Fig. 2: Quantum-frequency discrimination.
a Experimental setup: the spin state of a single nitrogen-vacancy (NV) center in diamond responds to magnetic fields by a Zeeman shift of the spin levels, which can be optically initialized and readout with a confocal microscope. b Experimentally measured quantum phase difference \(\varphi _{\Delta}\) as a function of coherent interaction time and as log–log plot (inset). Solid lines are a fit to \(\varphi _{\Delta} = \frac{2}{\pi }B\omega _{\Delta}t^2\), where \(\omega _{\Delta} = |\omega _1 - \omega _2| = \left( {2\pi } \right) \cdot 2\) kHz. c Measured spin population as a function of coherent interaction time. The probability to be in state \(\left| 0 \right\rangle\) is: \({\mathrm{P}}\left( {\left| 0 \right\rangle \left| {\omega _1} \right.} \right) = \frac{1}{2} - \frac{1}{2}\sin \left( {\frac{{B\omega _{\Delta}t^2}}{\pi }} \right)\), \({\mathrm{P}}\left( {\left| 0 \right\rangle \left| {\omega _2} \right.} \right) = \frac{1}{2} + \frac{1}{2}\sin \left( {\frac{{B\omega _{\Delta}t^2}}{\pi }} \right)\). For \(\varphi _{\Delta} = \pi\), the sensor is driven to one of two orthogonal eigenstates. All error bars correspond to SD of several independent repetitions.
