Observation of a broadband Lamb shift in a superconducting resonator*

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 narrowband modes. Here, we experimentally observe in high-quality superconducting resonators a Lamb shift of several megahertz, by externally tuning the coupling strength of an engineered broadband environment based on hybrid normal-metal–superconductor tunnel junctions. Our results may lead to improved control of dissipation in high-quality engineered quantum systems such as superconducting qubits.