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

Fri, 2011-02-18 16:37 - Daniel Rudolph
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Date: 
2010-11-30
Author(s): 

P. Forn-Díaz, J. Lisenfeld1, D. Marcos, J. J. García-Ripoll, E. Solano, C. J. P. M. Harmans, and J. E. Mooij

Reference: 

Phys. Rev. Lett. 105, 237001 (2010)

URL: 

http://link.aps.org/doi/10.1103/PhysRevLett.105.237001

We measure the dispersive energy-level shift of an LC resonator magnetically coupled to a superconducting qubit, which clearly shows that our system operates in the ultrastrong coupling regime. The large mutual kinetic inductance provides a coupling energy of ≈0.82  GHz, requiring the addition of counter-rotating-wave terms in the description of the Jaynes-Cummings model. We find a 50 MHz Bloch-Siegert shift when the qubit is in its symmetry point, fully consistent with our analytical model.