which is a superposition of all possible plane waves e i ( k x − − ℏ ℏ k 2 2 m t ) , which are eigenstates of the momentum operator with momentum p = ℏ ℏ k . The coefficients of the superposition are ψ ψ ^ ^ ( k , 0 ) , which is the Fourier transform of the initial quantum state ψ ψ ( x , 0 ) .