Ch 4.4, Ch 8.6 The operator $\hat{a}$ that lowers the occupation number by one: $\hat{a}|n\rangle = \sqrt{n}|n-1\rangle$ for the harmonic oscillator, and $\hat{a}|0\rangle = 0$. Also called the lowering or destruction operator. *See also:* Creation operator, Ladder operators, Number operator, Fock s