Geometric entanglement of a photon and spin qubits in diamond
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-12-15 |
| Journal | Communications Physics |
| Authors | Yuhei Sekiguchi, Yuki Yasui, Kazuya Tsurumoto, Yuta Koga, Raustin Reyes |
| Institutions | Yokohama National University |
| Citations | 17 |
| Analysis | Full AI Review Included |
Geometric Entanglement in Diamond: Technical Analysis and 6CCVD Solutions
Section titled âGeometric Entanglement in Diamond: Technical Analysis and 6CCVD SolutionsâThis document analyzes the research demonstrating geometric entanglement between a photon and a spin qubit in a diamond Nitrogen-Vacancy (NV) center under zero magnetic field. The findings highlight the critical role of high-ppurity, high-quality Single Crystal Diamond (SCD) for noise-resilient quantum network architectures.
Executive Summary
Section titled âExecutive Summaryâ- Breakthrough Entanglement: Geometric entanglement between a photon and an NV spin qubit was successfully generated via spontaneous emission under zero magnetic field conditions.
- High Fidelity Achieved: The experiment demonstrated an entanglement state fidelity of 86.8%, paving the way for noise-resilient quantum repeaters.
- Material Requirement: The work relied on high-purity, CVD-grown Type IIa diamond, emphasizing the necessity of ultra-low defect density substrates for coherent control.
- Geometric Qubit Advantage: Utilizing the geometric spin qubit (defined in the degenerate |ms = ±1> subspace) allows for manipulation insensitive to time, frequency, and space mode matching, crucial for scaling quantum networks.
- Zero-Field Operation: Operating at zero magnetic field enhances noise resilience, a key advantage over dynamic qubits manipulated under strong external fields.
- Primary Limitation: The main fidelity limitation (6%) was attributed to NV-axis misalignment in the <100> oriented substrate, suggesting a clear path for material optimization using <111> orientation.
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points were extracted from the experimental results:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Entanglement State Fidelity | 86.8 | % | Measured fidelity of the entangled state |
| Operating Temperature | 5 | K | Required for coherent electron orbital control |
| Microwave Rabi Frequency | 2.5 | MHz | Limited frequency used for GRAPE pulse shaping |
| ZPL Gate Delay Time | 10 | ns | Width of the APD gate for ZPL measurement |
| Green Excitation Wavelength | 515 | nm | Used for charge and spin initialization |
| Red Excitation Wavelength (ZPL) | 637 | nm | Used for A2> state preparation and spin measurement |
| Primary Material | Type IIa SCD | N/A | High-purity CVD diamond |
| Crystal Orientation Used | <100> | N/A | Resulted in 53.0° NV axis tilt |
| Dominant Error Source | 6 | % | NV-axis misalignment (Coherent Error) |
| Spin Measurement Error | 2 | % | Limited by microwave manipulation fidelity |
Key Methodologies
Section titled âKey MethodologiesâThe experiment utilized a highly controlled environment and advanced quantum control techniques:
- Material Selection and Cooling: A single, naturally occurring NV center in a high-purity Type IIa CVD diamond (<100> orientation) was cooled to 5 K to maintain electron orbital coherence.
- Zero Magnetic Field Setup: Residual magnetic fields, including the geomagnetic field, were canceled using a three-dimensional coil system. The zero-field condition was confirmed by maximizing the spin-echo coherence time.
- Microwave Manipulation: Two orthogonal copper wires were attached to the diamond surface to apply polarized microwaves. The GRAPE algorithm was used to generate modulated waveforms, achieving 97% fidelity for geometric spin qubit state preparation.
- Optical Excitation and Readout: A confocal microscope system was used. A 515 nm green laser initialized the NV center. Two 637 nm red lasers were used for resonant excitation to the A2> state and subsequent spin measurement via the Ey> state.
- Temporal Separation: A two-stage electro-optic modulator (EOM) was used to temporally eliminate reflected excitation light from the emitted photon signal, although overall extinction ratio remained a challenge.
- Conditional Measurement: An FPGA (100 MHz clock) processed signals from two Avalanche Photodiodes (APDs) in real time, enabling spin state measurement conditioned on the ZPL photon detection.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & Capabilitiesâ6CCVD provides the specialized MPCVD diamond materials and engineering services necessary to replicate, optimize, and scale this groundbreaking research into robust quantum network nodes.
Applicable Materials for Optimization
Section titled âApplicable Materials for OptimizationâThe research identified NV-axis misalignment in the <100> substrate as the primary fidelity limiter. 6CCVD offers materials specifically engineered to mitigate this issue:
| Material Specification | 6CCVD Capability | Research Application | Optimization Benefit |
|---|---|---|---|
| Optical Grade SCD | SCD plates up to 500 ”m thick, high purity (< 1 ppb N). | Host for NV centers and quantum memories. | Minimizes background defects and strain, maximizing coherence time. |
| <111> Crystal Orientation | Standard offering for SCD growth. | Required to eliminate NV-axis tilt (53.0° tilt observed in <100>). | Enables normally oriented NV centers, eliminating the 6% fidelity loss due to misalignment. |
| High-Quality Polishing | Ra < 1 nm (SCD). | Essential for ZPL collection and minimizing optical scattering. | Improves photon collection efficiency and reduces background noise, critical for conditional measurements. |
Customization Potential for Scaling
Section titled âCustomization Potential for ScalingâTo transition this proof-of-concept into scalable quantum hardware, 6CCVD offers critical customization services:
- Custom Dimensions: While the experiment used a single NV center, scaling requires larger platforms. 6CCVD provides SCD plates up to 10 mm thick and PCD wafers up to 125 mm for integration into complex photonic circuits and quantum repeaters.
- Advanced Metalization: The experiment used simple copper wires. 6CCVD offers in-house metalization (Ti/Pt/Au, W, Cu) services for creating high-fidelity, integrated microwave delivery structures directly on the diamond surface, improving Rabi frequency and manipulation fidelity.
- Precise Thickness Control: 6CCVD can supply SCD layers with thickness control from 0.1 ”m to 500 ”m, allowing researchers to optimize the diamond membrane thickness for coupling NV centers to photonic cavities or mechanical resonators (as suggested in the Discussion).
- Laser Cutting and Shaping: Custom laser cutting services ensure precise geometry for integration into optical setups (e.g., waveguides or micro-lenses) to maximize photon collection efficiency.
Engineering Support
Section titled âEngineering Supportâ6CCVDâs in-house PhD team specializes in material science for quantum applications. We offer consultation and support for projects focused on geometric quantum computing and quantum repeaters:
- Material Selection: Assistance in selecting the optimal crystal orientation (<111> vs. <100>) and nitrogen concentration for specific NV creation methods (e.g., implantation vs. in-situ doping).
- Strain Engineering: Consultation on minimizing non-axial strain (which limited fidelity by 1%) through optimized growth and post-processing techniques.
- Interface Optimization: Support for designing metalization layers and surface preparation protocols to ensure robust coupling between the NV spin and external microwave fields.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Abstract Geometric nature, which appears in photon polarization, also appears in spin polarization under a zero magnetic field. These two polarized quanta, one travelling in vacuum and the other staying in matter, behave the same as geometric quantum bits or qubits, which are promising for noise resilience compared to the commonly used dynamic qubits. Here we show that geometric photon and spin qubits are entangled upon spontaneous emission with the help of the spin â orbit entanglement inherent in a nitrogen-vacancy center in diamond. The geometric spin qubit is defined in a degenerate subsystem of spin triplet electrons and manipulated with a polarized microwave. An experiment shows an entanglement state fidelity of 86.8%. The demonstrated entangled emission, combined with previously demonstrated entangled absorption, generates purely geometric entanglement between remote matters in a process that is insensitive of time, frequency, and space mode matching, which paves the way for building a noise-resilient quantum repeater network or a quantum internet.