Toward the Realization of Single-Photon Sources for Radiometry Applications at Room Temperature

At a Glance

Metadata	Details
Publication Date	2023-01-01
Journal	IEEE Transactions on Instrumentation and Measurement
Authors	K.S. Hong, Hee‐Jin Lim, Dong-Hoon Lee, In‐Ho Bae, Kwang-Yong Jeong
Institutions	Korea Research Institute of Standards and Science, Saarland University
Citations	2
Analysis	Full AI Review Included

Executive Summary

This study characterizes room-temperature single photon emitters (SPEs) across three material platforms—silicon vacancy in diamond (vc-SiV), defects in GaN (df-GaN), and vacancy in hBN (vc-hBN)—to assess their suitability for quantum radiometry applications based on photon number metrology.

Core Objective: To evaluate photon number fluctuation (stability) and measurement repeatability for SPEs intended as standard light sources for photon flux metrology.
Stability vs. Brightness Trade-off: A fundamental trade-off was observed: vc-hBN offers high brightness (> 10⁶ cps) but suffers from slow blinking dynamics (long τ₂ ~ 1.0 µs), leading to high photon number fluctuation (v₂ ~ 1.3 x 10^-3).
High Stability Achieved: df-GaN demonstrated superior stability due to fast internal relaxation times (τ₂ ~ 53 ns). This resulted in low fluctuation noise (v₂ ~ 7.9 x 10^-5) and high repeatability.
Exceptional Repeatability: The highest repeatability error recorded for df-GaN was 30 ppm (parts per million) for photon count measurements, approaching the current standard repeatability of laser-based radiometry.
Radiometry Validation: A bright hBN emitter was used to successfully demonstrate repeatable radiant flux (S) measurements over a wide range (tens of femtowatts to one picowatt), validating the application of existing calibration parameters (DE(C)) derived from classical light sources to non-classical SPEs.
Collection Efficiency Limitation: The current setup achieved a low collection efficiency (< 6.4%), which flattened the photon number statistics toward a Poisson distribution, limiting the ability to exploit the advantages of sub-Poisson statistics necessary for advanced few-photon metrology.

Technical Specifications

Parameter	Value	Unit	Context
Operating Temperature	Room Temperature	°C	All SPE platforms tested.
Maximum Photon Count Rate (hBN)	> 1.0 x 10⁶	cps	Vacancy in hBN (vc-hBN), highest brightness observed.
Maximum Photon Count Rate (GaN)	< 3.0 x 10⁵	cps	Defects in GaN (df-GaN), limited by total internal reflection.
Zero-Time Correlation (df-GaN)	0.24 ± 0.14	(unitless)	Lowest g⁽²⁾(0) observed, confirming single photon emission.
Blinking/Relaxation Time (τ₂, df-GaN)	53 ± 5	ns	Fast relaxation, leading to high stability.
Blinking/Relaxation Time (τ₂, vc-hBN)	1.0 ± 0.1	µs	Slow relaxation, causing severe photon count fluctuations.
Photon Number Variance Coefficient (v₂, df-GaN)	7.9 x 10^-5	(unitless)	Low fluctuation noise, close to shot-noise limit at short Δt.
Photon Number Variance Coefficient (v₂, vc-hBN)	1.3 x 10^-3	(unitless)	High fluctuation noise (approximately 16x higher than GaN).
Radiant Flux Measurement Range	Few tens of fW to 1 pW	W	Demonstrated using bright hBN emitter (C ~ 2 x 10⁶ cps).
Highest Repeatability Error (df-GaN)	30	ppm	Achieved with M=20 repetitions for a qualified df-GaN emitter.
Theoretical Collection Efficiency	< 6.4	%	From sample-air interface to Single Mode Fiber (SMF) output.
SPAD Time Jitter Limit	0.3	ns	Instrumental resolution boundary for anti-bunching time (τ₁).
GaN Film Thickness	4	µm	GaN crystal grown on sapphire substrate.
SiV Nano-diamond Diameter	50-100	nm	Size of SiV nano-diamonds on iridium substrate.

Key Methodologies

The study utilized a specialized confocal microscopy setup integrated with a dual detection system to characterize SPEs and perform radiometry conversions.

Sample Preparation and Synthesis:
- vc-SiV: Silicon-free Chemical Vapor Deposition (CVD) process used to grow nano-diamonds (50-100 nm) on an iridium substrate, followed by silicon implantation to achieve high SiV purity.
- df-GaN: Commercial 4 µm GaN crystal grown on sapphire. Emitters are defects (charge-trapped dots) located at lattice dislocation intersections.
- vc-hBN: Commercial nano-flakes dispersed on oxidized silicon. Required special annealing treatment: 800 °C for 30 minutes in 1 Torr Argon (Ar) gas.
Optical and Collection Setup:
- Confocal microscopy was used to confine fluorescence signals from randomly distributed emitters.
- Single Mode Fibers (SMFs) were employed for photon collection, providing identical beam profiles for subsequent analysis modules.
- The overall collection efficiency was calculated to be less than 6.4%, limited by mode coupling efficiency of electrical dipole radiation to the SMF mode.
Dual Detection Radiometry Module:
- A custom module was constructed featuring two detection stages sharing the same single photon input via SMF:
  - Stage 1 (Photon Counting): Silicon Single Photon Avalanche Detector (SPAD) measured photon count rate (C, cps).
  - Stage 2 (Radiant Power): Photocurrent-generating photodiode measured radiant flux (S, W).
- This setup allowed for direct comparison and cross-checking of C and S measurements without external optical path loss corrections.
Photon Statistics Measurement:
- A Hanbury Brown-Twiss (HBT) interferometer was used to measure the normalized correlation g⁽²⁾(τ).
- The Time-Tag Correlation (TTC) method was used to derive reliable g⁽²⁾(0) values and fit the data to a model incorporating anti-bunching (τ₁) and meta-stable dark state trapping (τ₂).
Stability and Repeatability Assessment:
- Fluctuation: Photon number uncertainty (σ(N)/<N>) was measured from 10 s streaming acquisitions using 10 ms time bins (Δt). Variance (<ΔN²>) was modeled quadratically: <ΔN²> = <N> + v₂<N>².
- Repeatability: The average photon number (<N>) was measured repeatedly (M=5 to 20 shots), and repeatability error was calculated as σ(<N>)/√M.

Commercial Applications

The characterization of stable and bright single photon emitters at room temperature is crucial for advancing quantum technologies, particularly in metrology and communication.

Quantum Radiometry and Metrology:
- Establishing a new, counting-based standard for radiative flux, competing with traditional standards based on electrical current and temperature.
- Providing highly stable, traceable single photon sources for the calibration of high-sensitivity detectors (e.g., SPADs, superconducting nanowire detectors) used in low-light environments.
Quantum Communications and Information Processing:
- Developing reliable, room-temperature SPEs for encoding single data bits in encrypted communication systems.
- Utilizing stable emitters (like df-GaN) to improve the accuracy and reliability of photon-based quantum computing components.
Advanced Photonics and Material Engineering:
- Guiding the development of high-efficiency photon collection techniques (e.g., solid immersion lenses, meta-surfaces) to overcome total internal reflection limitations, especially in high refractive index materials like GaN.
- Informing material selection for applications requiring specific trade-offs between brightness (vc-hBN) and stability/low noise (df-GaN).
Quantum Sensing:
- Providing characterized sources for quantum sensing applications that rely on precise, low-uncertainty photon flux measurements.

View Original Abstract

A single photon source with high repeatability and low uncertainties is the key element for few-photon metrology based on photon numbers. While low photon number fluctuations and high repeatability are important figures for qualification as a standard light source, these characteristics are limited in single photon emitters by some malicious phenomena like blinking or internal relaxations to varying degrees in different materials. This study seeks to characterize photon number fluctuations and repeatability for radiometry applications at room temperature. For generality in this study, we collected photon statistics data with various single photon emitters of $g^{(2)}(0) < 1$ at low excitation power and room temperature in three material platforms: silicon vacancy in diamond, defects in GaN, and vacancy in hBN. We found common factors related with the relaxation times of the internal states that indirectly affect photon number stability. We observed a high stability of photon number with defects in GaN due to faster relaxations compared with vacancies in hBN, which on the other hand produced high rates ($> 10^6$) of photons per second. Finally, we demonstrate repeatable radiant flux measurements of a bright hBN single photon emitter for a wide radiant flux range from a few tens of femtowatts to one picowatt.

Tech Support

Original Source

DOI: https://doi.org/10.1109/tim.2023.3289550

References

2022 - Single photon sources for radiometry applications at room temperature