I had a question regarding some text in a paper that I was reading. It had to do with ramsey interferometry, where they initialized a particle in spin up (lets say up along the z axis on the bloch sphere), after which they applied a pi/2 pulse to rotate it 90 degrees around the x-axis, they wait a time tau. Now, the applied RF pulses are off resonant, so in the rotating frame of the RF, the particle precesses around the z-axis in a superposition in the x-y plane. It does so with a frequency equal to the difference of the Rabi frequency of the particle and the frequency of the RF pulse, so the superposition picks up a phase related to this Rabi frequency times tau, which the experiment wanted to determine. (The rabi frequency). The experiment then goes on to say they apply yet another pi/2 pulse to rotate by 90 degrees around the x axis, after which the accumulated phase is mapped onto the population of the spin up and down states. The paper then says that this phase can be measured. My question is quite literally, how can this phase be measured? there is no mention of it in the paper, and I simply do not know how this is done. I'd be very grateful if someone would be able to explain to me how they measure this phase. The paper is http://www.nature.com/nnano/journal/v7/n2/full/nnano.2011.225.html and the section I am talking about is in the methods section, in case it is not clear what I am describing.