The nearly perfect transfer, or complete blockade, of the target spin for the open, or closed, gate as shown in Fig. 1d was obtained for the system parameters as in Fig. 2a. It is however important to quantify the sensitivity of the spin transistor to uncontrolled fluctuations of the parameters. In Fig. 3a we show the fidelities of transfer at time tout=π/|J1| versus the amplitude of random noise affecting the trapping potential or the (effective) magnetic field. We observe that coherent transfer is quite robust with respect to moderate variations in V0, but is rather sensitive to small variations in U and h since they detrimentally affect the gate resonant conditions. Importantly, the gate blockade is virtually unaffected by uncertainties in U, V0, h since it relies on the (large) energy mismatch. In Fig. 3b, we show the dependence of the blockade fidelity on κ which determines the asymmetry parameter Δ of the XXZ model. Clearly, the spin transistor cannot operate when Δ=0, that is, in the absence of the spin–spin interactions, since then the control spin can leak out of the gate, as mentioned above and in Supplementary Note 2.