title: Observation of the Bloch-Siegert shift in a qubit-oscillator system in the ultrastrong coupling regime.
abstract: 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.
authors.affiliation: Quantum Transport Group, Delft University of Technology, Lorentzweg 1, 2628CJ Delft, The Netherlands.
authors.forename: P
authors.initials: P
authors.lastname: Forn-Díaz
authors.forename: J
authors.initials: J
authors.lastname: Lisenfeld
authors.forename: D
authors.initials: D
authors.lastname: Marcos
authors.forename: J J
authors.initials: JJ
authors.lastname: García-Ripoll
authors.forename: E
authors.initials: E
authors.lastname: Solano
authors.forename: C J P M
authors.initials: CJ
authors.lastname: Harmans
authors.forename: J E
authors.initials: JE
authors.lastname: Mooij