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. The measured change in the fundamental frequency of a superconducting resonator coupled to a tunnel junction reveals a broadband constant Lamb shift, which is typically inaccessible in atomic systems.