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6CCVD Research

Verification of single-photon path entanglement using a nitrogen vacancy center

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2024-11-13
JournalApplied Optics
AuthorsAlice I. Smith, Christine Steenkamp, Mark Tame
InstitutionsStellenbosch University, National Institute for Theoretical Physics
Citations2
AnalysisFull AI Review Included

Technical Documentation & Analysis: Single-Photon Path Entanglement via NV Centers

Section titled “Technical Documentation & Analysis: Single-Photon Path Entanglement via NV Centers”

This document analyzes the research detailing the generation and verification of single-photon path entanglement using a neutral Nitrogen Vacancy (NVÂș) center in a nanodiamond. The findings confirm the viability of solid-state emitters for complex photonic quantum information protocols, validating the use of continuous-wave (CW) excitation and a time-window method.

6CCVD is uniquely positioned to supply the high-purity, custom-engineered MPCVD diamond substrates required to scale this research from nanodiamonds on coverslips to integrated, high-efficiency quantum photonic circuits.

Executive Summary

Section titled “Executive Summary”
  • Core Achievement: Successful experimental generation and verification of bipartite single-photon path-entangled states using a solid-state Nitrogen Vacancy (NVÂș) center emitter.
  • Source Characterization: The emitter was identified as an isolated NVÂș center in a nanodiamond, confirmed by a low second-order correlation value (g(2)(0) = 0.173 ± 0.039), operating firmly in the single-photon regime.
  • Methodology Innovation: Path entanglement was achieved using a stable, room-temperature NVÂș center excited by a Continuous-Wave (CW) 532 nm laser, utilizing a novel ‘time-window’ state generation method, simplifying previous pulsed-laser techniques.
  • High Coherence: The analysis Mach-Zehnder (MZ) interferometer demonstrated high self-interference visibility (V = 0.9329 ± 0.0069), indicating excellent coherence across the NV emission bandwidth.
  • Entanglement Quantification: The normalized concurrence (CN) reached a maximum of 0.44 ± 0.07 at a 2 ns state-generation window, verifying the presence of entanglement.
  • Scaling Potential: The research explicitly points toward future improvements requiring high-efficiency collection optics (Solid Immersion Lenses, cavities) and on-chip integration, areas where 6CCVD’s SCD substrates are essential.

Technical Specifications

Section titled “Technical Specifications”

The following hard data points were extracted from the experimental characterization and entanglement verification stages:

ParameterValueUnitContext
NV Center TypeNVÂș (Neutral)N/AIdentified via ZPL and spectrum analysis.
Zero-Phonon Line (ZPL)575nmCharacteristic wavelength for NVÂș.
Peak Emission Wavelength~655nmMeasured at room temperature.
Excitation Wavelength532nmContinuous Wave (CW) laser source.
Pump Power~100”WUsed for NV center excitation.
Nanodiamond Diameter (Avg)40nmMaterial used for single-photon source.
Single-Photon Purity (g(2)(0))0.173 ± 0.039N/AConfirms operation in the single-photon regime (< 0.5).
Coincidence Time Window (w)1nsUsed for g(2) measurement.
Mean Visibility (V)0.9329 ± 0.0069N/AQuantifies single-photon self-interference coherence.
Maximum Normalized Concurrence (CN)0.44 ± 0.07N/AAchieved at the minimum 2 ns state-generation window.
Minimum State-Generation Window (ÎŽt)2nsLimited by count rates and setup stability.
Detection Efficiency (ηD)0.0402 ± 0.0069N/ALumped detection efficiency measurement.

Key Methodologies

Section titled “Key Methodologies”

The experiment utilized a laser-scanning confocal microscope setup combined with specialized optical components for state generation and analysis.

  1. Source Preparation: Nanodiamonds (40 nm average diameter, containing 1-4 NV centers) were diluted and spin-coated onto a high-precision #1.5 coverslip.
  2. Excitation & Location (Section A): A 532 nm CW laser (~100 ”W) was used for excitation. A 2D Galvo mirror system performed a fluorescence scan to locate an isolated NV center (identified by high intensity relative to background).
  3. Source Characterization (Section B):
    • Fluorescence scan (Box 1).
    • Second-order correlation (g(2)) measurement (Box 2) using two SPADs and a time-tagging unit (PicoQuant TimeHarp) to confirm single-photon emission (g(2)(0) < 0.5).
    • Fluorescence decay lifetime measurement (Box 3) using a pulsed 532 nm laser (23.8 MHz pulse rate).
    • Spectrum analysis (Box 4) confirmed the NVÂș state (ZPL at 575 nm, peak at ~655 nm).
  4. Entanglement Generation (Section C): The single photon was sent through a non-polarizing beamsplitter (NPBS) and Neutral Density (ND) filter to balance transmission and reflection, generating the path-entangled state |Κ>.
  5. Analysis & Verification (Section D): The setup completed a Mach-Zehnder interferometer configuration.
    • A Half-Wave Plate (HWP) and Polarizing Beamsplitter (PBS) were used for path-to-polarization tagging.
    • Visibility (V) was measured by rotating a motorized HWP to induce interference.
    • Population measurements (p0, p1, p2) were taken using the novel ‘state-generation window’ (ÎŽt) method to calculate the degree of contamination (yc).
    • Normalized concurrence (CN) was calculated from V and yc to verify entanglement.

6CCVD Solutions & Capabilities

Section titled “6CCVD Solutions & Capabilities”

The research successfully demonstrates a fundamental quantum resource using a solid-state emitter. However, the low overall detection efficiency (ηD ~ 4%) and the need to operate at extremely short time windows (2 ns) highlight the limitations of the current setup (nanodiamonds on a coverslip).

6CCVD provides the high-performance MPCVD diamond materials and engineering services necessary to transition this research to high-efficiency, integrated quantum devices.

Applicable Materials for Replication and Scaling

Section titled “Applicable Materials for Replication and Scaling”

To maximize collection efficiency, reduce contamination, and enable on-chip integration (as suggested by references [54], [55], [56], and [58]), the following 6CCVD materials are required:

6CCVD MaterialSpecificationApplication in NV Research
Optical Grade SCDUltra-high purity, low nitrogen content (< 1 ppb).Ideal substrate for deterministic NV creation (e.g., ion implantation) and subsequent fabrication of high-coherence, low-loss photonic structures.
Thick SCD SubstratesThicknesses up to 10 mm. Polishing: Ra < 1 nm.Essential for fabricating high-efficiency Solid Immersion Lenses (SILs) or micro-cavities directly onto the diamond surface to boost collection efficiency (currently a major limitation).
Polycrystalline Diamond (PCD)Plates up to 125 mm diameter.Suitable for large-area quantum sensor arrays or heat spreading in high-power excitation setups.
Boron-Doped Diamond (BDD)Custom doping levels (p-type).Required for integrated electrical control, gate electrodes, or creating p-n junctions for charge state control of the NV center (NVÂș vs NV-).

Customization Potential for Integrated Quantum Circuits

Section titled “Customization Potential for Integrated Quantum Circuits”

6CCVD’s in-house capabilities directly address the engineering challenges required to advance this path entanglement research:

  • Custom Dimensions and Thickness: We supply SCD wafers up to 10 mm thick, enabling the fabrication of deep, high-aspect-ratio structures (like SILs or photonic crystal cavities) necessary to increase the photon collection efficiency (ηD).
  • Ultra-Low Surface Roughness: Our SCD polishing achieves Ra < 1 nm. This is critical for minimizing scattering losses in integrated photonic waveguides and ensuring high visibility (V) is maintained in on-chip interferometers.
  • Precision Metalization Services: While the current paper did not require it, future integrated quantum circuits often require electrical contacts for NV charge state control or microwave delivery. 6CCVD offers custom metalization stacks including Ti/Pt/Au, W, and Cu, patterned to customer specifications.
  • Laser Cutting and Shaping: We provide precision laser cutting services for creating custom geometries, including small chips or specific shapes required for mounting in cryogenic or vacuum systems, or for defining the boundaries of on-chip interferometers.

Engineering Support

Section titled “Engineering Support”

6CCVD’s in-house PhD team specializes in MPCVD growth optimization and material science for quantum applications. We offer consultation services to assist researchers in:

  • Selecting the optimal diamond grade (SCD purity, PCD grain size) to minimize background fluorescence and maximize NV coherence time.
  • Designing substrate geometries for maximum photon extraction efficiency (e.g., optimizing SIL curvature or cavity dimensions).
  • Developing metalization recipes for robust, low-resistance electrical contacts on BDD or SCD surfaces for advanced NV control.

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

View Original Abstract

Path entanglement is an essential resource for photonic quantum information processing, including in quantum computing, quantum communication, and quantum sensing. In this work, we experimentally study the generation and verification of bipartite path-entangled states using single photons produced by a nitrogen vacancy center within a nanodiamond. We perform a range of measurements to characterize the photons being generated and verify the presence of path entanglement. The experiment is performed using continuous-wave laser excitation and a novel, to our knowledge, state-generation ‘time-window’ method. This approach to path entanglement verification is different from previous work as it does not make use of a pulsed laser excitation source.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2010 - Quantum Computation and Quantum Information

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