Direct experimental observation of nonclassicality in ensembles of single-photon emitters

At a Glance

Metadata	Details
Publication Date	2017-11-27
Journal	Physical review. B./Physical review. B
Authors	Ekaterina Moreva, P. Traina, J. Forneris, Ivo Pietro Degiovanni, S. Ditalia Tchernij
Institutions	Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Istituto Nazionale di Ricerca Metrologica
Citations	15
Analysis	Full AI Review Included

Technical Documentation: Nonclassicality Detection in NV Center Ensembles

This documentation analyzes the experimental demonstration of a robust nonclassicality criterion ($\theta^{(N)}$) using Nitrogen-Vacancy (NV) centers in nanodiamond, highlighting the direct applicability and necessary material requirements met by 6CCVD’s MPCVD diamond capabilities.

Executive Summary

The attached research paper validates a novel nonclassicality criterion ($\theta^{(N)}$) highly effective for characterizing single-photon emitters (SPSs), particularly those operating in large ensembles or environments dominated by background noise.

Core Achievement: Direct experimental validation of the $\theta^{(N)}$ function, which proves independent of Poissonian noise (background light contribution) at any order $N$.
Noise Robustness: The $\theta^{(N)}$ function is shown to be significantly more robust than the traditional second-order autocorrelation function, $g^{(2)}(0)$, allowing nonclassical signatures to be detected even when emitters are drowned in 25,000 counts/s of noise.
Material System: Nonclassical emission was observed from clusters of Nitrogen-Vacancy (NV) centers fabricated in synthetic High-Pressure High-Temperature (HPHT) nanodiamond powders.
NV Fabrication Method: Vacancies were introduced via 2 MeV $H^{+}$ ion irradiation ($5 \times 10^{12}$ protons/cm² fluence), followed by $800^{\circ}$C thermal annealing in a controlled $N_{2}$ atmosphere.
Key Result (Single Emitter Equivalent): A single NV emitter (Item-1) showed $g^{(2)}(0) = 0.407 \pm 0.012$ without noise, confirming clear sub-Poissonian light emission necessary for quantum technologies.
Experimental Configuration: Single-photon-sensitive confocal microscopy coupled to a 4-detector tree configuration for simultaneous measurement of six two-fold coincidences.

Technical Specifications

Extracted quantitative parameters and experimental results from the research.

Parameter	Value	Unit	Context
Nanodiamond Type	HPHT Synthetic, Ib	N/A	Source material for NV centers
Nanodiamond Size Distribution	10 to 250	nm	Powder size range
Substitutional N Concentration	10 to 100	ppm	Nominal range for Ib type
Irradiation Energy	2	MeV	$H^{+}$ beam energy
Irradiation Fluence	$5 \times 10^{12}$	protons/cm²	Optimized for single NV center creation
Thermal Annealing Temperature	800	°C	1 hour duration, 800 mbar $N_{2}$ atmosphere
Excitation Wavelength	532	nm	Solid-state laser source
Excitation Repetition Rate	5	MHz	Pulsed laser regime
Excitation Pulse Width (FWHM)	50	ps	Used for signal generation
Poisson Noise Wavelength	685	nm	Simulating background light
Detection Spectral Window	640 - 800	nm	Fluorescence collection range
Coincidence Temporal Window	40	ns	Set compatible with NV center lifetime (approx. 25 ns)
Max Poisson Noise Level	25,000	counts/s	Highest level of simulated noise tested
Item-1 $g^{(2)}(0)$ (No Noise)	$0.407 \pm 0.012$	N/A	Substantial anti-bunching (nonclassicality)
Classical $g^{(2)}(0)$ (Reflected Light)	$0.996 \pm 0.005$	N/A	Near-unity value, confirming coherent classical source

Key Methodologies

A step-by-step summary of the NV center material preparation and the optical measurement setup used to validate the nonclassicality criterion.

A. Nanodiamond Preparation and NV Center Creation

Source Material: Synthetic HPHT Ib-type nanodiamond powders (10-250 nm, 10-100 ppm substitutional N) were used as the base material.
Initial Cleaning: Powders underwent a 72-hour acid bath ($H_{2}SO_{4} : HNO_{3} = 9:1$ solution at $100^{\circ}$C) to remove graphite and organic surface contamination.
Irradiation: Samples were irradiated with a 2 MeV $H^{+}$ beam at $5 \times 10^{12}$ protons/cm² fluence to generate vacancies necessary for NV formation.
Thermal Annealing: Irradiated powders were annealed at $800^{\circ}$C for 1 hour under 800 mbar of $N_{2}$ atmosphere to mobilize vacancies and form NV centers.
Final Purification: A secondary cleaning step involved a 30-minute sonic bath in $H_{2}SO_{4}$, followed by Piranha solution ($H_{2}SO_{4}: H_{2}O_{2} = 3:1$) to remove metallic oxides and residuals.
Substrate Mounting: The purified nanodiamonds were dispersed onto cover-slip glass substrates for optical investigation.

B. Confocal Microscopy and Detection Setup

Excitation: A pulsed 532 nm solid-state laser (5 MHz repetition rate, 50 ps FWHM) provided excitation light.
Focusing and Collection: Excitation light was coupled into a single-mode fiber and focused via a high numerical aperture (NA = 1.3) oil immersion objective.
Filtering: Fluorescence (640-800 nm) was collected, passed through a long-pass filter (570 nm) and dichroic mirror for pump rejection (> $10^{12}$ attenuation).
Detection Tree: The filtered signal was coupled into a 50 µm multimode optical fiber leading to a detector-tree configuration realized by two cascaded 50:50 beam-splitters.
Detectors: Four Single Photon Avalanche Photo-diodes (SPADs) operating in Geiger mode were used, allowing for the simultaneous measurement of six independent two-fold coincidences ($g^{(2)}$ and $\theta^{(2)}$ estimations).
Noise Simulation: Poissonian noise was simulated using a separate pulsed laser (685 nm) electronically synchronized with the main excitation laser and coupled directly into the detection pinhole.

6CCVD Solutions & Capabilities

This research demonstrates the increasing sophistication of quantum photonic applications requiring diamond materials with extreme control over purity, geometry, and surface quality. 6CCVD provides the specialized MPCVD diamond necessary to transition this research from nanodiamond powders to scalable, integrated quantum devices.

Applicable Materials

To replicate or extend this research into high-fidelity, integrated quantum platforms, researchers require single-crystal diamond (SCD) that offers superior structural quality, minimal strain, and controlled defect creation compared to nanodiamond powders.

Application Requirement	Recommended 6CCVD Material	Material Specification
High-Fidelity NV Emitter Arrays	Low-Strain Optical Grade SCD	Required for low spectral diffusion and optimized NV coherence times ($T_{2}$). Provides a large, uniform substrate surface.
Scalable Integrated Quantum Devices	High-Purity Polycrystalline Diamond (PCD)	Cost-effective option for large-area fabrication (up to 125mm) requiring excellent thermal management and controlled thickness.
Creation of NV Precursors	High-Purity SCD/PCD (Low N)	Necessary for precise, post-synthesis doping (e.g., nitrogen implantation) to control NV density and location, replacing the less controllable native N found in HPHT Ib materials.

Customization Potential

The integration of quantum emitters often necessitates complex geometries, precise thickness control, and hybrid material integration. 6CCVD’s comprehensive in-house capabilities directly support these advanced engineering demands.

Research Need	6CCVD Capability	Technical Advantage
Integrated Photonic Structures	Custom Thickness & Dimensions	Offers SCD and PCD plates from 0.1 µm to 500 µm, allowing precise slab waveguide construction and critical thickness tuning for photonic applications.
On-Chip Control/Sensing	In-House Custom Metalization	We apply and pattern high-quality metal stacks (Au, Pt, Pd, Ti, W, Cu) for microwave control lines, electrodes, or contact pads essential for NV spin manipulation and charge state control.
High-NA Optical Interface	Ultra-Smooth Polishing	Our capability achieves surface roughness Ra < 1 nm (SCD) and Ra < 5 nm (Inch-size PCD), minimizing scattering losses critical for coupling light out of high-NA solid immersion lenses (SILs).
Complex Geometries	Precision Laser Cutting	Ability to dice, mill, or cut custom shapes (e.g., custom SIL structures, specific chip sizes) to perfectly match detector arrays and cryostat stages.

Engineering Support

6CCVD’s in-house PhD team provides authoritative support, ensuring researchers select the optimal MPCVD material parameters for applications requiring highly stable and controllable quantum emitters. We assist with:

Material Selection: Determining the appropriate substrate (SCD vs. PCD) based on required purity, size, and budget for single-photon source or quantum sensing projects.
Surface Preparation: Consulting on polishing requirements and final surface preparation necessary for optimal NV creation (via implantation) and high-efficiency optical coupling.
Global Logistics: Offering reliable global shipping, DDU default with DDP available, ensuring materials arrive safely and quickly worldwide.

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

View Original Abstract

In this work we experimentally demonstrate a recently proposed criterion addressed to detect nonclassical behavior in the fluorescence emission of ensembles of single-photon emitters. In particular, we apply the method to study clusters of nitrogen-vacancy centers in diamond characterized with single-photon-sensitive confocal microscopy. Theoretical considerations on the behavior of the parameter at any arbitrary order in the presence of Poissonian noise are presented and, finally, the opportunity of detecting manifold coincidences is discussed.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevb.96.195209