Decoherence and the emergence of classical behavior

4 problems

• Do generic interactions copy a system's state redundantly into its environment?

  Open

  Quantum Darwinism is the proposal that observers agree on classical facts because the environment holds many independent copies of the same information about an object, so many observers can read it without disturbing it. Whether ordinary physical interactions produce such copies, or only specially designed ones, is open.

• Which set of decoherent histories describes the quasi-classical world?

  Open

  The consistent-histories approach assigns probabilities to sequences of events only when the sequences do not interfere with each other. Many incompatible families of such sequences pass this test, and no agreed rule picks the family that matches the classical world.

• Is entropy production of a decohering chaotic system set by Lyapunov exponents?

  Open

  When a quantum system whose classical version is chaotic (extremely sensitive to initial conditions) is weakly disturbed by its surroundings, Zurek and Paz predicted that it loses information at a rate fixed by its chaos, independent of how strong the disturbance is. Whether and when this holds is not settled.

• Does the Hamiltonian alone determine which subsystems behave classically?

  Open

  Decoherence arguments start by splitting the world into an object and its environment, but quantum theory does not say where that split lies. The question is whether the energy function of the whole system (its Hamiltonian) by itself picks out one natural split in which classical behavior appears.