Entanglement of dark electron-nuclear spin defects in diamond

At a Glance

Metadata	Details
Publication Date	2021-06-09
Journal	Nature Communications
Authors	Maarten Degen, S. J. H. Loenen, H. P. Bartling, C. E. Bradley, Aletta Meinsma
Institutions	QuTech, Element Six (United Kingdom)
Citations	43
Analysis	Full AI Review Included

Technical Documentation & Analysis: Entanglement of Dark Electron-Nuclear Spin Defects in Diamond

This document analyzes the research paper “Entanglement of dark electron-nuclear spin defects in diamond” (Nature Communications, 2021) to highlight the critical material requirements and demonstrate how 6CCVD’s advanced MPCVD diamond solutions meet and exceed the specifications necessary for replicating and scaling this quantum computing research.

Executive Summary

Core Achievement: Demonstrated heralded initialization, coherent control, and entanglement of individual P1 dark electron-nuclear spin defects (qubits) in a high-purity diamond spin bath.
Material Requirement: The success relies critically on ultra-high purity, isotopically enriched Single Crystal Diamond (SCD) with extremely low ¹³C concentration (0.01%) to minimize environmental decoherence.
Performance Metrics: Achieved a two-qubit CPHASE gate fidelity of F = 0.81(5) between two P1 centers (S1 and S2) via magnetic-dipole coupling.
Key Coherence Times: Measured long coherence times for the dark spins, including P1 electron spin T_2e = 1.00(4) ms and P1 nuclear spin T_2N = 4.2(2) ms, validating the material quality.
Methodology: Utilized a nearby optically addressable Nitrogen-Vacancy (NV) center to projectively measure and selectively control the P1 centers’ multiple degrees of freedom (Jahn-Teller axis, nuclear spin, and charge state).
6CCVD Value Proposition: 6CCVD provides the necessary Optical Grade SCD substrates, customized for isotopic purity and surface finish (Ra < 1 nm), essential for advanced quantum device fabrication (e.g., solid-immersion lenses and anti-reflection coatings).

Technical Specifications

The following hard data points were extracted from the experimental results, highlighting the performance achieved using high-quality CVD diamond material.

Parameter	Value	Unit	Context
Diamond Growth Method	Homoepitaxial CVD	N/A	(100) crystal orientation
Isotopic Purity (¹³C)	0.01% (Estimated)	Abundance	Minimizes ¹³C spin bath decoherence
Nitrogen Concentration (P1)	~75	ppb	Source of P1 dark spin qubits
Operating Temperature	3.3	K	Montana Cryostation
Magnetic Field (B)	45.5553(5)	G	Used for resolving P1 transitions
NV Electron T₁ (Relaxation)	> 30	s	Measured at 3.3 K
P1 Electron T_1e (Relaxation)	21(7)	s	Longitudinal relaxation time
P1 Electron T_2e (Coherence, Echo)	1.00(4)	ms	Spin-echo experiment
P1 Nuclear T_2N (Coherence, Echo)	4.2(2)	ms	Nitrogen nuclear spin-echo
Dipolar Coupling (J)	-2π· 17.8(5)	kHz	Between P1 centers S1 and S2
CPHASE Gate Fidelity (F)	0.81(5)	N/A	Two-qubit entangled state

Key Methodologies

The experiment successfully achieved entanglement by combining high-purity material with sophisticated control sequences:

Material Selection and Defect Engineering: Utilized homoepitaxial CVD diamond with ultra-low ¹³C concentration (0.01%) and controlled ¹⁴N concentration (~75 ppb) to host the NV center and the P1 dark spin bath.
Optical Interface Fabrication: A solid-immersion lens (SIL) was fabricated on top of the NV center, and a single-layer aluminum-oxide anti-reflection coating was deposited to enhance photon collection efficiency.
DEER Spectroscopy for Selection: Double electron-electron resonance (DEER) spectroscopy was employed to probe the spin bath and resolve the 12 main P1 electron-spin transitions (four Jahn-Teller axes and three ¹⁴N states).
Heralded Initialization: Projective measurements (DEER sequences) were used to selectively prepare the multiple degrees of freedom (Jahn-Teller axis, nuclear spin, and charge state) of individual P1 centers (S1, S2, S3/S4).
Active State Reset: Fast logic and a 515 nm laser pulse were used to actively reset the P1 centers to a random state following unsuccessful initialization attempts, significantly speeding up the experiment.
Coherent Control: Radio-frequency (RF) pulses were applied to implement electron-controlled CNOT gates on the P1 ¹⁴N nuclear spin.
Two-Qubit Entanglement: A double echo sequence (U_zz(t)) was applied for an interaction time 2τ = π/J to implement a two-qubit CPHASE gate between two selectively initialized P1 centers (S1 and S2).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the foundational diamond materials and advanced processing required to replicate and scale this critical quantum research. Our capabilities ensure the highest material quality and customization necessary for complex quantum registers.

Research Requirement	6CCVD Solution & Capability	Technical Advantage for Quantum Applications
Ultra-High Purity Diamond (0.01% ¹³C, controlled ¹⁴N)	Optical Grade Single Crystal Diamond (SCD)	We provide SCD with guaranteed low native defect concentrations (N, Si, B) and precise isotopic control. This is essential for achieving the long T₂ coherence times (> 1 ms) demonstrated for the P1 electron spins.
Custom Substrate Dimensions (for device integration)	Custom Dimensions & Thickness	SCD plates are available from 0.1 µm up to 500 µm thick, and substrates up to 10 mm. We support large-scale integration by offering PCD wafers up to 125 mm for scalable quantum architectures.
Surface Preparation (for SIL fabrication and AR coating)	Precision Polishing (Ra < 1 nm)	Our SCD material is polished to an atomic-scale finish (Ra < 1 nm). This ultra-smooth surface is critical for subsequent high-resolution lithography, solid-immersion lens (SIL) fabrication, and minimizing optical scattering losses.
Anti-Reflection Coating (Alumina)	Custom Metalization & Thin Film Deposition	6CCVD offers in-house deposition of thin films, including Ti, Au, Pt, Pd, W, and Cu. We can collaborate on custom dielectric stacks (e.g., Al₂O₃) required for specific quantum optical interfaces and enhanced photon extraction efficiency.
Controlled Defect Density (NV and P1 centers)	Engineered Doping and Implantation Support	We supply SCD material optimized for post-growth processing (e.g., electron irradiation or ion implantation) necessary to create high-quality NV centers and control the density of P1 centers for scalable quantum registers.

Engineering Support

6CCVD’s in-house PhD team specializes in the material science of diamond quantum defects. We can assist researchers and engineers with material selection, orientation, and purity specifications required for similar quantum sensing, computation, and network projects involving NV and P1 centers. Our expertise ensures that the starting material maximizes the potential for achieving long coherence times and high gate fidelities.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41467-021-23454-9