Pointer basis

A basis of apparatus records robust under the relevant environmental interaction. Its meaning is dynamical and depends on the coupling and the time scale, rather than on an arbitrary coordinate choice for the apparatus Hilbert space.

If record projectors commute with an apparatus-environment interaction, their populations are preserved while the corresponding environmental states can become distinguishable. Their conditional environment overlap then suppresses interference in the apparatus reduced density matrix. A nondegenerate record coupling can thereby select robust sectors; commutation alone does not select a unique basis in a degenerate or trivial interaction.^[ref]