Generation of entangled photon strings using NV centers in diamond
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2015-08-06 |
| Journal | Physical Review B |
| Authors | D. D. Bhaktavatsala Rao, Sen Yang, Jörg Wrachtrup |
| Institutions | University of Stuttgart |
| Citations | 33 |
| Analysis | Full AI Review Included |
Technical Documentation and Analysis: Entangled Photon Generation via Diamond NV Centers
Section titled âTechnical Documentation and Analysis: Entangled Photon Generation via Diamond NV CentersâExecutive Summary
Section titled âExecutive SummaryâThe analyzed research paper presents a groundbreaking scheme utilizing Nitrogen-Vacancy (NV) centers in CVD diamond to generate long, entangled photon strings (GHZ and cluster states), crucial for scalable optical Quantum Information Processing (QIP).
- Core Mechanism: Entanglement is mediated by the ultra-long coherence time of the intrinsic $^{14}$N nuclear spin (T$_{2}$ up to minutes), overcoming the rapid decoherence inherent in using electron spins alone.
- Material Requirement: The protocol demands ultra-high-purity, low-strain single crystal diamond (SCD) operated at cryogenic temperatures (T < 8 K) with NV centers aligned precisely along the [111] direction.
- Scalability Demonstrated: By incorporating the NV system into a photonic crystal cavity, the estimated generation rate for a 10-photon entangled state increases significantly to 10 photons per second, a prerequisite for scalable quantum networks.
- Methodology: The scheme relies on repetitive cycling involving electron-photon entanglement followed by C-NOT gate operations to transfer the entanglement to the nuclear spin, which acts as a robust quantum memory.
- 6CCVD Advantage: Replication and scaling of this research require high-fidelity SCD substrates, custom metalization for spin control (MW/RF fields), and ultra-smooth polishing for photonic integration, all core capabilities provided by 6CCVD.
Technical Specifications
Section titled âTechnical SpecificationsâThe following critical hard data points and physical requirements were extracted from the analysis:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Required Operating Temperature | T < 8 | K | Essential for efficient initialization and highly-resolved optical transitions (resonant excitation). |
| Required Strain Environment | ~1.2 | GHz | Low strain environment necessary for maintaining ground state degeneracy and coherence. |
| NV Center Orientation | [111] | Direction | Alignment critical for optimal optical properties and reduced phase errors. |
| Nuclear Spin Coherence (T**2**) | ~1 | Minute | Required for generating entangled strings involving a large number of photons (N > 10). |
| Base Operation Cycle Time ($\tau$) | 1 | ”s | Time separation required between subsequent photon emissions. |
| Target Entanglement Length | $\ge 10$ | Photons | Minimum chain length demonstrated feasible for generation per second with optimized cavity coupling. |
| Optimized Cavity Coupling | 70 | % | Emission directed into the Zero Phonon Line (ZPL) using a photonic crystal cavity. |
| Gate Error Tolerance | $\le 10$ | ° | Maximum phase error used in modeling fidelity loss for Hadamard/C-NOT gates. |
Key Methodologies
Section titled âKey MethodologiesâThe generation of entangled photon strings relies on high-fidelity spin control and repetitive absorption/emission cycles mediated by the long-lived nuclear spin.
- Material and Environment Setup: Utilize high-purity Single Crystal Diamond (SCD) with NV centers aligned along the [111] axis. Maintain cryogenic operation (T < 8 K) and switch off external magnetic fields to maintain ground state degeneracy.
- Hybrid Spin Preparation: Initialize the NV electron spin (S=1) in a superposition state. The nuclear spin ($^{14}$N, I=1) serves as the long-lived quantum memory.
- Electron-Photon Entanglement: Send a polarized photon resonant with the A$_{2}$ transition into the NV center. Absorption acts as a Bell-type measurement, projecting the system into an electron-photon entangled state.
- Entanglement Transfer (C-NOT Gate): Immediately after emission of the first photon, apply a C-NOT gate operation (via customized MW/RF fields) between the electron spin and the nuclear spin. This transfers the entanglement from the rapidly decohering electron spin to the robust nuclear spin memory.
- Repetitive Generation: The NV system is re-pumped with a second photon pulse ($\tau \approx 1 \mu\text{s}$ later). The nuclear spin retains the entanglement from the first photon, mediating further entanglement with the second emitted photon.
- Quantum State Projection: Through manipulation of the electron spin using Hadamard and C-NOT gates (as shown in Fig. 1(b)), the $N$-photon state can be projected onto the desired GHZ or Cluster state via a final measurement on the nuclear spin.
- Fidelity Maintenance: Errors arising from imperfect gate operations and interaction with the surrounding $^{13}$C spin bath must be mitigated, potentially requiring Hahn echo sequences on the electron spin.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThis research highlights the critical dependence of scalable QIP on material quality and precise engineering integration. 6CCVD is uniquely positioned to supply and service the foundational materials required to replicate and advance this NV-based quantum protocol.
Applicable Materials
Section titled âApplicable MaterialsâTo replicate the ultra-long coherence times and high fidelity required, researchers must utilize the highest quality diamond substrates:
- Optical Grade SCD (Single Crystal Diamond): Needed to host coherent NV centers. 6CCVD provides SCD material with low defect density and high purity, essential for achieving the requisite low-strain environment ($\approx 1.2$ GHz) and maximizing nuclear spin coherence (T$_{2}$ > 1 minute).
- Precision [111] Orientation: 6CCVD offers custom-cut SCD wafers grown and polished precisely along the [111] direction, ensuring optimal alignment for NV fabrication and maintaining the necessary ground state degeneracy during the optical cycles.
Customization Potential
Section titled âCustomization PotentialâThe experimental scheme requires sophisticated control mechanisms and photonic integration (e.g., the high-efficiency photonic crystal cavity).
| Research Requirement | 6CCVD Solution | Technical Advantage |
|---|---|---|
| Photonic Integration & Low Scattering | Precision Polishing (Ra < 1 nm for SCD) | Ultra-smooth surfaces are crucial for minimizing scattering loss and ensuring high-efficiency coupling (up to 90% collection efficiency) into integrated optical elements or fibers. |
| Spin Control (MW/RF Gates) | Custom Metalization Services | We offer internal deposition of thin films (including Au, Pt, Ti, W, Cu) required for fabricating microwave antennas and radio-frequency transmission lines necessary to perform high-fidelity Hadamard and C-NOT gate operations on the electron and nuclear spins. |
| Scalable Device Footprint | Custom Dimensions (Wafers up to 125mm) | Supports the fabrication of large-scale integrated photonic circuits or chip-based cryogenic quantum systems, essential for translating single-NV research into scalable architectures. |
| Specific Layer Thickness | SCD Thickness Control (0.1 ”m - 500 ”m) | Provides material necessary for various integration depths, whether researchers require thin membranes for cavity fabrication or robust substrates for mechanical stability in cryogenic environments. |
Engineering Support
Section titled âEngineering SupportâNV-based quantum research relies heavily on optimizing defect incorporation and managing spin bath environment ($^{13}$C concentration).
- 6CCVDâs in-house PhD engineering team specializes in CVD growth parameter tuning. We can assist with material selection and post-growth processing to ensure target propertiesâsuch as controlled nitrogen doping and low strainâare optimized for similar quantum computing (QC) and quantum cryptography projects. We help clients maximize NV center creation yield and nuclear spin T$_{2}$ coherence times, mitigating the fidelity loss observed due to spin-bath effects.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
We present a scheme to generate entangled photons using nitrogen vacancy (NV) centers in diamond. We show how the long-lived nuclear spin in diamond can mediate entanglement between multiple photons, thereby increasing the length of the entangled photon string. With the proposed scheme one could generate both n-photon Greenberger-Horne-Zeilinger and cluster states. We present an experimental scheme realizing the same and estimate the rate of entanglement generation both in the presence and absence of a cavity. © 2015 American Physical Society.