Quantum probe of an on-chip broadband interferometer for quantum microwave photonics

TL;DR

This paper demonstrates how a superconducting transmon qubit integrated into an on-chip interferometer can serve as a sensitive spectrometer to probe broadband electromagnetic environments relevant for quantum microwave photonics.

Contribution

It introduces a novel on-chip broadband interferometer with a transmon qubit used as a quantum-level spectrometer to analyze electromagnetic noise spectra.

Findings

High-frequency environment described by harmonic oscillators

Low-frequency noise exhibits colored quasi-static Gaussian characteristics

Probes broadband electromagnetic environment using qubit relaxation and dephasing rates

Abstract

Quantum microwave photonics aims at generating, routing, and manipulating propagating quantum microwave fields in the spirit of optical photonics. To this end, the strong nonlinearities of superconducting quantum circuits can be used to either improve or move beyond the implementation of concepts from the optical domain. In this context, the design of a well-controlled broadband environment for the superconducting quantum circuits is a central task. In this work, we place a superconducting transmon qubit in one arm of an on-chip Mach-Zehnder interferometer composed of two superconducting microwave beam splitters. By measuring its relaxation and dephasing rates we use the qubit as a sensitive spectrometer at the quantum level to probe the broadband electromagnetic environment. At high frequencies, this environment can be well described by an ensemble of harmonic oscillators coupled to the transmon qubit. At low frequencies, we find experimental evidence for colored quasi-static Gaussian noise with a high spectral weight, as it is typical for ensembles of two-level fluctuators. Our work paves the way towards possible applications of propagating microwave photons, such as emulating quantum impurity models or a novel architecture for quantum information processing.