Strong Field Effects in Rabi Oscillations of the dc SQUID Phase Qubit*

In the phase qubit, Rabi oscillations between the two lowest metastable zero-voltage states can be driven with a microwave current. At the high microwave powers needed to perform fast single-qubit operations, multilevel and multiphoton effects lead to an ac Stark shift of the resonant drive frequency and modification of the Rabi frequencies. We have observed these effects in an asymmetric Nb/AlOx/Nb dc SQUID at 25 mK, where one junction (with a roughly 20 $\mu$A critical current) behaves as a phase qubit and the other provides isolation from the bias line. We found quantitative agreement between experimental results and theoretical predictions obtained with a three-level multiphoton analysis.