In situ characterization of qubit control lines: a qubit as a vector network analyzer

TL;DR

This paper introduces a novel method to use a qubit as a vector network analyzer for in situ measurement of control line transfer functions, enhancing calibration and field probing in quantum processors.

Contribution

The paper presents a technique to characterize control lines using a qubit as a vector network analyzer, enabling in situ calibration and electromagnetic field probing.

Findings

Successfully characterized flux control lines from 8 to 400 MHz.

Improved qubit control line calibration for higher fidelity entangling gates.

Demonstrated qubit as a microscopic electromagnetic field probe.

Abstract

We present a technique to measure the transfer function of a control line using a qubit as a vector network analyzer. Our method requires coupling the line under test to the the longitudinal component of the Hamiltonian of the qubit and the ability to induce Rabi oscillations through simultaneous driving of the transversal component. We used this technique to characterize the 'flux' control of a superconducting Transmon qubit in the range of 8 to 400\,MHz. Our method can be used for the qubit 'flux' line calibration to increase the fidelity of entangling gates for the quantum processor. The qubit can be also used as a microscopic probe of the electro-magnetic fields on a chip.