Wednesday, January 02, 2013
Review about quantum coherence
Happy 2013 to all! The Proceedings of the Royal Society A has a special issue from this past fall about the issue of decoherence in quantum mechanics. When last I looked, the content from this issue was free (!) for download. I need to read through the rest of it, but I enjoyed Philip Stamp's article about decoherence and the possibility of intrinsic decoherence. The point is, in ordinary quantum mechanics the state function of a quantum system evolves with time according to the Schroedinger equation, also called unitary time evolution. However, that seems at odds with what happens when we perform measurements - in that situation, it seems like the state of the system "collapses" into an eigenstate of a measured observable, so that coherence appears to be lost, and we don't observed, e.g., Schroedinger's cat to be in a superposition of alive and dead. The now standard treatment says that the apparent decoherence of a quantum system of interest when we "perform a measurement" results from the coupling of the system with many environmental degrees of freedom (a "bath"), the states of which we then "trace over". When we do this, looking only at the system, we see what looks like decoherence of the system, but the idea is that this is an approximation of the true situation, in which the whole system + environment is still obeying the Schroedinger equation. (This skirts other aspects of the "measurement problem", like what really picks out the particular classical states that we seem to observe.) Intrinsic decoherence would imply either that there is something genuinely wrong with unitary time evolution (i.e., quantum mechanics is incomplete), or there are environmental degrees of freedom out there in the very fabric of the universe that are impossible to avoid, such as the quantum degrees of freedom of curved spacetime itself.
Posted by Douglas Natelson at 9:41 AM