Time operators for continuous-time and discrete-time quantum walks.

We study time operators for both continuous and discrete-time quantum walks. Because the continuous-time quantum has a Hamiltonian, we can define its time operators by the canonical commutation relation to the Hamiltonian. On the other hand, the discrete-time quantum walk has no Hamiltonian and its evolution is described by a unitary operator. Hence, it is natural to define the time operators for the iscrete-time quantum walk by a commutation relation to the unitary operator. In this paper, we construct concrete examples of the time operators for both continuous and discrete-time quantum walks. We also separate time operators for a unitary operator into two classes and give their basic properties.