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Aug
27
comment Reversing gravitational decoherence
@RonMaimon, I mean that gravity couples the system to its environment, and so you get a change in the environment over time depending on the state of the system, and hence the reduced density matrix becomes mixed. As regards unitaries, I was refering to the general case. Even if you think about a static EM field, this induces a unitary transformation on the system (no decoherence, but it is altered) and to recover the initial state you need to apply another unitary. In the general case, states can become delocalised, mixing the reduced density matrix, even if the global state remains pure.
Aug
27
answered Reversing gravitational decoherence
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May
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comment What can the D-Wave quantum computer do?
@episanty: That is true of every problem in PSPACE. It might be that P=PSPACE, but we're pretty sure it's not true. But again, the only real evidence is that we can't prove equality.
Apr
18
comment Higgs mass and the hierarchy problem
I fixed your latex. You can include it yourself by enclosing it in dollar signs (like an inline equation in latex).