plane waves eigenstate waves momentum
which is a superposition of all possible plane waves 




e

i
(
k
x
−



ℏ

k

2




2
m



t
)




{\displaystyle e^{i(kx-{\frac {\hbar k^{2}}{2m}}t)}}

, which are eigenstates of the momentum operator with momentum 



p
=
ℏ
k


{\displaystyle p=\hbar k}

. The coefficients of the superposition are 






ψ
^



(
k
,
0
)


{\displaystyle {\hat {\psi }}(k,0)}

, which is the Fourier transform of the initial quantum state 



ψ
(
x
,
0
)


{\displaystyle \psi (x,0)}

.
