Thanks, that makes sense to me. As far as I know, the momentum operator in 3 dimensions should be
\hat{P}=-i\hbar \int d^3 x a^\dagger (x) \nabla a(x)
Your equation slightly modified gives
\int \int dx_1\, dx_2\, a^\dagger (x_2)\langle x_2|\left(\sum \hat{P}_i |p_i\rangle \langle...