Resonant Excitation and Purcell Enhancement of Coherent Nitrogen-Vacancy Centers Coupled to a Fabry-Perot Microcavity

At a Glance

Metadata	Details
Publication Date	2021-02-19
Journal	Physical Review Applied
Authors	M Ruf, M.J. Weaver, S. B. van Dam, Hanson R, M Ruf
Institutions	QuTech, Delft University of Technology
Citations	66
Analysis	Full AI Review Included

Technical Analysis and Documentation: Purcell Enhanced NV Centers in MPCVD Diamond

This document analyzes the research paper “Resonant Excitation and Purcell Enhancement of Coherent Nitrogen-Vacancy Centers Coupled to a Fabry-Pérot Micro-Cavity” and outlines how 6CCVD’s specialized MPCVD diamond materials and fabrication services can directly support and advance this critical quantum networking research.

Executive Summary

The research successfully demonstrates the resonant operation and Purcell enhancement of coherent Nitrogen-Vacancy (NV) centers embedded in thin diamond membranes coupled to a fiber-based micro-cavity, a crucial step toward scalable quantum networks.

Core Achievement: Demonstrated resonant optical addressing and Purcell-enhanced coherent photon emission from individual NV centers in a closed-cycle cryostat (T ~ 4 K).
Material Requirement: Utilized thin (~ 5.8 µm) electron-irradiated and annealed diamond membranes to maintain optical coherence.
Performance Metrics: Achieved measured Purcell factors (F^ZPL) up to 4, consistent with theoretical models, and spectrally resolved NV center transitions with linewidths as narrow as (190 ± 9) MHz.
Key Limitation Identified: System performance is currently limited by low excitation probability, diffraction losses (limiting finesse to 1000-2500), and significant cavity length fluctuations due to cryostat vibrations (0.1-0.2 nm RMS).
Future Projection: Modeling predicts that combining existing achievements (spin pumping, 20x vibration reduction, higher finesse) can increase the collected ZPL photons per excitation pulse to ~ 10%.
6CCVD Value Proposition: 6CCVD provides the high-purity Single Crystal Diamond (SCD) substrates, precise thickness control (down to 0.1 µm), and ultra-low roughness polishing (Ra < 1 nm) necessary to overcome current finesse and vibration limitations, enabling the projected 10% efficiency goal.

Technical Specifications

The following hard data points were extracted from the experimental results and theoretical projections:

Parameter	Value	Unit	Context
Operating Temperature	~ 4	K	Closed-cycle cryostat operation
Diamond Membrane Thickness	~ 5.8	µm	Etched down in the cavity region
NV Center Lifetime (Off-Resonant, $\tau_0$)	11.8 ± 0.2	ns	Consistent with bulk diamond
NV Center Lifetime (Purcell Enhanced)	9.77 ± 0.08	ns	Measured under low vibration conditions
Measured Purcell Factor (F^ZPL)	Up to 4	N/A	Enhancement of ZPL emission
Estimated Maximum Purcell Factor	~ 7	N/A	Theoretical maximum for current cavity design
Cavity Finesse (Design)	6200	N/A	Target value
Cavity Finesse (Operating Range)	1000 - 2500	N/A	Limited by diffraction/clipping losses
NV Center Linewidth (Centered)	190 ± 9	MHz	Spectral diffusion limited coherence
Cavity Vibration (RMS)	0.1 - 0.2	nm	Root mean squared amplitude
Projected ZPL Collection Efficiency	~ 10	%	Achievable with proposed improvements

Key Methodologies

The experiment relied on advanced diamond material processing and precise cryogenic optical control:

Diamond Preparation: High-purity diamond was subjected to electron irradiation and annealing to create optically coherent NV centers.
Membrane Fabrication: The diamond was etched down to a final thickness of ~ 5.8 µm to form a thin membrane suitable for cavity integration, a process critical for preserving NV center optical coherence.
Fabry-Pérot Cavity Construction: An open, fiber-based micro-cavity was formed using a flat, super-polished mirror bonded to the diamond membrane and a laser-ablated fiber mirror.
Cryogenic Operation and Tuning: The system was housed in a closed-cycle cryostat (T ~ 4 K) and utilized a piezo stage for in situ tuning of the cavity length, compensating for thermal and mechanical drifts.
Stabilization Protocol: A ~ 637 nm laser was used for frequency stabilization, serving as a reference for locking the cavity length during measurement sequences.
Resonant Excitation and Detection: Pulsed green light (~ 532 nm) was used for state initialization, followed by short resonant red pulses (~ 637 nm) to excite the NV center. Detection occurred simultaneously in the Zero-Phonon Line (ZPL) and Phonon-Sideband (PSB) paths.
Vibration Analysis: Cavity transmission measurements were synchronized with the cryostat coldhead cycle to quantify the root mean squared (RMS) vibration amplitude (0.1-0.2 nm) and its direct influence on NV center lifetime and counts.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced MPCVD diamond materials and custom fabrication required to replicate this research and achieve the projected 10% ZPL collection efficiency.

Applicable Materials

To replicate and extend this research, the highest quality SCD material is essential, particularly for membrane fabrication and achieving ultra-low surface roughness.

Optical Grade Single Crystal Diamond (SCD): Required for the thin, high-coherence membranes. 6CCVD offers high-purity SCD wafers suitable for electron irradiation and annealing processes.
- Thickness Control: We provide SCD plates with custom thicknesses ranging from 0.1 µm up to 500 µm, allowing researchers to precisely target the required ~ 5.8 µm thickness or explore thinner membranes for enhanced Purcell coupling.
Polishing Excellence: The paper highlights that diffraction losses limit the current finesse. Ultra-smooth surfaces are critical to minimize scattering.
- Polishing Specification: 6CCVD guarantees Ra < 1 nm surface roughness on SCD, significantly reducing scattering losses and enabling the high finesse (up to 11000) required for future systems.

Customization Potential

The experimental setup relies heavily on precise geometry and specialized coatings, areas where 6CCVD offers direct, in-house support.

Research Requirement	6CCVD Customization Capability	Technical Advantage
Precise Membrane Geometry	Custom laser cutting and etching services.	Ensures precise alignment and optimal overlap with the Gaussian cavity mode.
High Finesse Mirror Coatings	Internal metalization capability (Au, Pt, Pd, Ti, W, Cu).	Allows researchers to integrate high-reflectivity coatings directly onto the diamond surface, simplifying the cavity structure and potentially reducing vibration sensitivity.
Vibration Reduction	Provision of ultra-flat, highly polished substrates (Ra < 1 nm).	Minimizes surface scattering losses, which become critical when targeting the 20x vibration reduction required to reach 10% ZPL efficiency.
Large-Scale Integration	Plates/wafers up to 125 mm (PCD) and large SCD substrates (up to 10 mm thick).	Supports scaling up the quantum network nodes beyond single-site experiments.

Engineering Support

6CCVD’s in-house team of PhD material scientists and engineers specializes in optimizing diamond properties for quantum applications.

Material Selection for Quantum Projects: Our experts can assist researchers in selecting the optimal SCD purity, orientation, and post-growth processing parameters (e.g., surface termination, irradiation dose) necessary for similar NV-center quantum networking projects.
Global Logistics: We offer reliable Global Shipping (DDU default, DDP available), ensuring timely delivery of sensitive materials worldwide, supporting international collaborations.

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

View Original Abstract

<p>The nitrogen-vacancy (N-V) center in diamond has been established as a prime building block for quantum networks. However, scaling beyond a few network nodes is currently limited by low spin-photon entanglement rates, resulting from the N-V center’s low probability of coherent photon emission and collection. Integration into a cavity can boost both values via the Purcell effect, but poor optical coherence of near-surface N-V centers has so far prevented their resonant optical control, as would be required for entanglement generation. Here, we overcome this challenge, and demonstrate resonant addressing of individual, fiber-cavity-coupled N-V centers, and collection of their Purcell-enhanced coherent photon emission. Utilizing off-resonant and resonant addressing protocols, we extract an enhancement of the zero-phonon line emission by a factor of up to 4, consistent with a detailed theoretical model. This model predicts that the probability of coherent photon detection per optical excitation can be increased to 10% for realistic parameters - an improvement over state-of-the art solid immersion lens collection systems by 2 orders of magnitude. The resonant operation of an improved optical interface for single coherent quantum emitters in a closed-cycle cryogenic system at T∼4 K is an important result towards extensive quantum networks with long coherence.</p>

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.15.024049