SiV / SnV Color Center Qubit

Figure

Description

Silicon-vacancy (SiV⁻) and tin-vacancy (SnV⁻) centers in diamond are group-IV color center qubits with inversion symmetry that protects their optical transitions from electric field noise. This gives them spectrally stable, nearly transform-limited optical emission — a critical advantage over NV centers for quantum networking, where photon indistinguishability is essential for remote entanglement.

Group-IV color centers (SiV, GeV, SnV, PbV) share a split-vacancy structure: the impurity atom sits at an inversion center between two vacant carbon sites. This $D_{3d}$ symmetry eliminates first-order coupling to electric fields (Stark effect), producing narrow, stable optical lines even in nanostructures.

SiV⁻: Ground state is a spin-1/2 doublet with ~48 GHz splitting in the orbital ground state. Optical transition at 737 nm with >90% emission into the zero-phonon line (ZPL), vs. ~3% for NV centers. The challenge: the orbital degree of freedom makes the spin sensitive to phonons, requiring operation below ~100 mK for long coherence.

SnV⁻: Larger spin-orbit splitting (~850 GHz) pushes phonon-mediated relaxation to higher temperatures, enabling spin coherence at ~1–2 K — accessible with a standard ³He cryostat rather than a dilution refrigerator. Optical transition at 619 nm.

Both are promising for quantum repeater nodes, where spin-photon entanglement and photon-mediated remote spin-spin entanglement are the core operations.

Hamiltonian

Ground-state spin Hamiltonian in an external magnetic field:

$H={\lambda }_{\text{SO}}{L}_z{S}_z+g_e{\mu }_B\mathbf{B}\cdot \mathbf{S}+g_L{\mu }_B\mathbf{B}\cdot \mathbf{L}+H_{\text{JT}}$

where ${\lambda }_{\text{SO}}$ is the spin-orbit coupling (~48 GHz for SiV, ~850 GHz for SnV), $L_z$ is the orbital angular momentum projection, and $H_{\text{JT}}$ captures dynamic Jahn-Teller effects.

Qubit states are typically the spin-up and spin-down states of the lower orbital branch, split by the Zeeman interaction.

Motivation

• Spectral stability: Inversion symmetry eliminates first-order Stark effect, giving ~100× narrower optical lines than NV centers — critical for photon-mediated entanglement.
• High ZPL fraction: >70% (SiV) and >60% (SnV) of emission into the zero-phonon line vs. ~3% for NV — dramatically higher photon collection efficiency.
• Quantum networking: Best-in-class spin-photon interface for diamond platforms; SiV demonstrated metropolitan-scale entanglement.
• Temperature tradeoff spectrum: SiV (100 mK) → SnV (1.5 K) → PbV (potentially higher) — SnV already accessible without dilution refrigerator.
• Nanophotonic integration: High ZPL fraction enables efficient coupling to photonic crystal cavities with >100× Purcell enhancement.

Experimental Status

SiV spin coherence — Sukachev et al. (2017):

• Demonstrated SiV spin qubit as quantum memory exceeding 10 ms coherence
• Single-shot spin readout with >99% fidelity at millikelvin temperatures

SiV quantum network — Nguyen et al. (2019):

• Quantum network nodes based on SiV qubits with efficient nanophotonic interface
• Demonstrated spin-photon entanglement and remote two-node entanglement

Metropolitan entanglement — Knaut et al. (2024):

• Entanglement of SiV quantum memory nodes across a deployed telecom fiber network in Boston
• Remote entanglement fidelity of 94%

SnV coherence — Debroux et al. (2021):

• Quantum control of SnV spin qubit at 1.7 K (³He cryostat)
• Spin echo $T_2$ = 0.3 ms — first demonstration of SnV as viable qubit

Key Metrics

Metric	Value	Notes	Fidelity reference
ZPL fraction (Debye-Waller)	>70% (SiV), >60% (SnV)	vs. ~3% for NV center	Nguyen et al. 2019
T₂ (spin echo, SiV)	13 ms	At 100 mK in diamond	Sukachev et al. 2017
T₂ (spin echo, SnV)	0.3 ms	At 1.7 K	Debroux et al. 2021
Spectral diffusion	<100 MHz (SiV)	100× narrower than NV	—
Remote entanglement fidelity	94%	SiV, Harvard 2024	Knaut et al. 2024
Optical linewidth	~100 MHz (SiV)	Near transform-limited	—
Operating temperature	<100 mK (SiV), ~1.5 K (SnV)	SnV advantage for scalability	—

Scaling Considerations

• Quantum networking: Best-in-class spin-photon interface for diamond platforms. SiV demonstrated Boston-to-Harvard entanglement over deployed fiber.
• Nanophotonic integration: High ZPL fraction enables efficient coupling to photonic crystal cavities (Purcell enhancement >100×).
• Temperature tradeoff: SiV needs dilution refrigerator; SnV works at ³He temperatures. PbV may work even higher.
• Fabrication: Requires implantation + annealing in high-purity diamond. Yield and placement precision improving but below semiconductor standards.

References

Key experiments

• D. D. Sukachev et al., “Silicon-Vacancy Spin Qubit in Diamond: A Quantum Memory Exceeding 10 ms with Single-Shot State Readout,” Phys. Rev. Lett. 119, 223602 (2017) — arXiv:1708.08852
• C. T. Nguyen et al., “Quantum Network Nodes Based on Diamond Qubits with an Efficient Nanophotonic Interface,” Phys. Rev. Lett. 123, 183602 (2019) — arXiv:1907.13199
• C. M. Knaut et al., “Entanglement of nanophotonic quantum memory nodes in a telecom network,” Nature 629, 573 (2024) — arXiv:2310.01316
• R. Debroux et al., “Quantum Control of the Tin-Vacancy Spin Qubit in Diamond,” Phys. Rev. X 11, 041041 (2021) — arXiv:2106.00723

Linked Papers

• nguyen-2019-siv-network
• sukachev-2017-siv-coherence
• debroux-2021-snv-coherence
• knaut-2024-siv-entanglement

Related Entries

• t-center-qubit — Silicon-based spin-photon interface with native telecom emission
• nv-center-qubit — Nitrogen-vacancy center; more mature but worse optical properties
• rare-earth-ion-qubit — rare-earth ions in crystals; alternative solid-state spin-photon interface
• dual-rail-photonic-qubit — Photonic encoding for quantum networking