All-optical initialization, readout, and coherent preparation of single silicon-vacancy spins in diamond

Abstract: The silicon-vacancy ($\mathrm{SiV}^-$) color center in diamond has attracted attention due to its unique optical properties. It exhibits spectral stability and indistinguishability that facilitate efficient generation of photons capable of demonstrating quantum interference. Here we show high fidelity optical initialization and readout of electronic spin in a single $\mathrm{SiV}^-$ center with a spin relaxation time of $T_1=2.4\pm0.2$ ms. Coherent population trapping (CPT) is used to demonstrate coherent preparation of dark superposition states with a spin coherence time of $T_2^\star=35\pm3$ ns. This is fundamentally limited by orbital relaxation, and an understanding of this process opens the way to extend coherences by engineering interactions with phonons. These results establish the $\mathrm{SiV}^-$ center as a solid-state spin-photon interface.

• The paper demonstrates high-fidelity optical initialization and readout of SiV spins, achieving a T1 relaxation time of 2.4 ± 0.2 ms using controlled spin transitions.
• The paper employs coherent population trapping to prepare dark superposition states, achieving a measured spin coherence time (T2*) of 35 ± 3 ns despite orbital relaxation effects.
• The paper reveals resolved hyperfine interactions with 29Si nuclear spins, offering potential for longer quantum information storage in scalable network applications.

Overview of All-Optical Initialization, Readout, and Coherent Preparation of Single Silicon-Vacancy Spins in Diamond

The paper presents a detailed exploration of silicon-vacancy (SiV) color centers in diamond, highlighting their potent capabilities for quantum information processing. The authors demonstrate how these centers, known for their excellent spectral properties and emission of indistinguishable photons, can be leveraged to initialize, readout, and coherently prepare electron spin states. The implications of this work are significant, as SiV centers exhibit both optical stability and ease of integration, making them promising candidates for scalable quantum networks.

Key Findings and Technical Details

1. Optical Initialization and Readout: The authors successfully achieved high fidelity optical initialization and readout of SiV electronic spin states, showcasing a spin relaxation time of $T_1 = 2.4 \pm 0.2 \, \text{ms}$. The process involved utilizing spin-flipping and spin-conserving transitions to induce and detect spin polarization, exploiting the distinguishable coherent photon emission from diamond's silicon-vacancy defects.
2. Coherent Preparation of Dark Superposition States: Utilizing coherent population trapping (CPT), the research presents coherent preparation of dark superposition states, with a measured spin coherence time $T_2^\star = 35 \pm 3 \, \text{ns}$. The spin-coherence time, although limited by orbital relaxation through electron-phonon interactions, underscores significant potential for manipulation at cryogenic temperatures.
3. Hyperfine Interactions with ${}^{29} \text{Si}$ Nuclear Spin: The study unveils interactions between electronic spins and ${}^{29} \text{Si}$ nuclear spin, suggesting that the hyperfine structure can be resolved into distinguishable coherent spin manipulations—a potential asset for storing quantum information longer than electron spins permit.

Practical and Theoretical Implications

The research establishes SiV centers as viable spin-photon interfaces due to their promising optical properties—the narrow zero-phonon line emissions are a considerable advantage over other color centers like NV centers. The work illustrates how SiV centers’ properties might overcome NV centers' challenges such as phonon sidebands and spectral diffusion, enhancing the viability of SiV-based quantum nodes for network applications.

From a theoretical perspective, understanding the mechanisms limiting coherence times—especially the orbital relaxation result—offers a pathway to longer coherence durations. Strategic phonon engineering, either by downscaling the environment or exploiting phononic properties, could conceivably enhance the utility of these centers in quantum technologies.

Future Directions and Conclusion

The study provides a basis for future exploration into extending coherence times in SiV centers, potentially through advanced cooling techniques or phononic band gap engineering. The identification of hyperfine interactions proposes a compelling case for further research into utilizing nuclear spins within quantum networks. Additionally, the SiV center's capacity for coherent spin state manipulation and high-fidelity readout posits it as a crucial component for future scalable quantum computing architectures.

In conclusion, the paper offers a comprehensive investigation into the functionalities of SiV centers in diamond. The insights from this study are poised to catalyze further innovations in quantum optics and information sciences, bridging theoretical concepts with practical implementations in quantum technologies.