Observation of the Bloch-Siegert shift in a driven quantum-to-classical transition

TL;DR

This paper experimentally observes the Bloch-Siegert shift in a driven quantum-to-classical transition, revealing how counter-rotating terms affect cavity frequency in a transmon system across different drive regimes.

Contribution

It demonstrates the first direct measurement of the Bloch-Siegert shift during a quantum-to-classical transition in a circuit QED system, combining experimental data with numerical simulations.

Findings

Bloch-Siegert shift appears as a small constant in the quantum regime.

In the classical regime, the shift oscillates with photon number.

Experimental results agree with quasienergy spectrum simulations.

Abstract

We show that the counter-rotating terms of the dispersive qubit-cavity Rabi model can produce relatively large and nonmonotonic Bloch-Siegert shifts in the cavity frequency as the system is driven through a quantum-to-classical transition. Using a weak microwave probe tone, we demonstrate experimentally this effect by monitoring the resonance frequency of a microwave cavity coupled to a transmon and driven by a microwave field with varying power. In the weakly driven regime (quantum phase), the Bloch-Siegert shift appears as a small constant frequency shift, while for strong drive (classical phase) it presents an oscillatory behaviour as a function of the number of photons in the cavity. The experimental results are in agreement with numerical simulations based on the quasienergy spectrum.