Speaker: Luciano Pereira
Affiliation: IFF-CSIC
Date: Tuesday, 5 March 2024 at 12:00
Location: Seminar Room, Serrano 121 (CFMAC)
Quantum non-demolition measurements (QND) allow repeated evaluation of an observable without destroying the quantum system. Efficient characterization of these measurements is an essential part of certifying, improving, and scaling up quantum processors. In this thesis, we address the challenge of simulating and characterizing QND measurements through quantum tomography. First, we introduce a self-consistent tomography scheme to perform a complete physical characterization of an arbitrary QND detector, including the reconstruction of the measurement processes and the extraction of relevant quantifiers such as readout fidelity, QNDness, and destructiveness. This framework is a diagnostic tool for the dynamics of QND detectors, allowing us to identify errors, and to improve their calibration and design. We illustrate this on a realistic Jaynes-Cummings simulation of dispersive superconducting qubit readout. We characterize non-dispersive errors, quantify the backaction introduced by the readout cavity, and calibrate the optimal measurement point. Then, we show that this procedure can be efficiently parallelized when addressing 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 and generalized measurements such as parity or measurement-reset-feedback schemes. We also show how to quantify measurement crosstalk and use it to certify the quality of simultaneous readout on multiple qubits. Our method is an efficient alternative for characterizing and understanding QND measurements in current quantum devices from theoretical and experimental points of view.