Microwave response of an Andreev level

We measured the microwave response of an Andreev level formed in a semiconducting nanowire quantum dot connecting two superconducting leads using a circuit quantum electrodynamics probe. This probe gave us unprecedented access to the physics of Andreev levels by making it possible to separately measure the response of different many-body configurations of the system. Comparing these responses, we observed the violation of the particle-hole symmetry that points to an appreciable Coulomb interaction in the dot. We also found that the energy-phase relation of a state with a single electron at the dot had a strong π-shifted phase dependence, which can be attributed to contribution of the continuum states to the supercurrent. Our results are in a good agreement with a minimal theoretical model.