Phase shifting control for IQ separation in qubit state tomography

Achieving high fidelity in the readout of superconducting qubits through homodyne detection requires a sufficient signal-to-noise ratio during detection, the data of which ideally land on a circular path in the IQ quadrature plane. Extraneous imbalances in the IQ mixer deform the path into an ellipse and decrease the detection fidelity. We present a novel approach that utilizes phase-shift control to rotate the state vectors on the IQ plane. By optimizing the phase shift, the detection data are symmetrically distributed about the elliptical minor axis where the readout signal-to-noise ratio is maximized. An 11% enhancement in the readout signal-to-noise ratio has been demonstrated through both theoretical analysis and experimental verification, resulting in improved fidelity in qubit state tomography.