All-Optical Control of the Silicon-Vacancy Spin in Diamond at Millikelvin Temperatures

The silicon-vacancy center in diamond offers attractive opportunities in quantum photonics due to its favorable optical properties and optically addressable electronic spin. Here, we combine both to achieve all-optical coherent control of its spin states. We utilize this method to explore spin dephasing effects in an impurity-rich sample beyond the limit of phonon-induced decoherence: Employing Ramsey and Hahn-echo techniques at temperatures down to 40 mK we identify resonant coupling to a substitutional nitrogen spin bath as limiting decoherence source for the electron spin.

Figure 1

(a) Electronic level scheme of the ${\mathrm{SiV}}^{-}$ at $B=0$ (left) and $B=0.21\mathrm{T}$ (right). Optical transitions are indicated by blue arrows, splittings by red arrows. (b) Photoluminescence excitation spectrum of the transitions between the lowest ground and excited state spin doublets at 3.7 K.

Figure 2

All-optical Rabi oscillations between the two lowest ground state spin sublevels $|1⟩$ and $|2⟩$ using a bichromatic Raman pulse at 40 mK (effective ${\mathrm{SiV}}^{-}$ temperature extracted from $T_1$ measurements). As expected for two-photon Rabi oscillations, the Rabi frequencies scale linearly [$\mathrm{\Omega }(20\mathrm{nW})=1.54\mathrm{MHz}$, $\mathrm{\Omega }(40\mathrm{nW})=2.48\mathrm{MHz}$, $\mathrm{\Omega }(80\mathrm{nW})=4.13\mathrm{MHz}$] with power. Blue lines represent simulations using a four-level Bloch equation model [10].

Figure 3

(a) Ramsey interference envelope measured at 40 mK by applying two subsequent 4 ns long $\pi /2$ Raman pulses (average power $P=0.5\mathrm{nW}$) for a number of fixed delays and maximizing the fluorescence in the second pulse by fine-tuning their temporal spacing. (b) Spin echo measurement using a simple three pulse echo sequence with a single refocusing $\pi$ pulse (average power $P=1\mathrm{nW}$). Blue lines are simulations using a density matrix model with the power ratio of both Raman components and the ground state decoherence rate as free parameters.

Figure 4

(a) Spin relaxation measured via a two-pulse optical pumping sequence on transitions $B1$ and $B2$ for different temperatures between 3.7 K and 40 mK and an average optical power of $P=50\mathrm{nW}$. An exponential fit to the decay of the fluorescence signal yields the spin relaxation time $T_1^{\text{spin}}$. (b) Temperature-dependent spin (blue) and orbital relaxation rates (red) indicating single-phonon spin relaxation processes below 300 mK and above 2.3 K as well as nonlinear multiphonon processes in between. Inset shows an enlargement of the low temperature region. Further details can be found in the main text. The gray temperature region (1–3.7 K) is inaccessible with the cryostat used in this study. (c) Temperature-dependent spin echo measurements according to the scheme in Fig. 3. Blue line corresponds to an analytical model including a residual decoherence rate at 0 K and a temperature- and magnetic field-dependent decoherence rate describing field fluctuations generated by flip-flop processes in a spin bath ($C=2\pi \times 6.34\mathrm{kHz}$, ${\mathrm{\Gamma }}_{\text{res}}=2\pi \times 1.15\mathrm{MHz}$, $T_Z=280\mathrm{mK}$).