Fig. 3.

Dissipative decoupling of target spins during classical storage on quantum memory. a During storage (green background) the sensor-spin flips stochastically into a magnetic state $∣\pm 1⟩$ and an orange repumping Laser (594 nm) excites the NV and repolarizes the sensor spin predominantly into the non-magnetic state $∣0⟩$. We perform Ramsey measurements on target spin A ₁ and record the $T_2^{*}$ time depending on the applied optical excitation rate. The shown errors correspond to the standard error of the exponential fit to the decay of the spin state. We fitted simulated coherence lifetimes to the measured results using three parameters, (i) the conversion from applied laser power to excitation rate, (ii) the ionization probability, and (iii) the probability for the $∣\pm 1⟩$ excited state to decay via the metastable state into the $∣0⟩$ ground state (see “Methods”). Three distinct regions are observed. The first decrease is induced by a slow excitation and depolarization into the $∣\pm 1⟩$ spin state. As soon as the excitation rate exceeds the coupling strength, the coherence time starts to increase. At a certain excitation rate, the sensor is ionized before the measurement can be concluded, resulting in loss of signal. b Longest living Ramsey oscillation of target A ₁ for an applied laser power of 6.3 μW. The solid line is a fit of a decaying cosine oscillation, with a decay time $T_2^{*}=17.4\pm 4.2$ ms. c, d Real (imaginary) part of the Fourier transformed signal of (b). Although the dark gray (orange) dots represent the raw transform, the bold, light gray (orange) line represents the transformation of the zero-filled input signal. The thin, green (orange) line shows a fit, where the real and imaginary parts of the Fourier transform are fitted simultaneously resulting in a linewidth of 18.3 ± 4.3 Hz