(a) Refocusing of the geometric echo signal by fixing the first evolution time at 35 μs and sweeping the second evolution time ∼35 μs. The solid line shows the best theoretical fit with a Gaussian envelope. (b) Echo decays of the electron spin under magnetic fields of 0 mT (red squares), 0.04 mT (green circles) and 0.12 mT (blue triangles) measured along the NV axis. Solid lines show the best fitting curves integrated with T1 decay. (c) Echo coherence decay time T2 as a function of the magnetic field. The solid line shows the theoretical fits to the data (Methods). Error bars in a,b are defined as the s.d. of the photon shot noise. Error bars in c are given by least squares fitting.