Schrödinger's Cat — Quantum Decoherence

A superposition of alive/dead states loses quantum coherence as the system interacts with the environment. Watch the off-diagonal density matrix elements decay.

Density Matrix ρ

ρ₀₀ (P(alive))—
ρ₁₁ (P(dead))—
|ρ₀₁| coherence—
Purity Tr(ρ²)—

State

Time elapsed0.00s
Coherence time—
von Neumann S—
PhaseQuantum

Decoherence (Zurek, 1981) explains why quantum superpositions are not observed at macroscopic scales. The environment acts as a measuring apparatus — entanglement with environmental degrees of freedom destroys the off-diagonal elements of the reduced density matrix, leaving a classical mixture.