High-Fidelity Electron Spin Gates for Scaling Diamond Quantum Registers
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-05-27 |
| Journal | Physical Review X |
| Authors | Timo Joas, Florian Ferlemann, Roberto Sailer, Philipp J. Vetter, Jingfu Zhang |
| Institutions | Center for Integrated Quantum Science and Technology, National Institute for Materials Science |
| Citations | 1 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: High-Fidelity Electron Spin Gates
Section titled âTechnical Documentation & Analysis: High-Fidelity Electron Spin GatesâExecutive Summary
Section titled âExecutive SummaryâThis research demonstrates a critical breakthrough in solid-state quantum computing, achieving record two-qubit gate fidelity in diamond NV centers under ambient conditions. 6CCVDâs expertise in high-purity MPCVD diamond is essential for replicating and scaling this performance.
- Record Fidelity: Achieved a two-qubit entangling gate fidelity ($F_{2q}$) of (96.0 ± 2.5)% at room temperature, surpassing previous state-of-the-art results.
- Material Foundation: Success relies on a $^{12}$C enriched, epitaxially grown MPCVD diamond layer (20 ”m thick) with ultra-low nitrogen concentration (< 1 ppb) to maximize electron spin coherence.
- Coherence Performance: Demonstrated long electron spin coherence times ($T_{2,XY8}$) averaging 465 ”s per qubit, operating close to the decoherence limit.
- Gate Speed: The robust $\sqrt{ZZ}$ entangling gate was executed in an ultra-fast time of $2.17 \text{ ”s}$.
- Error Analysis: 90% of observed errors are coherent and correctable, primarily stemming from unpolarized $^{14}\text{N}$ nuclear spins and magnetic field misalignment.
- Scaling Pathway: The model projects that mitigating these coherent errors and extending coherence times (e.g., via advanced dynamical decoupling or cryogenic operation) can achieve fidelities of 99.6%, exceeding the threshold required for fault-tolerant quantum error correction.
Technical Specifications
Section titled âTechnical SpecificationsâThe following table summarizes the critical performance metrics and material parameters achieved in the study, highlighting the requirements for high-fidelity NV quantum registers.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Two-Qubit Gate Fidelity ($F_{2q}$) | $96.0 \pm 2.5$ | % | Randomized Benchmarking, Ambient Conditions |
| Gate Infidelity (EPC) | $14.9 \pm 2.7$ | % | Error Per Clifford gate |
| Entangling Gate Time ($t_{\sqrt{ZZ}}$) | $2.17 \pm 0.02$ | ”s | Calibrated evolution time |
| Electron Spin Coherence ($T_{2,XY8}$) | $454 \pm 58$ (NV1); $476 \pm 30$ (NV2) | ”s | Measured using XY8 dynamical decoupling sequence |
| Dipolar Coupling Strength ($\nu_{\text{dip}}$) | $119.8 \pm 1.0$ | kHz | Between two NV centers (distance $\sim 9.5 \text{ nm}$) |
| Rabi Frequency ($\Omega_{\text{Rabi}}/(2\pi)$) | $23.7$ | MHz | Optimized setting for magnetic field 2 |
| CVD Film Thickness | 20 | ”m | Epitaxially grown $^{12}\text{C}$ enriched layer |
| Isotopic Purity ($^{12}\text{C}$) | $99.95$ | % | Used to suppress $^{13}\text{C}$ effects on coherence |
| Nitrogen Concentration | < 1 | ppb | High-purity requirement for long $T_2$ |
| Magnetic Field Strength ($ | B | $) | $105.33$ |
Key Methodologies
Section titled âKey MethodologiesâThe experimental success relies on precise material engineering and advanced quantum control techniques:
- Material Growth: A Type-IIa (100) single-crystalline diamond film was grown homoepitaxially via Microwave Plasma-Assisted CVD (MPCVD) onto a Type-Ib substrate. The film was highly enriched with $^{12}\text{C}$ (99.95%) and maintained ultra-low nitrogen concentration (< 1 ppb).
- NV Center Fabrication: Ion implantation was performed using ionized $\text{C}_5\text{N}_4\text{H}_n$ (adenine) molecules accelerated to 65 keV, achieving a fluence of $10^{8} \text{cm}^{-2}$.
- Annealing and Recovery: Post-implantation annealing was conducted at 1000 °C in forming gas (4% $\text{H}_2$ in Ar) to create NV centers and recover the diamond lattice structure.
- Spin Initialization and Readout: Spin states were initialized using a 552 nm green laser pulse (3 ”s) and read out optically via confocal microscopy. Charge state initialization utilized a weak 594 nm orange laser pulse (3.5 ms) combined with photon thresholding post-selection.
- Microwave Control: Electron spin states were manipulated using sine-envelope shaped microwave pulses applied via a 20 ”m copper wire antenna placed on the diamond surface.
- Gate Implementation: The $\sqrt{ZZ}$ entangling gate was embedded in a decoherence-protecting dynamical decoupling sequence (XY8-1) applied robustly to both NV electron spins.
- Fidelity Quantification: Gate performance was rigorously quantified using Repetitive Benchmarking (to separate gate error from SPAM) and Two-Qubit Randomized Benchmarking (to obtain the average gate set infidelity, EPC).
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & Capabilitiesâ6CCVD is uniquely positioned to supply the foundational diamond materials required to replicate and advance this high-fidelity quantum register research. Our custom MPCVD capabilities directly address the stringent material specifications necessary for scaling NV-based quantum processors.
Applicable Materials
Section titled âApplicable MaterialsâTo achieve the high coherence times ($T_{2,XY8} \approx 465 \text{ ”s}$) demonstrated in this work, researchers require diamond with exceptional purity and precise geometry.
- Optical Grade SCD (Single Crystal Diamond): 6CCVD provides high-purity, low-strain SCD films, epitaxially grown with $^{12}\text{C}$ enrichment (up to 99.999%) and ultra-low nitrogen concentration (< 1 ppb). This material is the direct equivalent of the layer used in the study, ensuring maximal electron spin coherence and minimal decoherence from nuclear spin baths.
- Custom Substrates: We offer high-quality Type-Ib or Type-IIa substrates (up to 10 mm thick) optimized for subsequent epitaxial growth, ensuring a stable foundation for the quantum register layer.
Customization Potential
Section titled âCustomization PotentialâThe scaling of quantum registers demands precise control over material dimensions and integration features. 6CCVD offers tailored solutions to meet these requirements:
| Research Requirement | 6CCVD Customization Capability | Value Proposition |
|---|---|---|
| Film Thickness (20 ”m) | Custom SCD film thickness from 0.1 ”m up to 500 ”m. | Allows researchers to optimize film thickness for specific implantation depths (65 keV used here) and optical collection efficiency. |
| Surface Quality (Optical Readout) | Ultra-low roughness polishing: SCD Ra < 1 nm. | Essential for integrating micro-optical components (e.g., solid immersion lenses, SILs) mentioned in the outlook for enhanced photon collection and single-shot readout. |
| Microwave Control Integration | Internal Metalization Services: Au, Pt, Pd, Ti, W, Cu. | Enables the fabrication of integrated microwave structures (coplanar waveguides, antennas) directly onto the diamond surface for scalable, high-speed spin manipulation, mitigating crosstalk errors. |
| Custom Dimensions | Plates/wafers up to 125 mm (PCD) and large-area SCD. | Supports the transition from proof-of-concept devices to large-scale quantum processors requiring increased qubit density and register size. |
Engineering Support
Section titled âEngineering SupportâThe paper identifies several complex coherent error sources (unpolarized $^{14}\text{N}$ spins, magnetic field misalignment, crosstalk) that require advanced material and control solutions.
- 6CCVDâs in-house PhD team specializes in the material science of NV centers and can assist researchers in selecting the optimal diamond specifications (e.g., isotopic purity, nitrogen doping profiles) for similar NV-NV Entangling Gate projects.
- We provide consultation on material preparation protocols, including surface termination and annealing optimization, crucial for maximizing the yield and charge-state fidelity ($F_{\text{NV}^{-}, \text{NV}^{-}} = 83 \pm 6%$) required for robust operation.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Diamond is a promising platform for quantum information processing as it can host highly coherent qubits that could allow for the construction of large quantum registers. A prerequisite for such devices is a coherent interaction between nitrogen-vacancy (NV) electron spins enabling scalable entanglement. Entanglement between dipolar-coupled NV spin pairs has been demonstrated but with a limited fidelity, and its error sources have not been characterized. Here, we design and implement a robust two-qubit gate between NV electron spins in diamond and quantify the influence of multiple error sources on the gate performance. Experimentally, we demonstrate a record gate fidelity of <a:math xmlns:a=âhttp://www.w3.org/1998/Math/MathMLâ display=âinlineâ><a:mrow><a:msub><a:mrow><a:mi>F</a:mi></a:mrow><a:mrow><a:mn>2</a:mn><a:mi mathvariant=ânormalâ>q</a:mi></a:mrow></a:msub><a:mo>=</a:mo><a:mo stretchy=âfalseâ>(</a:mo><a:mn>96.0</a:mn><a:mo>±</a:mo><a:mn>2.5</a:mn><a:mo stretchy=âfalseâ>)</a:mo><a:mi>%</a:mi></a:mrow></a:math> under ambient conditions. Our identification of the dominant errors paves the way towards NV-NV gates beyond the error correction threshold.
Tech Support
Section titled âTech SupportâOriginal Source
Section titled âOriginal SourceâReferences
Section titled âReferencesâ- 2022 - Proceedings of the Annual International Symposium on Microarchitecture : MICRO 2022 [Crossref]