  The process of measuring a two-level quantum system was examined by applying
Hamiltonian formalism. For the measurement of an observable that does not
commute with the system Hamiltonian, a non-trivial relationship among the
strength of interaction, the time interval of the process, and the accuracy of
the measurement was obtained. Particularly, to achieve an error-free
measurement of such an observable, a condition stating that the interaction
Hamiltonian does not commute with the system Hamiltonian needs to be satisfied.
