Searching for an exotic spin-dependent interaction with a single electron-spin quantum sensor

At a Glance

Metadata	Details
Publication Date	2018-02-15
Journal	Nature Communications
Authors	Xing Rong, Mengqi Wang, Jianpei Geng, Xi Qin, Maosen Guo
Institutions	Hefei National Center for Physical Sciences at Nanoscale, University of Science and Technology of China
Citations	72
Analysis	Full AI Review Included

Technical Documentation: Quantum Sensing of Exotic Spin-Dependent Interactions

Document Title: Ultra-Sensitive Search for Exotic Monopole-Dipole Forces Using Near-Surface MPCVD Diamond NV Centers Source Paper DOI: 10.1038/s41467-018-03152-9 Prepared By: 6CCVD Advanced Materials Engineering Team

Executive Summary

This research successfully demonstrates the use of a near-surface Nitrogen-Vacancy (NV) center in diamond as an exceptionally sensitive quantum sensor to explore fundamental physics beyond the Standard Model. The core value proposition lies in the use of high-purity Single Crystal Diamond (SCD) to enable ultra-short range force detection.

Core Achievement: Set a stringent constraint on the hypothetical axion-mediated electron-nucleon monopole-dipole coupling ($\text{g}^e_g \text{g}^N_p$) over micrometer ranges.
Force Range Breakthrough: Successfully searched for interactions in the $0.1 - 23$ µm range, previously challenging due to sensor size limitations in macroscopic setups.
Constraint Set: The upper bound of the coupling at a $20$ µm force range was determined to be $\text{g}^e_g \text{g}^N_p < 6.24 \times 10^{-15}$.
Methodology: Utilized an Optically Detected Magnetic Resonance (ODMR) setup combined with an Atomic Force Microscope (AFM) tuning fork to sinusoidally modulate a fused silica ($\text{SiO}_2$) source mass.
Noise Suppression: A Spin Echo pulse sequence, synchronized precisely with the source mass vibration, was employed to suppress environmental noise (e.g., magnetic field drift, Overhauser field fluctuation) and enhance coherence time.
Material Necessity: The high sensitivity and long coherence time ($T_2 = 8.3$ µs) rely fundamentally on the low impurity and structural integrity afforded by high-quality MPCVD Single Crystal Diamond (SCD).

Technical Specifications

Hard data extracted from the experimental setup and results:

Parameter	Value	Unit	Context
NV Center Depth	< 10	nm	Near-surface requirement for short-range detection
Source Material (M)	$\text{SiO}_{2}$	N/A	Fused silica half-ball lens
Source Radius (R)	250 ± 2.5	µm	Geometric parameter of the source mass
Minimum Distance ($d_{0}$)	0.5 ± 0.1	µm	Shortest distance between NV sensor (S) and source (M)
Modulation Amplitude (A)	41.1 ± 0.1	nm	Vibration amplitude of the source mass
Modulation Frequency ($\omega_{m}$)	$2\pi \times 187.29$	kHz	Angular vibration frequency of the tuning fork
Static Magnetic Field ($B_{0}$)	$\sim 300$	G	Applied along the NV symmetry axis
Force Range Explored ($\lambda$)	$0.1 - 23$	µm	Range for constraint setting
Spin Coherence Time ($T_{2}$)	8.3 ± 0.8	µs	Enhanced coherence using spin echo sequence
DC Dephasing Time ($T^*_2$)	0.67 ± 0.04	µs	Measured without spin echo
Upper Bound Constraint	6.24 x $10^{-15}$	N/A	Electron-nucleon coupling ($\text{g}^e_g \text{g}^N_p$) at $20$ µm
Accumulated Phase ($\phi$)	0.000 ± 0.018	rad	Phase shift due to electron-nucleon interaction
Microwave $\pi/2$ Pulse Duration	59	ns	Used in the spin echo sequence
Microwave $\pi$ Pulse Duration	118	ns	Used in the spin echo sequence

Key Methodologies

The experiment relies on precise material synthesis and synchronized quantum control:

NV Center Generation:
- Bulk diamond, [100] oriented, was implanted with $10$ keV $\text{N}^+$ ions.
- Annealing performed at $800$ °C in vacuum for 2 hours.
- Oxidative etching performed at $580$ °C for 4 hours to control surface termination and achieve a near-surface depth ($<10$ nm).
- Nanopillars were fabricated to enhance Photoluminescence (PL) detection efficiency ($100$ kcounts $\text{s}^{-1}$).
Quantum Sensing and Control (ODMR):
- A static magnetic field ($B_{0} \sim 300$ G) was applied along the NV axis to split the $|m_s = \pm 1\rangle$ states.
- The NV electron spin was initialized and read out using a $532$ nm green laser pulse, monitoring PL ($650-800$ nm fluorescence).
- Microwave pulses (generated via IQ modulation and amplified) were delivered via a copper wire to manipulate the spin state between $|m_s = 0\rangle$ and $|m_s = -1\rangle$.
Source Modulation and Synchronization:
- The source mass (M, $\text{SiO}_2$ half-ball) was mounted on an AFM tuning fork actuator.
- The AFM positioned M close to the diamond surface ($d_{0} = 0.5$ µm) and drove M to vibrate sinusoidally at $\omega_m$.
- The quantum sensing pulse sequence (Spin Echo: $\pi/2 - \tau - \pi - \tau - \pi/2$) was precisely synchronized with the mechanical vibration of M ($\tau = \pi / \omega_m$). This synchronization modulates the effective magnetic field ($\text{B}_{eff}$) in phase with the spin echo, allowing detection of the oscillating signal while canceling static noise.
Data Extraction:
- The final accumulated phase ($\phi$) was extracted by measuring the PL intensity ($\text{I}{\text{PL}}$) as a function of the final microwave phase ($\phi{mw}$), yielding $\phi = \phi_2 - \phi_1$ (phase with M minus phase without M).
- The resulting phase shift ($\phi = 0.000 \pm 0.018$ rad) was used to calculate the upper bound constraint on the coupling constant $\text{g}^e_g \text{g}^N_p$.

6CCVD Solutions & Capabilities

The advanced search for exotic spin-dependent interactions fundamentally requires high-quality, ultra-low defect diamond material and specialized post-processing. 6CCVD provides the foundational materials and engineering support necessary to replicate, improve, and extend this critical quantum sensing research.

Applicable Materials

To replicate and enhance the results achieved in this paper, 6CCVD recommends materials optimized for quantum sensing:

Optical Grade Single Crystal Diamond (SCD): Required for low background PL and structural homogeneity essential for creating stable, coherent NV centers. Our SCD wafers ensure low intrinsic defect density, minimizing unwanted noise sources.
- For Replication: Standard High-Purity SCD substrates (up to 500 µm thickness).
- For Next-Generation Sensing: Isotopically Engineered $\text{}^{12}\text{C}$ SCD: The paper notes that coherence time ($T_2$) can be enhanced by synthesizing $\text{}^{12}\text{C}$-enriched diamond. 6CCVD specializes in custom MPCVD growth recipes to deliver substrates with $>99.99%$ carbon-12 purity, crucial for maximizing $T_2$ coherence and improving future constraints by orders of magnitude.
PCD & BDD Substrates: While not used as the primary sensor, 6CCVD’s Polycrystalline Diamond (PCD) and Boron-Doped Diamond (BDD) materials can serve as robust, thermally conductive substrates for complex integration or as alternate source masses (M) requiring specific mechanical or electrical properties.

Customization Potential

The experiment relies on meticulous control over material geometry and surface quality. 6CCVD offers capabilities tailored to these exacting requirements:

Requirement from Paper/Future	6CCVD Capability	Technical Specification
Substrate Geometry/Size	Custom Dimensions & Laser Cutting	Plates/wafers up to $125$ mm (PCD), custom shapes available via in-house laser cutting (e.g., precise micro-structures or alignment features for AFM integration).
NV Center Support	Substrates up to $10$ mm thick	Provides rigid, stable foundation for sensitive AFM/ODMR setups.
Surface Preparation	Atomic-Scale Polishing	Achievable Ra < $1$ nm (SCD) and Ra < $5$ nm (Inch-size PCD). Critical for minimizing surface defects that limit near-surface NV performance and ensuring ultra-close proximity to the source mass (0.5 µm $d_{0}$).
Advanced Source Materials	Metalization Services	Internal capability for depositing Au, Pt, Pd, Ti, W, Cu. Useful if researchers pivot to testing interactions mediated by specific metallic source masses.

Engineering Support

6CCVD’s in-house PhD team provides authoritative support for quantum technology projects:

NV Protocol Consultation: We assist engineering teams in selecting the optimal SCD substrate (purity, orientation, isotopic composition) required for successful NV implantation, annealing, and surface etching protocols necessary for ultra-shallow NV creation ($<10$ nm).
Material Selection for Enhanced Sensitivity: We offer guidance on selecting materials for similar Exotic Force Detection projects, particularly focusing on how isotopic purity or crystallographic orientation affects spin $T_2$ and overall sensor sensitivity.
Global Logistics: We guarantee reliable Global Shipping, managing logistics via DDU default (DDP available), ensuring critical materials arrive safely and on schedule for sensitive laboratory work.

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/s41467-018-03152-9