Maksym Serbyn : Probing superconductivity and emergent excitations with microwaves

Microwave spectroscopy provides a sensitive probe of superconducting systems by direct measurement of kinetic inductance (and therefore the superfluid density) and by efficient coupling to low-energy collective modes.
I will overview recent experimental examples where microwaves provide insight into superconductivity and emergent excitations, and discuss the theoretical understanding and open questions motivated by these experiments. First, motivated by experiments [1], I will discuss how heterostructures proximitized by superconductors can be probed via measurements of their superfluid density in microwave resonators. I will summarize our theoretical results [2,3] showing how the magnetic-field dependence of the superfluid density reveals the role of spin–orbit coupling, disorder, and the emergence of gapless superconductivity.
In the second part, I will show how microwave spectroscopy is used experimentally to probe plasmon modes in Josephson junction chains [4]. Motivated by these experiments, I will present our current theoretical understanding [5] of the intrinsic linewidth of plasmons arising from their nonlinear interactions, and outline open theoretical questions about nonequilibrium regimes.

1.    D. Phan et al., “Detecting induced p±ip pairing at the Al-InAs interface with a quantum microwave circuit”,
arXiv:2107.03695 , PRL 128, 107701 (2021).

2.    S.S. Babkin et al., “Proximity-induced gapless superconductivity in 2d Rashba semiconductor in magnetic field“,
arXiv:2311.09347 , SciPost Phys. 16, 115 (2024).

3.    S.S. Babkin et al., “Superconducting Proximity Effect in Two-Dimensional Hole Gases”,
arXiv:2412.04084 , PRB 111, 214518 (2025).

4.    A.V. Bubis et al., “Nonequilibrium plasmon liquid in a Josephson junction chain”,
arXiv:2504.09721 , Sci. Adv. 12, eady7222 (2026).

5.    L. Vigliotti et al., “Intrinsic decay rates and steady states of driven Josephson junction chains cavities”,
arXiv:2603.06371 (2026).