No-go result for quantum postselection measurements of rank-m degenerate subspace

We present a no-go result for postselection measurements where the conditional expectation value of a joint system-device observable under postselection is nothing else than the conventional expectation value. Such a no-go result relies on the rank-m degenerate of the joint observable, where m is the dimension of the device subspace. Remarkable, we show that the error and disturbance in quantum measurements obey the no-go result, which implies that the error-disturbance uncertainty is unaffected under postselection measurements.