We present a new regime of quantum electrodynamics where a single photon acquires a finite lifetime, due to spontaneous decay into many lower-frequency photons. This phenomenon is a hallmark of ultra-strong coupling between a sufficiently non-linear quantum system (the quantum impurity) and a continuum of 1D bosonic modes [1, 2]. Although there is no practical analog for a single photon decay in traditional quantum optics, this phenomenon is ubiquitous in the bosonic description of strongly correlated 1D electronic systems. We demonstrate the phenomenon with a generic quantum impurity, consisting of a weak Josephson junction that is galvanically embedded into a long section of a high-impedance transmission line, and with a bosonic version of the Kondo impurity implemented by a fluxonium qubit. The photon lifetime data provides a unique resource for the analog quantum simulation of difficult computational problems.
[1] R. Kuzmin et al., PRL 126, 197701 (2021); [2] A. Burshtein et al., PRL 126, 137701 (2021).