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NICT REPORT 49Research HighlightsFig.1 : The deep-strong-coupling circuit used in the experi-ments, consisting of an aluminum superconducting artifi-cial atom (outlined in red) and an LC resonator circuitThe physics of extremely strong interaction between light and matter has not been well un-derstood due to the lack of suitable ex-perimental means. To find and under-stand new phenomena in this unexplored regime, a research group comprising Senior Researcher Fumiki Yoshihara, Senior Researcher Tomoko Fuse, and Executive Researcher Kouichi Semba of the NICT Advanced ICT Research Institute has been work-ing on superconducting artificial at-oms*1 that can interact very strongly with an electromagnetic field mode in a resonator circuit. In 2016, they suc-cessfully implemented a new regime of very strong interactions between light and matter (deep-strong-coupling (DSC) regime), and became the first to demonstrate the existence of stable states molecule-like consisting of pho-tons and artificial atoms.In the DSC regime, interactions with just a single photon can cause tremen-dous changes in the energy levels of an artificial atom. Until now, there have been no reports of systematic experi-ments to explore phenomena (Lamb shift, Stark eect) in this regime.In collaboration with NTT Corpora-tion, Qatar Environment & Energy Re-search Institute, Tokyo Medical and Dental University, and Waseda Univer-sity, this research group has become the first to successfully generate a very large energy change (optical shift) in artificial atoms interacting with pho-tons. This experiment was conducted using the superconducting circuit shown in Fig.1. The superconducting artificial atoms (outlined in red) were prepared using microfabrication tech-niques. Artificial atoms have quantum properties equivalent to atoms, and can confine photons in the superconduct-ing resonator circuit. New experiments using double resonance spectroscopy showed that the observable energy range became wider, and the team succeeded in observing a huge relative light shift about 100 times larger than in conventional experiments (Fig.2).The Lamb shift is caused by interac-tions with the vacuum electromagnetic field in resonator circuits. The effect was first discovered as a difference in the fine energy levels of hydrogen at-oms. Since then, it has brought dramat-“The relative shift we observed is 6 orders of magni-tude larger than that initially ob-served in the hy-drogen atom.”ic developments in quantum electrody-namics, and plays a key role in the sophisticated electronics technology supporting modern society. In the words of Kouichi Semba (Executive Re-searcher at the Frontier Research Lab-oratory), “The huge Lamb shift we ob-served this time is 6 orders of magnitude larger than the energy shift initially observed in the hydrogen atom, due to the eect of the zero point fluc-tuation current in the superconducting circuit. In this way, quantum technology using superconducting artificial atoms has lead to an era when it is necessary to change the idea in circuit design, to make use of the quantum fluctuation which was only a very small correction term in the past in a leading role”Enhancing the energy control-lability using strong interac-tion with photonsObserved huge Lamb shift shows potential of utilizing vacuum uctuation in design-ing quantum circuit DEVELOP