# Characterization of hidden modes in networks of superconducting qubits

## TL;DR

This paper introduces a measurement technique using a superconducting qubit as a sensitive phase meter to detect and characterize electromagnetic modes, including high-Q and spurious modes, that are otherwise hard to observe.

## Contribution

The authors develop a novel method based on measurement-induced dephasing to detect and characterize electromagnetic modes coupling to superconducting qubits, surpassing traditional transmission measurements.

## Key findings

- Successfully detected high-Q coupling resonators
- Identified spurious modes in 3D cavities
- Provided characterization parameters like quality factor and coupling strength

## Abstract

We present a method for detecting electromagnetic (EM) modes that couple to a superconducting qubit in a circuit-QED architecture. Based on measurement-induced dephasing, this technique allows the measurement of modes that have a high quality factor (Q) and may be difficult to detect through standard transmission and reflection measurements at the device ports. In this scheme the qubit itself acts as a sensitive phase meter, revealing modes that couple to it through measurements of its coherence time. Such modes are indistinguishable from EM modes that do not couple to the qubit using a vector network analyzer. Moreover, this technique provides useful characterization parameters including the quality factor and the coupling strength of the unwanted resonances. We demonstrate the method for detecting both high-Q coupling resonators in planar devices as well as spurious modes produced by a 3D cavity.