broadband electromagnetic vacuum fluctuations—the Lamb
shift—has been central for the development of quantum electrodynamics
and for the understanding of atomic spectra1–6.
Identifying the origin of small energy shifts is still important
for engineered quantum systems, in light of the extreme precision
required for applications such as quantum computing7,8.
However, it is challenging to resolve the Lamb shift in its
original broadband case in the absence of a tuneable environment.
Consequently, previous observations1–5,9 in non-atomic
systems are limited to environments comprising narrowband
modes10–12. Here, we observe a broadband Lamb shift in highquality
superconducting resonators, a scenario also accessing
static shifts inaccessible in Lamb’s experiment1,2. We measure
a continuous change of several megahertz in the fundamental
resonator frequency by externally tuning the coupling
strength to the engineered broadband environment, which is
based on hybrid normal-metal–insulator–superconductor tunnel
junctions13–15. Our results may lead to improved control of
dissipation in high-quality engineered quantum systems and
open new possibilities for studying synthetic open quantum
matter16–18 using this hybrid experimental platform.