Inversion of Qubit Energy Levels in Qubit-Oscillator Circuits in the Deep-Strong-Coupling Regime

作者：Fumiki Yoshihara, T. Fuse, Zhimin Ao, Sahel Ashhab, Kosuke Kakuyanagi, Shiro Saito, T. Aoki, Kazuki Koshino, Kouichi Semba · 发表于：Physical Review Letters · 年份：2018 · DOI：10.1103/physrevlett.120.183601 · 被引用次数：114 · 研究领域：Quantum Information and Cryptography、Quantum Computing Algorithms and Architecture、Quantum and electron transport phenomena

We report on experimentally measured light shifts of superconducting flux qubits deep-strongly coupled to LC oscillators, where the coupling constants are comparable to the qubit and oscillator resonance frequencies. By using two-tone spectroscopy, the energies of the six lowest levels of each circuit are determined. We find huge Lamb shifts that exceed 90% of the bare qubit frequencies and inversions of the qubits' ground and excited states when there are a finite number of photons in the oscillator. Our experimental results agree with theoretical predictions based on the quantum Rabi model.