Energy Control of a Superconducting Flux Qubit with an Oscillating Magnetic Field

We control energy of a superconducting flux qubit by an oscillating magnetic field. Our device consists of a flux qubit inductively coupled to a frequency tunable resonator. The tunable resonator is implemented with an on-chip LC circuit containing a superconducting quantum interference device (SQUID). Hamiltonian of the system is represented as follows.
H = ½[Δσ_x+(ε+δεa^†a)σ_z]+hfa^†a
In the experiment, we control energy bias ε and number of photons a^†a in the LC circuit by a static magnetic field and an oscillating magnetic field (microwave driving), respectively. We first investigate energy bias ε dependence of the energy shift. As we expect from the model, we observe positive (negative) energy shift for positive (negative) detuning bias ε. We also confirm linear dependence of the energy shift to the number of photons a^†a far from the optimal point (ε»Δ). The tuning range of the energy shift exceed 1.5 GHz. This range is larger than the energy gap of the flux qubit (∼ 1.3 GHz), and much larger than the typically reported value of AC Stark shift using circuit QED system in a dispersive coupling regime.