Circuit Quantum Electrodynamic Spectroscope: a new superconducting microwave quantum sensor

Project Information

cQEDscope

Grant agreement ID: 101075962

DOI

10.3030/101075962

EC signature date 1 December 2022

Start date 1 January 2023

End date 31 December 2027

Funded under

• European Research Council (ERC)

Total cost

€ 1 480 000,00

EU contribution

€ 1 480 000,00

Coordinated by

MAX-PLANCK-GESELLSCHAFT ZUR FORDERUNG DER WISSENSCHAFTEN EV
Germany

Periodic Reporting for period 1 - cQEDscope (Circuit Quantum Electrodynamic Spectroscope: a new superconducting microwave quantum sensor)

Reporting period: 2023-01-01 to 2025-06-30

Summary of the context and overall objectives of the project

In recent years, a growing collection of superconducting materials is being discovered. These materials show a wide range of exotic properties and are expected to lead to future technological breakthroughs, but there are still many open question our understanding of the underlying structure of the superconducting phase in these materials. This is especially true of the superconductors that can only be isolated in atomically thin and micron-sized samples such as thin films van-der-Waals flake, with which conventional measurement technique are incompatible.

At the same time, microwave circuits based on superconducting thin film have evolved significantly to become a leading platform for implementing coherent quantum effect. While efforts are mostly focused on the promise of quantum computing, the high coherence and tunability of these circuits also makes them excellent sensors, particularly for exploring novel superconductors.

The main goal of the cQEDscope project is to utilize superconducting microwave circuits as sensors for exploring novel superconductors. We do so by incorporating the material as part of the circuit, and measure the response of the circuit to learn about the superconducting gap structure.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

cQEDscope is divided into 3 milestones that highlight different geometries of the hybrid microwave circuit as well as measurement techniques of growing complexity.

Here we will highlight the main achievements so far, focusing on milestones 1 and 3.

Milestone 1: Hybrid microwave resonators for studying the gap structure of micron-sized samples

The main objectives of Milestone 1 are the fabrication and measurement of microwave resonators which combine a novel superconductor as part of the circuit. We have initially focused on van-der-Waals superconductors. In the first two years of the project, we have developed a process for fabricating a thin film resonator and hybridizing it with a van-der-Waals superconducting flake by a capacitive link through a thin oxide barrier. We have shown that we can generate high quality circuits and maintain the pristine quality of the material. This method is now commonly used in the research group to create devices and explore various van-der-Waals materials, and is starting to be utilized by additional groups as well.

Our first hybrid circuits incorporated a flake of the cuprate superconductor Bi2Sr2CaCu2O8+x, and our measurements have observed strong coupling to a nonlinear mode, a surprising result that be useful both to understand the superconductivity in these materials and for future devices for quantum technology. These results have been recently published in Jin et al., Nano Letters (2025).

Milestone 3: Superconducting qubit based on effects in novel superconductors

The objective of Milestone 3 is to design a novel superconducting qubit based on the interaction between the models of novel superconductor with the microwave circuits. While considering the design of this circuit, we have made a theoretical breakthrough that led to a new type of superconducting circuit that is protected by the unique "d-wave" symmetry which appears in a certain class of novel superconductors. This new design can protect the underlying quantum state from noise in the environment - a key limited of current quantum devices. These results have been published in Brosco et al. Phys. Rev. Lett (2024).

This circuit, which we nicknamed "flowermon", can be seen as the first of a new class of circuits that utilize the effects of novel superconductors. We have begun the first theoretical steps in exploring this class of qubits, and this work is published in Coppo et al. Appl. Phys. Lett. (2024).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We had made breakthroughs beyond the state of the art in these two ways:

1) The design of hybrid superconducting circuits (Jin et al., Nano Letters (2025)) is an entirely new way to make circuits that can be applied to a broad class of materials, and will be used in many future works by our group and others.

2) The discovery of the “flowermon” protected qubit (Brosco et al. Phys. Rev. Lett (2024)). This qubit design could be revolutionary to the development of fault tolerant quantum circuits and will lead to new theoretical and experimental development in hybrid circuits.