Cooperatively enhanced dipole forces from artificial atoms in trapped nanodiamonds

At a Glance

Metadata	Details
Publication Date	2016-11-14
Journal	Nature Physics
Authors	Mathieu L. Juan, Carlo Bradac, Benjamin Besga, Mattias Johnsson, Gavin K. Brennen
Institutions	ARC Centre of Excellence for Engineered Quantum Systems, Macquarie University
Citations	51
Analysis	Full AI Review Included

6CCVD Technical Documentation: Cooperative Dipole Forces in NV Nanodiamonds

Executive Summary

This research paper successfully demonstrates the observation of cooperatively enhanced atomic dipole forces arising from high-density Nitrogen-Vacancy (NV) color centers embedded in optically trapped nanodiamonds (NDs). The findings provide a critical bridge between bulk optical trapping and single-atom quantum optics, enabling highly controlled nanoscale manipulation.

Core Achievement: Observation of a 10% modification in overall optical trapping stiffness near the NV Zero-Phonon Line (ZPL) (639 nm).
Quantum Mechanism Validation: The observed enhancement cannot be explained by independent NV centers; modeling requires the inclusion of collective (cooperative) quantum effects (Dicke model).
Magnitude of Enhancement: Collective effects enhance the predicted trap stiffness by an estimated factor of 50 compared to theoretical models assuming non-interacting NVs.
Material Foundation: The study relies on high-purity synthetic Type Ib diamond (300 ppm N) processed via proton irradiation and high-temperature vacuum annealing (700 °C) to create high concentrations ($\approx 9,500$ per ND) of stable NV^- centers.
Future Trajectory: The results directly motivate the use of defects with stronger dipole moments, such as Silicon-Vacancy (SiV) centers, to achieve a quantum optomechanics regime where the nanocrystal behaves as a “superatom,” crucial for strong single-photon coupling and room-temperature side-band cooling.
6CCVD Relevance: This work necessitates highly controlled diamond synthesis and post-processing, aligning perfectly with 6CCVD’s core capabilities in MPCVD growth, custom doping, and precision fabrication of quantum-grade diamond substrates.

Technical Specifications

Parameter	Value	Unit	Context
Nanodiamond Average Size	150 ± 23	nm	Measured via Dynamic Light Scattering/AFM
Source Diamond Nitrogen Concentration	300	ppm	Synthetic Type Ib powder
High NV Concentration Estimate	$\approx 9,500$	NVs/ND	Determined by fluorescence scaling
NV Formation Annealing Temperature	700	°C	Held for 2 hours in vacuum
Proton Irradiation Dose	$1 \times 10^9$	ions per cm²	Used for creating high NV density
NV ZPL Average Position ($\lambda_{ref}$)	639.08 ± 0.65	nm	Zero-Phonon Line (measured in NDs)
Trapping Laser Wavelength Range	629 - 648	nm	Used for resonant stiffness measurement
Re-pump Laser Wavelength	532	nm	Pulsed green laser (40 MHz)
Observed Trapping Stiffness Change	10	%	Relative modification around ZPL
Trapping Medium	Deionized water	None	Nanodiamonds suspended in microfluidic chamber
Objective Numerical Aperture (NA)	1.2	None	Used for focusing Gaussian Standing Wave (GSW) trap

Key Methodologies

The experiment required meticulous material preparation and a highly stable optical setup utilizing a Gaussian Standing Wave (GSW) trap to observe resonant dipole forces.

A. Nanodiamond Preparation (High NV Density)

Purification: Synthetic Type Ib diamond powder was purified by nitration in concentrated sulfuric and nitric acid (H₂SO₄-HNO₃) to remove excess sp² carbon phase.
Rinsing: Nanodiamonds were rinsed in deionized water.
Irradiation: Samples were subjected to a 3-MeV proton beam at a dose of $1 \times 10^9$ ions per cm² to create vacancies.
Annealing: Irradiated NDs were annealed in vacuum at 700 °C for 2 hours to promote the diffusion of nitrogen and form stable NV centers.
Characterization: Samples were characterized using combined confocal/AFM microscopy and spectroscopy to confirm NV- ZPL position and concentration ($\approx 9,500$ NVs per ND).

B. Optical Trapping and Stiffness Measurement

Trap Configuration: A GSW trap was created by focusing a Gaussian laser beam through a water-immersion objective (NA 1.2) onto a silver-coated mirror, housed within a static micro-fluidic chamber containing the NDs in deionized water.
Laser Sources: Five temperature-stabilized diode lasers were used, providing a constant off-resonant 660 nm trap (6 mW power) and a 532 nm re-pump laser (30 µW power) to maintain the NV^- charge state.
Resonant Measurement: Three additional diode lasers (operating between 633 nm and 642 nm, 4 mW power each) were tuned across the 639 nm NV ZPL.
Data Acquisition: A 50-second continuous time trace was segmented into 10-second intervals for measurement at different wavelengths. The position of the trapped particle was tracked using a quadrant photodiode (QPD).
Stiffness Extraction: Trap stiffness ($\kappa$) was obtained from the corner frequency ($f_c$) derived from a Lorentzian fit of the power spectral density (PSD) of the position signal. Measurements were normalized to a reference wavelength ($\lambda_{ref} = 639.13 \text{ nm}$) to eliminate dependence on ND size and drag coefficient.

6CCVD Solutions & Capabilities: Enabling Advanced Quantum Optomechanics

This groundbreaking research confirms that engineered diamond defect ensembles are viable platforms for large-scale, cooperative quantum dynamics. To replicate, optimize, or extend this work—particularly into the SiV ‘superatom’ regime suggested by the authors—requires diamond materials and engineering precision that only 6CCVD can provide.

Applicable Materials & Doping Optimization

Research Requirement	6CCVD Recommended Material	Technical Advantage
High Density NV/SiV Precursor	MPCVD Single Crystal Diamond (SCD) or Polycrystalline (PCD)	We control nitrogen incorporation (Type Ib precursor equivalent) or silicon incorporation during growth. Custom low-strain SCD is ideal for post-processing and creating narrow ZPL distributions.
SiV-Based “Superatom” Regime	SCD Substrates with Controlled Si-Doping	The paper notes SiV centers offer higher densities and stronger transition dipole moments. 6CCVD offers precise, in-situ silicon doping via silane precursor during MPCVD growth, delivering uniform high-quality SiV precursors in plate formats.
Electrode Integration/Sensing	Boron-Doped Diamond (BDD) / Metalized SCD	BDD provides conductive surfaces necessary for applying electric fields for Stark shift tuning or integration into nanodevices. We offer custom metalization stacks (Au, Pt, Ti) for electrodes directly on NV/SiV active layers.
Future Integrated Devices (Optomechanics)	Large, Ultra-Low Roughness SCD Wafers (up to 125mm)	While this study used ND powder, future devices will require polished bulk diamond for integrated optics. We guarantee SCD polishing to Ra < 1 nm and PCD polishing to Ra < 5 nm.

Customization Potential & Engineering Support

To transition this proof-of-concept into scalable quantum hardware, precise material control and micro-fabrication are essential.

Custom Dimensions: While the paper used NDs, 6CCVD provides quantum-grade diamond as plates or wafers up to 125 mm (PCD) and substrates up to 10 mm thick, suitable for integration into complex optical platforms (e.g., waveguides, fiber coupling).
Precision Fabrication: We offer custom laser cutting and shaping services necessary for defining optical features or packaging components required for GSW traps or micro-fluidic chambers referenced in the methodology.
Metalization Services: We provide internal thin-film metal deposition capabilities (Au, Pt, Pd, Ti, W, Cu). Replication of the GSW setup could benefit from custom patterned mirror surfaces or integrated electrodes on high-purity SCD.
Engineering Support: 6CCVD’s in-house PhD team specializes in defect engineering and MPCVD growth optimization. We can assist researchers and engineers in selecting the optimal diamond type (SCD or PCD), doping level, and post-growth processing parameters (like specific annealing recipes) needed to maximize NV/SiV density and minimize strain for similar Quantum Sensing and Nanomanipulation projects.
Global Logistics: We ensure reliable, secure global shipping (DDU default, DDP available) to minimize lead times for critical materials globally.

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/nphys3940