  Correctly characterizing state preparation and measurement (SPAM) processes
is a necessary step towards building reliable quantum processing units (QPUs).
In this work, we discuss the subtleties behind separately measuring SPAM
errors. We propose a protocol that can separately estimate SPAM errors, in the
case where quantum gates are ideal. In the case where the quantum gates are
imperfect, we derive bounds on the estimated SPAM error rates, based on gate
error measures which can be estimated independently of SPAM processes. Our
method shows that the gauge ambiguity in characterizing SPAM operations can be
resolved, by assuming that there exists one qubit whose initial state is
uncorrelated with other qubits in a QPU. We test the protocol on a publicly
available five-qubit QPU and demonstrate its validity by comparing our results
with simulations.
