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Quantum decoherence arises when different system alternatives correlate with distinguishable environmental states. For an initially uncorrelated state, a measurement-like unitary time evolution can produceTaking the environmental partial trace givesThe environmental overlap suppresses locally observable quantum coherence. Scattered photons, surrounding gas and uncontrolled apparatus variables can carry away such records. The interaction often singles out robust states forming a pointer basis. Decoherence can be much faster than thermal energy relaxation because recording a macroscopic distinction need not transfer much energy.

The overall state can remain pure and evolve unitarily even when the system alone looks like a classical mixture. Discarding environmental information is therefore not, by itself, a proof of fundamental wavefunction collapse or a solution to the definite-outcome measurement problem. A small controlled environment can return coherence, as in the single-qubit environment dephasing model; practical irreversibility arises when recovering the many dispersed records becomes inaccessible. In an interferometer, varying gas pressure or an independently controllable path marker tests the dependence of fringe contrast on this environmental coupling. Quantum computation must suppress or correct such unwanted system–environment entanglement to preserve the intended interference.

Write $H=g{I}_S\otimes Z_D\otimes X_E$, with $Z_D={\Lambda }_{\uparrow }-{\Lambda }_{\downarrow }$. The orthogonal projection operators obey ${\Lambda }_{\uparrow }{\Lambda }_{\downarrow }=0$ and ${\Lambda }_j^2={\Lambda }_j$. Hence $[Z_D,{\Lambda }_j]=0$, and the remaining tensor factors commute with detector operators. ThereforeThe environment interaction preserves these detector populations, making their basis a candidate pointer basis. It can change their relative coherence without flipping the stored labels.

Published in Physical Review D 24, starting at page 1516, on 15 September 1981. The apparatus-environment interaction is used to identify robust recorded observables and their pointer basis.