Coherent control of a flux qubit by phase-shifted resonant microwave pulses

The quantum state of a flux qubit was successfully pulse controlled by using a resonant microwave. We observed Ramsey fringes by applying a pair of phase-shifted π∕2 microwave pulses without introducing detuning. With this method, the qubit state can be rotated on an arbitrary axis in the x‐y plane of the Bloch sphere in a rotating frame. We successfully observed qubit Larmor precession stroboscopically with an angular velocity of 2π×11.4Grad∕s. In combination with Rabi pulses, this method enables us to achieve full control of single qubit rotation with shorter operation time.