(color online) Controllable effects of quantum fluctuations on the free-induction decay (FID) of an NV center spin: (a) the dephasing time , and (b) the exponential decay index n, for three NV centers (A, B, and C). Experimental data are shown in circle, square, and diamond symbols with error bars, and numerical data are shown in solid, dashed, and dash–dotted lines. By tuning the strength of the external magnetic field, the contribution of quantum fluctuation can be suppressed or enhanced, as evidenced by the vibration of n ( represents a non-Gaussian envelop decay, which is a signature of the quantum fluctuation). Reproduced from Ref. [9], NPG.