  Vacuum fluctuations fundamentally affect an atom by inducing a finite excited
state lifetime along with a Lamb shift of its transition frequency. Here we
report the reverse effect: modification of vacuum modes by a single atom in
circuit quantum electrodynamics. Our one-dimensional vacuum is a long section
of a high wave impedance (comparable to resistance quantum) superconducting
transmission line. It is directly wired to a transmon qubit circuit. Owing to
the combination of high impedance and galvanic connection, the transmon's
spontaneous emission linewidth can greatly exceed the discrete transmission
line modes spacing. This condition defines a previously unexplored
"superstrong" coupling regime of quantum electrodynamics where many
frequency-resolved vacuum modes hybridize with a single atom. We establish this
regime by observing the spontaneous emission line of the transmon, revealed
through the mode-by-mode measurement of the vacuum's density of states. The
linewidth as well as the atom-induced dispersive photon-photon interaction are
accurately described by a physically transparent Caldeira-Leggett model, with
the transmon's quartic non-linearity treated as a perturbation.
Non-perturbative modification of vacuum, including inelastic scattering of
single photons, can be enabled by the superstrong coupling regime upon
replacing the transmon by more anharmonic qubits, with broad implications for
simulating quantum impurity models of many-body physics.
