3. Postulates: Measurement

Postulate 3: Measurement

This is where things get weird. When we look at the system, we change it.

• The Rule: Quantum measurements are described by a collection of measurement operators $\{M_m\}$. The probability of outcome $m$ is $P(m) = \langle \psi | M_m^{\dagger} M_m | \psi angle$.
• Collapse: Immediately after measurement, the state of the system collapses to the outcome state.

The Takeaway: Measurement is irreversible. Once you look, you destroy the superposition.