Single Photon Randomness based on a Defect Center in Diamond

At a Glance

Metadata	Details
Publication Date	2019-12-05
Journal	Scientific Reports
Authors	Xing Chen, Johannes Greiner, Jörg Wrachtrup, Ilja Gerhardt
Institutions	University of Stuttgart, Center for Integrated Quantum Science and Technology
Citations	10
Analysis	Full AI Review Included

Single Photon Quantum Randomness (QRNG) in NV-Diamond

Technical Analysis and Material Sourcing Solutions by 6CCVD

Executive Summary

This research successfully demonstrates a stable, single-photon Quantum Random Number Generator (QRNG) utilizing a Nitrogen-Vacancy (NV) defect center in diamond, operating continuously under ambient conditions.

Core Technology: Single NV-center in high-purity Single Crystal Diamond (SCD) acting as a stable, non-classical single-photon source (SPS).
Quantum Certification: Non-classical light validated via robust anti-bunching measurements, achieving a critical zero-delay auto-correlation $g^{\text{2}}(0) = 0.15$.
Operational Stability: The system demonstrated true 24/7 operation over a 7-day period (608,125 seconds) under ambient laboratory conditions, enabled by continuous refocusing procedures.
Data Volume: Acquired a massive raw data stream totaling 832 GiB (over 55 billion raw bits).
Randomness Output: Employing a conservative third-model extraction strategy (Conditional Min-Entropy, $H_{\infty}$), the generation speed of unbiased quantum random bits achieved 34.37 bits/second.
Key Efficiency Element: High photon collection efficiency facilitated by a custom-fabricated Solid-Immersion Lens (SIL) around the NV center.
Material Requirement: The system relies critically on ultra-pure, low-strain SCD to ensure the superior stability and optical properties of the NV-center.

Technical Specifications

Parameter	Value	Unit	Context
Material Source	Single NV-Center in Diamond	-	Single Photon Source (SPS) for QRNG.
Excitation Wavelength	532	nm	CW Laser (Stabilized).
Optimal Excitation Power	26	µW	Used for primary data analysis.
Anti-bunching Quality	0.15	-	Zero-delay auto-correlation $g^{\text{2}}(0)$. Lower is better for quantum certification.
Raw Data Acquisition Time	608,125	seconds	Continuous 7-day operation duration.
Raw Bit Count (Total)	5.579 x 10¹⁰	bits	Total raw data collected (832 GiB).
Average Detection Rate	91.7	kcps	Including periodic refocusing periods.
Detection Timing Resolution	100	ps	Time-tagger (FPGA) resolution.
Start-Stop Resolution ($\tau_{\text{rs}}$)	500	ps	Timing resolution for anti-bunching measurement.
Detector A Dead Time ($\tau_{\text{dead}}$)	43.5	ns	Technical parameter influencing click rate models.
Detector B Dead Time ($\tau_{\text{dead}}$)	42.9	ns	Technical parameter influencing click rate models.
Detection Efficiency ($\eta_{A}$, $\eta_{B}$)	60	%	Estimated efficiency for both APDs.
Conditional Min-Entropy ($H_{\infty}$) (Model 2)	0.5168	bits/raw bit	Conservative bound for single-photon events.
Final QRNG Output Speed (Model 3)	34.37	bits/second	Conservative quantum randomness generation speed (11.5$\sigma$ error bound).

Key Methodologies

The experiment implemented a Quantum Random Number Generator based on detecting single photons from a single NV-center in a Hanbury Brown and Twiss (HBT) configuration.

Diamond Preparation: The starting material was a millimeter-sized diamond hosting NV-centers at natural abundance.
NV Localization & Enhancement: A single NV-center was localized using confocal microscopy. To enhance collection efficiency, a Solid-Immersion Lens (SIL) was fabricated around the center using Focused Ion Beam (FIB) milling.
Excitation: A continuous wave (CW) 532 nm laser was used for optical excitation. Laser intensity was stabilized using a PID-controller and an acousto-optical modulator (AOM) to suppress power fluctuations.
Detection Setup: Emitted fluorescence was collected, filtered (640 nm long-pass filter), and spatially separated using a symmetric non-polarizing beam splitter (BS: T=0.39, R=0.61).
Single Photon Detection: Photons in the two output paths were recorded by two Avalanche Photo Diodes (APDs). The two outputs (A and B) were defined as raw bit ‘0’ and ‘1’.
Time-Tagging and Data Acquisition: All detection events were recorded as time-tags using a commercial FPGA-based time-tagger with 100 ps resolution.
Real-Time Stability Control: To counter mechanical and thermal drift, the NV-center position was repeatedly re-centered via medial and lateral scanning every $\Delta t$ = 8 minutes.
Quantum Certification: Anti-bunching measurements ($g^{\text{2}}(\tau)$) were continuously monitored to confirm the non-classicality and single-photon nature of the source ($g^{\text{2}}(0) = 0.15$).
Randomness Extraction: Three increasingly conservative entropy estimation models were applied, culminating in Model 3 which limits randomness to only anti-correlated start-stop events below the classical limit line ($g^{\text{2}}(\tau) \le 1.0$) to certify against external adversaries (“Eve”).

6CCVD Solutions & Capabilities

The successful replication and future scaling of this high-stability, certified QRNG fundamentally depends on the quality and engineering precision of the diamond substrate. 6CCVD is an expert technical partner specializing in the necessary MPCVD materials and processing services.

Applicable Materials

To achieve the highly localized, stable single photon emission required for this research, the following 6CCVD material is essential:

Optical Grade Single Crystal Diamond (SCD):
- Requirement Match: SCD is mandatory for hosting isolated NV centers with superior stability and optical properties. High-purity, low-strain material is critical to minimize background noise (which impacts the $g^{\text{2}}(0)$ value) and maximize the spin coherence time of the NV center.
- 6CCVD Advantage: We supply high-quality, optical-grade SCD wafers, ensuring low defect density and precise material control necessary for reliable NV creation (e.g., via ion implantation or during growth).
- Alternative: For research focusing on ensemble QRNG, High-Quality Polycrystalline Diamond (PCD) wafers up to 125mm may be considered for large-area industrial scalability, though SCD remains preferred for certification experiments utilizing deterministic single emitters.

Customization Potential

The experimental setup relied on a microscopically engineered feature: the Solid-Immersion Lens (SIL). 6CCVD offers the necessary fabrication precision for scaling and optimizing these components.

Custom Dimensions: The experiment used a mm-sized diamond. 6CCVD supplies SCD plates and wafers in custom dimensions and shapes, allowing for tailored sizes necessary for specific cryostats or microscopy stages.
Ultra-Polishing and Surface Preparation: NV-centers close to the surface, especially when covered by a custom SIL structure, require extremely low surface roughness to minimize scattering and non-radiative decay.
- Capability Match: 6CCVD guarantees ultra-low roughness ($\text{Ra } < 1\text{nm}$) on SCD, essential for maintaining high photon collection efficiency and the long-term stability crucial for 24/7 operation.
Microstructure Fabrication Support (SIL): While FIB was used in the paper, 6CCVD can assist researchers needing high-precision laser cutting and shaping of diamond plates required for advanced optical components, including pre-cursors for custom SIL fabrication.

Engineering Support

The complexity of QRNG requires tight integration between material science (defect engineering) and quantum optics (entropy analysis).

Material Selection and QA/QC: 6CCVD’s in-house PhD engineering team can assist clients with material selection (e.g., optimizing nitrogen concentration or post-growth processing) to achieve targeted NV center densities and optical performance metrics, directly impacting the final $g^{\text{2}}(0)$ value and background fraction.
Stability and Performance: We provide guidance on how substrate parameters (purity, strain) influence the long-term stability and non-classicality of the NV-source, aiding in replicating or extending this research into certificable Quantum Random Number Generation projects.

Call to Action

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

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41598-019-54594-0