In the Schrödinger picture, the state | ψ ψ ⟩ ⟩ at time t is related to the state | ψ ψ ⟩ ⟩ at time 0 by a unitary time-evolution operator, U ( t ) : | ψ ψ ( t ) ⟩ ⟩ = U ( t ) | ψ ψ ( 0 ) ⟩ ⟩ . If the Hamiltonian does not vary with time, then the time-evolution operator can be written as U ( t ) = e − − i H t / ℏ ℏ , where H is the Hamiltonian and ħ is the reduced Planck constant. Therefore,