  The shift of energy levels owing to broadband electromagnetic vacuum
fluctuations, the Lamb shift, has been pivotal in the development of quantum
electrodynamics and in understanding atomic spectra. Currently, small energy
shifts in engineered quantum systems are of paramount importance owing to the
extreme precision requirements in applications such as quantum computing.
However, without a tunable environment it is challenging to resolve the Lamb
shift in its original broadband case. Consequently, the observations in other
than atomic systems are limited to environments comprised of narrow-band modes.
Here, we observe a broadband Lamb shift in high-quality superconducting
resonators, a scenario also accessing static shifts inaccessible in Lamb's
experiment. We measure a continuous change of several megahertz in the
fundamental resonator frequency by externally tuning the coupling strength of
the engineered broadband environment which is based on hybrid
normal-metal--superconductor tunnel junctions. Our results may lead to improved
control of dissipation in high-quality engineered quantum systems and open new
possibilities for studying synthetic open quantum matter using this hybrid
experimental platform.
