Parallel tomography of quantum non-demolition measurements in multi-qubit devices

An efficient characterization of QND measurements is an important ingredient toward certifying and improving the performance and scalability of quantum processors. In this work, we introduce a parallel tomography of QND measurements that addresses single- and two-qubit readout on a multi-qubit quantum processor. We provide an experimental demonstration of the tomographic protocol on a 7-qubit IBM-Q device, characterizing the quality of conventional qubit readout as well as generalized measurements such as parity or measurement-and-reset schemes. Our protocol reconstructs the Choi matrices of the measurement processes, extracts relevant quantifiers-fidelity, QNDness, destructiveness-and identifies sources of errors that limit the performance of the device for repeated QND measurements. We also show how to quantify measurement crosstalk and use it to certify the quality of simultaneous readout on multiple qubits.