Figure 2: Phase preparation and measurement.
From: Demonstration of entanglement-enhanced phase estimation in solid
(a) For each unknown state sitting on the equatorial plane of the Bloch sphere, we compare the amplitudes of the two Rabi signals from this state (under orthogonal microwave pulses driven) with extract the original phase information: ϕ=arctan (Ax/Ay). (b) Upper pane: pulse sequence to prepare and measure electron–nuclear spin entanglement state 
. Lower panel: pulse sequence to prepare and measure nuclear spin superposition state 
. (c–e) Nuclear Rabi signals for phase measurement, with 0.1, 0.4 and 2 M repetition of pulse sequence in (b). The input phase ϕ is 30°. Solid circle with blue fitting line is driven by 0° RF pulse (X measurement), and square with red dash fitting line is driven by 90° RF pulse (Y measurement). (f) Dependence of measured phase and its standard deviation on repeat number.
