Deep Three-Dimensional Solid-State Qubit Arrays with Long-Lived Spin Coherence

At a Glance

Metadata	Details
Publication Date	2019-12-03
Journal	Physical Review Applied
Authors	C. J. Stephen, B.L. Green, Y. N. D. Lekhai, Weng L, Hill P
Institutions	University of Strathclyde, Engineering and Physical Sciences Research Council
Citations	40
Analysis	Full AI Review Included

Technical Documentation & Analysis: 3D Solid-State Qubit Arrays in Diamond

This document analyzes the requirements and achievements detailed in the research paper, “Three-dimensional solid-state qubit arrays with long-lived spin coherence,” and maps them directly to the advanced material solutions and fabrication capabilities offered by 6CCVD.

Executive Summary

The research successfully demonstrates the creation of high-density, three-dimensional Nitrogen-Vacancy (NVC) qubit arrays in diamond using femtosecond laser writing, achieving performance metrics critical for scalable quantum computing and sensing platforms.

High Coherence: Achieved electron spin coherence times (T₂) up to 710 µs at room temperature, an order of magnitude longer than previously reported laser-written qubits.
3D Scalability: Demonstrated the potential for 5 million qubits within a 4.5x4.5x0.5 mm diamond volume, utilizing the upper 50 µm for 3D array fabrication.
Precision Placement: NVCs were positioned with high accuracy: ±200 nm in the transverse (XY) plane and ±250 nm in the vertical (Z) direction, enabled by adaptive optics and non-linear writing.
Material Requirement: The platform relies on Electronic (EL) grade diamond with natural isotopic abundance (1.1% ¹³C) to provide the necessary nuclear spin register for long-term quantum information storage.
Integrated Functionality: The methodology supports simultaneous 3D fabrication of NVC arrays and electrically conductive graphitic micro-wires for integrated control and Stark shifting.
6CCVD Value Proposition: 6CCVD specializes in providing the high-purity, low-nitrogen Single Crystal Diamond (SCD) substrates, custom dimensions, and advanced surface preparation (Ra < 1nm) necessary to replicate and scale this critical quantum research.

Technical Specifications

The following hard data points were extracted from the research paper, highlighting the critical performance metrics and fabrication parameters.

Parameter	Value	Unit	Context
Maximum Electron Spin Coherence Time (T₂)	710 ± 40	µs	Measured at room temperature on NV2
Longitudinal Lifetime (T₁)	3.0 ± 0.7	ms	Measured at room temperature on NV2
Minimum T₂ for Single NVCs	> 500	µs	Achieved by 16 out of 23 measured single NVCs
NVC Placement Precision (Transverse, XY)	± 200	nm	Limited by 1-5 ppb nitrogen concentration
NVC Placement Precision (Vertical, Z)	± 250	nm	Enabled by non-linear laser writing
Diamond Substrate Dimensions	4.5 x 4.5 x 0.5	mm	Commercially available EL grade diamond
Nitrogen Concentration (Target Purity)	1 to 5	ppb	Required for high-precision NVC creation
Laser Pulse Energy Range	14 to 19	nJ	Used for optimal vacancy generation
Array Pitch (Minimum)	2	µm	Used for 3D array stacking
Array Depth Span	50	µm	Upper layer of diamond used for 3D fabrication
Annealing Conditions	1000 °C for 3 hours	N/A	Performed in a nitrogen environment

Key Methodologies

The 3D NVC array fabrication relies on precise material preparation, advanced laser optics, and controlled thermal processing.

Substrate Preparation:
- Used Electronic (EL) grade SCD with natural ¹³C isotopic abundance (1.1%).
- Plasma etching was performed to remove 20 µm of sub-surface polishing damage, ensuring a pristine starting material for deep writing.
Laser Writing System:
- Single 250 fs pulses from a 790 nm laser were used to generate vacancy ensembles.
- Focusing achieved via a high numerical aperture (NA=1.4) oil objective.
Adaptive Optics Correction:
- A liquid crystal Spatial Light Modulator (SLM) was used for adaptive optics to correct depth-dependent spherical aberration caused by the refractive index mismatch at the oil-diamond interface. This ensured consistent fabrication resolution (theoretically 350 nm radially, 1.7 µm longitudinally) across all depths.
3D Translation:
- The diamond was mounted on a three-axis precision translation stage and moved relative to the fixed laser focus to fabricate 3D arrays (up to 21x20 2D arrays stacked up to five depths).
Thermal Annealing:
- The diamond was annealed at 1000 °C for 3 hours in a nitrogen environment to mobilize the laser-generated vacancies (V⁰) and promote bonding with native nitrogen dopants, forming the negatively charged nitrogen-vacancy centers (NVCs).
Integrated Wiring:
- Electrically conductive graphitic micro-wires (DC resistivity of 0.1 Ωcm) were laser-written simultaneously with the NVC arrays for future electrical control and Stark shifting.

6CCVD Solutions & Capabilities

6CCVD provides the foundational MPCVD diamond materials and advanced fabrication services required to replicate, optimize, and scale the 3D qubit array platform described in this research.

Applicable Materials

The research requires high-purity SCD with specific nitrogen and carbon isotopic control. 6CCVD offers tailored solutions:

Quantum Grade SCD (Natural ¹³C): To replicate the long-lived nuclear spin register demonstrated in the paper, 6CCVD supplies high-purity Single Crystal Diamond (SCD) with controlled, low nitrogen concentration (targeting the 1-5 ppb range) and natural ¹³C isotopic abundance (1.1%).
Isotopically Pure SCD (¹²C enriched): For applications requiring maximum electron spin coherence (T₂) where the ¹³C nuclear spin bath is the limiting factor, 6CCVD offers SCD enriched to >99.99% ¹²C. This material is essential for pushing T₂ times beyond the reported 710 µs limit.
Boron-Doped Diamond (BDD): For integrated electrical components or electrodes requiring higher conductivity than the laser-written graphitic wires, 6CCVD can supply highly conductive BDD substrates or layers.

Customization Potential

6CCVD’s in-house engineering and fabrication capabilities directly address the complex requirements of 3D quantum device manufacturing:

Research Requirement	6CCVD Capability	Technical Advantage
Substrate Size (4.5x4.5 mm used)	Custom Dimensions up to 125 mm	Enables scaling from R&D samples to production-scale wafers (PCD or SCD).
Surface Quality (20 µm damage removal required)	Ultra-Low Damage Polishing	Standard SCD polishing achieves Ra < 1 nm, minimizing the need for extensive pre-fabrication plasma etching.
Integrated Control (Laser-written graphitic wires)	Custom Metalization Services	Internal capability for depositing high-conductivity metal stacks (Au, Pt, Pd, Ti, W, Cu) for superior electrical contacts, waveguides, and integrated microwave control lines.
Thickness Control (0.5 mm substrate)	Precision Thickness Control	SCD plates available from 0.1 µm to 500 µm, and substrates up to 10 mm, allowing precise control over the volume available for 3D writing.
Post-Processing (1000 °C Annealing)	Material Stability Guarantee	6CCVD SCD is grown via MPCVD, ensuring high thermal stability and structural integrity required for high-temperature annealing processes (up to 1200 °C) necessary for NVC formation.

Engineering Support

6CCVD’s in-house PhD team provides expert consultation on material selection and optimization for complex quantum projects. We can assist researchers in defining the optimal nitrogen concentration, isotopic purity (¹²C vs. ¹³C), and surface orientation ([100] or [111]) required to maximize NVC yield, coherence, and integration efficiency for similar 3D Qubit Array and Quantum Sensing projects.

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

View Original Abstract

Three-dimensional arrays of silicon transistors increase the density of bits.\nSolid-state qubits are much larger so could benefit even more from using the\nthird dimension given that useful fault-tolerant quantum computing will require\nat least 100,000 physical qubits and perhaps one billion. Here we use laser\nwriting to create 3D arrays of nitrogen-vacancy centre (NVC) qubits in diamond.\nThis would allow 5 million qubits inside a commercially available 4.5x4.5x0.5\nmm diamond based on five nuclear qubits per NVC and allowing $(10 \mu m)^3$ per\nNVC to leave room for our laser-written electrical control. The spin coherence\ntimes we measure are an order of magnitude longer than previous laser-written\nqubits and at least as long as non-laser-written NVC. As well as NVC quantum\ncomputing, quantum communication and nanoscale sensing could benefit from the\nsame platform. Our approach could also be extended to other qubits in diamond\nand silicon carbide.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.12.064005