Strong Coupling of a Quantum Oscillator to a Flux Qubit at Its Symmetry Point

A flux qubit biased at its symmetry point shows a minimum in the energy splitting (the gap), providing protection against flux noise. We have fabricated a qubit of which the gap can be tuned fast and have coupled this qubit strongly to an LC oscillator. We show full spectroscopy of the qubit-oscillator system and generate vacuum Rabi oscillations. When the gap is made equal to the oscillator frequency ?osc we find the largest vacuum Rabi splitting of ?0.1?osc. Here being at resonance coincides with the optimal coherence of the symmetry point.Kavli Institute of NanoscienceApplied Science