3 ms·

Thanks! I was about ask: Why is cloning a problem, but not a downconverter crystal (specifically a Type-0 SPDC [0]) that creates two photons with the same pola

• archgoon 7y ago
  Hey! That's pretty cool! I wasn't familiar with that (and probably would have been stumped by the question :) ). Thanks for the solution! Also; thanks for a great example of something that looks like cloning (and could be mistaken as such) but isn't. I'll need to keep it in mind when explaining things in future. By the way; the math reason for the impossibility of cloning states boils down to the fact that Quantum State evolution is described by a unitary operator. The linear algebra[1] shows that this means that you can't have a process that takes a state Q(|psi> x |r>) => |psi> x |psi> For a general |psi>. So it'd be very surprising to discover a process that pulls this off. :) That said; I find it very surprising that causality gets preserved in regular quantum mechanics by a completely unexpected mechanism, namely unitary state evolution. Regular QM doesn't know about special relativity, or its needs, and by pure accident it seems to prevent a giant loophole that would play havoc with causality. [1] https://en.wikipedia.org/wiki/No-cloning_theorem#Theorem_and_proof https://en.wikipedia.org/wiki/No-cloning_theorem#Theorem_and...