# Expectation value of kinetic energy operator

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Homework Statement:
Relevant Equations:

The expectation value of the kinetic energy operator in the ground state ##\psi_0## is given by
$$<\psi_0|\frac{\hat{p^2}}{2m}|\psi_0>$$
$$=<\psi_0|\frac{1}{2m}\Big(-i\sqrt{\frac{\hbar mw}{2}}(\hat{a}-\hat{a^{\dagger}})\Big)^2|\psi_0>$$
$$=\frac{-\hbar w}{4}<\psi_0|\hat{a}^2+-\hat{a}\hat{a^{\dagger}}-\hat{a^{\dagger}}\hat{a}+\hat{a^{\dagger}}^2|\psi_0>$$
$$=\frac{-\hbar w}{4}<\psi_0|\Big(\hat{a}^2|\psi_0>-\hat{a}\hat{a^{\dagger}}|\psi_0>-\hat{a^{\dagger}}\hat{a}|\psi_0>+\hat{a^{\dagger}}^2|\psi_0>\Big)$$
$$=\frac{-\hbar w}{4}<\psi_0|\Big(|-\psi_0>+\sqrt{2}|\psi_2>\Big)$$
$$=\frac{-\hbar w}{4}<-\psi_0|\psi_0>+<\psi_0|\psi_2>$$

$$=\frac{\hbar w}{4}$$

PeroK