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

TL;DR

This paper reports the experimental observation of the Bloch-Siegert shift in a superconducting qubit-oscillator system operating in the ultrastrong coupling regime, confirming theoretical predictions with precise measurements.

Contribution

First experimental demonstration of the Bloch-Siegert shift in a superconducting qubit-oscillator system within the ultrastrong coupling regime, validating advanced theoretical models.

Findings

Measured a 50 MHz Bloch-Siegert shift at the qubit symmetry point.

Achieved a coupling energy of approximately 0.82 GHz in the system.

Confirmed the necessity of including counter-rotating terms in the model.

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 $\approx0.82$~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.