(a) (i–iv) Readout fluorescence as a function of the delay between π/2 rotations using the four allowed transitions to probe and prepare the electron spin, and for different scan directions. The level schemes indicate the probed transition. The non-sinusoidal shape is due to the polarization of nuclear spins in the quantum dot. The data are taken for Bext=4 T resulting in an electron spin splitting of 25.2 GHz. (b) Alternating pulse sequence to suppress nuclear spin polarization. Every second rotation sequence begins with an inverted spin state. (c) Count rates from the alternating sequence. The two out-of-phase signals with (blue circles) and without (red circles) a spin inversion produce a time-averaged signal without phase dependence (grey circles). The curves are sinusoidal fits to data. The lower count rates compared with (a) are a consequence of the hardware constraints in producing and measuring the alternating sequence. (d) Full free induction decay with alternating sequence. The probed transition is the same as in a (i). The black curve is a Gaussian envelope with a 1.93-ns decay.