Coherent Spin-Photon Interface of single PL6 Color Centers in Silicon Carbide

The PL6 color center in silicon carbide has recently emerged as a promising platform for quantum information processing, yet its coherent spin–photon interface has remained largely unexplored. Here we present a comprehensive investigation of single PL6 centers, combining spectroscopy with theoretical analysis. The excited-state fine structure is fully resolved using group-theoretical modeling and strain-dependent measurements. Under resonant excitation, we achieve a spin initialization fidelity of $99.69 \pm 0.03%$ and a readout contrast of $98.31 \pm 1.03%$. The spin–photon–entangled $A_2$ transition exhibits narrow optical linewidths ($\sim 180$~MHz) and a polarization visibility of $\sim 82%$. Coherent optical driving enables Rabi frequencies up to $2.895$~GHz, while dynamical decoupling extends the spin coherence time from $0.5$~ms to $5.70$~ms. Our results establish PL6 as a competitive solid-state spin–photon interface hosted in a commercially available semiconductor platform.

💡 Research Summary

This work presents a thorough investigation of the PL6 color center in 4H‑silicon carbide (SiC) as a solid‑state spin‑photon interface. By combining low‑temperature photoluminescence excitation (PLE) spectroscopy with group‑theoretical modeling, the authors fully resolve the excited‑state fine structure of the ³E manifold. The C₃ᵥ symmetry yields irreducible representations A₁, A₂, Eₓ, Eᵧ, E₁ and E₂, split by spin‑orbit coupling (λ ≈ 5.74 GHz) and spin‑spin interactions (D_ES ≈ 0.93 GHz, D₁ ≈ 0.026 GHz, D₂ ≈ 0.285 GHz). Strain‑dependent measurements on seven individual defects, with transverse strain δ⊥ ranging from 0.7 to 12.4 GHz, confirm the theoretical Hamiltonian and demonstrate that strain is the dominant knob for tuning the orbital mixing.

Optical linewidths of the A₂, A₁, and E₁/₂ transitions are as narrow as ~180 MHz at 100 nW excitation, remaining below 450 MHz even at 900 nW. These values are substantially tighter than those of electron‑irradiated 3‑divacancies in commercial SiC, indicating excellent spectral purity for high‑resolution addressing. The A₂ transition exhibits a markedly longer radiative lifetime than A₁ or E₁/₂, mirroring the NV‑center’s A₂ state and providing a natural Λ‑type spin‑photon entanglement pathway.

Spin initialization is performed via resonant excitation of the E₁/₂ transition. At an optimal laser power of 80 nW, the polarization fidelity reaches 99.69 ± 0.03 %, and stays above 99 % up to several microwatts. Spin‑flip rates measured on the Eₓ and Eᵧ manifolds are strain‑dependent, ranging from 105 kHz to 140 kHz—significantly lower than the ~330 kHz reported for typical divacancies. The reduced flip rate, together with the narrow optical lines, enhances the number of photons that can be collected before spin randomization, thereby improving single‑shot readout signal‑to‑noise.

Using an optimized pulse sequence (914 nm charge reset → E₁/₂ polarization → Eᵧ readout), the authors demonstrate coherent single‑spin Rabi oscillations with a contrast of 98.31 ± 1.03 % at 6.35 K under a 5.7 mT magnetic field. Optical Rabi experiments employ 15–20 ns resonant pulses generated by a high‑extinction electro‑optic modulator. The |mₛ = 0⟩↔|Eₓ⟩ transition reaches a Rabi frequency of 2.895 GHz at 60 µW, roughly 130 times the optical saturation power, comparable to the NV‑center’s fastest optical driving. The |mₛ = ±1⟩↔|A₂⟩ transition shows Rabi frequencies of 1.61 GHz and 2.12 GHz at 56 µW, with linear scaling Ω ∝ √P confirmed across a wide power range. Lifetime analysis yields T₁≈15–19 ns for the A₂ state, matching the inverse spin‑relaxation rates and confirming the symmetric decay to |mₛ = +1⟩ and |mₛ = −1⟩—a prerequisite for high‑fidelity spin‑photon entanglement.

Polarization visibility of the A₂ transition is probed by rotating a quarter‑wave plate while recording photon counts during optical π‑pulses. A cosine fit yields a visibility of ~82 %, demonstrating that the A₂ state forms the expected entangled state |σ⁻⟩⊗|+1⟩ + |σ⁺⟩⊗|−1⟩. This confirms that PL6 can serve as a deterministic source of spin‑polarized photons with well‑defined circular polarization.

Spin coherence is characterized by Ramsey and dynamical decoupling sequences. The free‑induction decay time T₂* is ~0.5 ms, limited by the surrounding nuclear spin bath. Applying an XY‑8 sequence extends the coherence time to 5.70 ms, a tenfold improvement and on par with the best reported SiC color centers. Such millisecond‑scale coherence, combined with the fast optical control and narrow linewidths, positions PL6 as a competitive platform for quantum networking, quantum repeaters, and spin‑photon entanglement protocols.

Overall, the paper establishes that PL6 color centers in commercially available 4H‑SiC simultaneously achieve near‑unity spin initialization fidelity, high readout contrast, sub‑200 MHz optical linewidths, strong polarization entanglement, GHz‑scale optical Rabi rates, and multi‑millisecond spin coherence. The authors discuss the practical advantages of using a mature semiconductor platform and ion‑implantation techniques, and outline future directions such as integration with photonic crystal cavities, electrical control of charge states, and scaling to multi‑defect entangled networks. This comprehensive demonstration makes PL6 a compelling candidate for next‑generation solid‑state quantum information technologies.