Broadband Lamb shift in an engineered quantum system

The shift of the energy levels of a quantum system owing to broadband electromagnetic vacuum fluctuations-the Lamb shift-has been central for the development of quantum electrodynamics and for the understanding of atomic spectra^1-6. 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 computing^7,8. However, it is challenging to resolve the Lamb shift in its original broadband case in the absence of a tuneable environment. Consequently, previous observations^1-5,9 in non-atomic systems are limited to environments comprising narrowband modes^10-12. Here, we observe a broadband Lamb shift in high-quality superconducting resonators, a scenario also accessing static shifts inaccessible in Lamb's experiment^1,2. 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 junctions^13-15. Our results may lead to improved control of dissipation in high-quality engineered quantum systems and open new possibilities for studying synthetic open quantum matter^16-18 using this hybrid experimental platform.