can obtain the key. Although, in a way quantum mechanics can be described as a
cosmic joke played on physicists. Somewhat akin to Kurt Godel's work being a joke played on mathematicians and logicians.
More like Godel shoving a stick up their asses. Godel's first incompleteness theorem is simple: First, if a system of logic is complicated enough to contain the statement "this statement is a lie" then the system contains contradictions [*]. Second, any system of mathematics which is complicated enough to include natural arithmetic is complicated enough to contain that statement. That's not quite how Godel put it, but it's much the same result. Of course everyone already knew that really, but pretended it was just a curiosity so they could look for a universal system for all mathematics; the shock was just Godel pointing it out in a way that could not be ignored. I suspect that one day quantum mechanics may be seen exactly the same way. [*] statements which cannot be assigned a true or false value. A bit like qubits... Peter Fairbrother