Instanton-induced inelastic scattering of a single-photon in a superconducting circuit

How would our world look like if the fine structure constant $\alpha$ were of order unity? While in our small $\alpha$ world an atom excited to the first excited state has negligible probability of decaying to the ground state while emitting more than a single photon, such processes are important in a large $\alpha$ world, making photon frequency conversion effective in the single-photon regime. We show how such behavior can be realized in a superconducting circuit QED system, where a transmon, which serves as an artificial atom, is galvanically coupled to a high-impedance Josephson junction array, which acts as a waveguide for microwave photons with a high effective $\alpha$. Instantons (phase slips) that occur in the transmon interact with the microwave photons, and lead to inelastic scattering probabilities which approach unity and greatly exceed the effect of the quartic anharmoncity of the Josephson potential [1]. The instanton-photon cross section is calculated using a novel formalism which allows to directly observe the dynamical properties of the instantons, and should be useful in other quantum field theoretical contexts. The calculated inelastic decay rates compare well with recent measurements from the Manucharyan group at Maryland [2].

[1] Photon-instanton collider implemented by a superconducting circuit, A. Burshtein, R. Kuzmin, V. E. Manucharyan and M. Goldstein, Phys. Rev. Lett. 126 137701 (2021)

[2] Photon decay in circuit quantum electrodynamics, R. Kuzmin, N. Grabon, N. Mehta, A. Burshtein, M. Goldstein, M. Houzet, L. I. Glazman and V. E. Manucharyan, arxiv:2010.02099 (2020)

Recording: https://us02web.zoom.us/rec/share/C4Ae2pDdEfvFNDICX5kR8Nr3k6O8_Uu4uo8oG…