Efficient characterization of correlated SPAM errors

State preparation and measurement (SPAM) errors limit the performance of gate-based quantum computers, but are partly correctable after a calibration step that requires, for exact implementation on a register of n qubits, 2n additional characterization experiments. Here we introduce an approximate but efficient method for SPAM error characterization requiring 2n(n − 1) measurements. The technique assumes that multi-qubit measurement errors are dominated by pair correlations, which are estimated with n(n − 1)/2 two-qubit experiments. We demonstrate this technique on the IBM and Rigetti online quantum computers, allowing comparison of their SPAM errors in both magnitude and degree of correlation. We find that the pair-correlation model is reasonably accurate on linear arrays of qubits. However qubits with more closely spaced two-dimensional geometries exhibit significant higher order (e.g., 3-qubit) SPAM error correlations.