[Cryptography] Quantum computers and the Government

[Sam Hartman]

    John> Pretty confident.  Some calculations are amenable to quantum
    John> methods, some aren't.  Only what one might call "reversible"
    John> computations are, but those happen to include
    John> multiplcation/factoring and exponentiation/logarithm.
    John> Hashing, on the other hand, is not reversible because multiple
    John> inputs hash to the same output and you can't tell which input
    John> you started with.  So it's relatively straightforward to tell
    John> whether a quantum computer could run an algorithm.

Can you give me some reading on this or places to start to understand
why we think it is that  some computations are not amenable to quantum
methods?
I'd really like to understand why it is we believe that if there is a
separation between P and NP, there is a separation between QP and NP.
I mean I guess what I'm probably asking is to get a better understanding
of the quantum computation model and how that relates to classical
models I studied back at MIT.

--Sam