Fig. 4: SUPER applied to spin qubits.

a Simulation of the temporal evolution of the ground and excited states of the spin qubit under an optimized magnetic field and SUPER pulses. Population exchange dynamics of \(|1,\downarrow \rangle \to |3,\downarrow \rangle\) and \(|1,\uparrow \rangle \to |3,\uparrow \rangle\) follow a similar trajectory. b Coherence transfer from the ground state superposition, to the excited state superposition: \(| {\rho }_{1},\downarrow \uparrow \,\,|=| \langle 1,\downarrow |\widehat{\rho }|1,\uparrow \rangle | \to | {\rho }_{3},\downarrow \uparrow \,\,|=| \langle 3,\downarrow |\widehat{\rho }|3,\uparrow \rangle |\). c Subsequentially applied spin initialization and readout pulses with a variable delay τ for estimating spin coherence. The observable peak at  ~ 0.13 ms is the fluorescence induced by the applied SUPER excitation. d The thermalization of the spin states when a SUPER pulse is applied. Decay time of the population to the mixed state yields the T_1,spin time. e The control measurement for spin population thermalization when no SUPER pulses are applied between initialization and readout pulses. Uncertainties are extracted from the 95% confidence intervals retrieved from the fit algorithm. f Illustration of the proposed entanglement scheme based on generating photonic qubits encoded in the frequency basis. Two SnVs are first prepared in an equal spin superposition in state. Next, using the SUPER scheme population is inverted to excited state, while maintaining the probability amplitudes of the spin levels. Finally, the two SnVs emit photons with frequencies entangled to their spin-state. Using Hong-Ou-Mandel interference, detecting two clicks on two detectors heralds the state when two photons are distinguishable and the spin states of two SnVs are anti-correlated.