Vector control of quantum superposition states of a single spin on surface using STM

The combination of scanning tunneling microscopy (STM) and electron spin resonance (ESR) provides a tool for qubit control on surfaces. So far, coherent control of such qubits has been achieved on individual atoms [1] and molecules [2], but without a phase control. Here we implemented phase-controlled radio-frequency (RF) pulses in our pulsed ESR experiments. Rabi oscillation data without a phase control demonstrated that our signal was dominated by DC detection, i.e., z-projected qubit state. Measurements on Ti (S=1/2) atoms on a MgO surface with the phase control scheme resulted in ESR signal that oscillated with the RF phase, which directly proved a coherent rotation of the qubit state controlled with the rotation axis. In addition, we performed CPMG sequence and obtained longer coherence time than that from a Spin-echo scheme. We also present a STM-based possible approach to quantum gate operation of a single qubit, paving a way for complete control of its superposition states.