What requirements must an observable A meet to be usefull as a qubit for quantum computers? I think that the wave function must have time independent coefficients in the base of A (when we are not applying quantum gates). This means that expected value <A> must be constant, so the necessary condition is [A,H]=0. But is this enough? What if the eigenstates of A have different energies (eigenvalues of H)? Then the phase of the two states is changing with different frequencies. Is quantum computing possible anyway or must both eigenstates have the same energy?