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 νosc we find the largest vacuum Rabi splitting of ∼0.1νosc. Here being at resonance coincides with the optimal coherence of the symmetry point.