Observation of the Bloch-Siegert Shift in a Qubit-Oscillator System in the Ultrastrong Coupling Regime

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Here, the Mooij group at TU-Delft in close collaboration with group of SOLID theorist, E. Solano have measured the dispersive energy-level shift of an LC resonator magnetically coupled to a super- conducting qubit. The results clearly show that the system operates in the ultrastrong coupling regime of the light matter interaction. The large mutual kinetic inductance provides a coupling energy of ≈0.82GHz, requiring the addition of counter-rotating-wave terms in the description of the Jaynes-Cummings model. A50 MHz Bloch- Siegert shift was found when the qubit is in its symmetry point, fully consistent with the analytical model that was developed.

P. Forn-Díaz, J. Lisenfeld1, D. Marcos, J. J. García-Ripoll, E. Solano, C. J. P. M. Harmans, and J. E. Mooij
Phys. Rev. Lett. 105, 237001 (2010)