Quantum Decoherence: When a quantum system |ψ⟩ = α|0⟩ + β|1⟩ interacts with an environment,
the reduced density matrix ρ_S evolves as ρ(t) = ρ(0) with off-diagonals decaying:
ρ₀₁(t) = ρ₀₁(0)·e^{−γt}. The pointer basis is the preferred basis selected by the environment
interaction — states that remain least entangled with the environment. For dephasing noise,
the pointer basis is the energy eigenbasis {|0⟩,|1⟩}. The density matrix ρ starts as a pure state
(Bloch vector on sphere surface), and decoherence drives the off-diagonal coherences to zero,
turning the pure state into a classical mixture — the Bloch vector shrinks inward. This is
environment-induced superselection (einselection): only pointer states survive monitoring.