\frac{1}{\sqrt{2 E_{\mathbf{p}}}} \left ( \hat{a}_{\mathbf{p}} e^{-\frac{i}{\hbar}E_\mathbf{p}t} e^{\frac{i}{\hbar} \mathbf{p} \mathbf{x}} + \hat{a}_{\mathbf{p}}^{\dagger} e^{\frac{i}{\hbar}E_\mathbf{p}t} e^{-\frac{i}{\hbar} \mathbf{p} \mathbf{x}} \right )