Proposal for the search for new spin interactions at the micrometer scale using diamond quantum sensors

At a Glance

Metadata	Details
Publication Date	2022-05-31
Journal	Physical Review Research
Authors	P.-H. Chu, Nathaniel Ristoff, Jānis Šmits, Nathan Jackson, Young Jin Kim
Institutions	University of New Mexico, Los Alamos National Laboratory
Citations	13
Analysis	Full AI Review Included

Executive Summary

Core Objective: Propose experiments utilizing Nitrogen-Vacancy (NV) centers in diamond to search for exotic spin interactions (mediated by hypothetical bosons) at the micrometer (µm) scale.
Sensitivity Projection: Projected constraints are expected to achieve a 5 orders-of-magnitude improvement over previous constraints in the micrometer range for several hypothetical spin-velocity and velocity-dependent spin-spin interactions (V₁₂₊₁₃, V₄₊₅, V₆₊₇, V₁₄, V₁₅).
Sensing Platform: Uses ensembles of NV electron spins in a near-surface diamond layer coupled to micron-scale test masses attached to high-frequency vibrating mechanical oscillators.
Quantum Protocol: Sensitivity is achieved using multi-pulse quantum sensing protocols (XY8-N sequence) synchronized with the 1 MHz oscillation frequency (f_m) of the test mass.
Test Mass Innovation: A spin-polarized test mass based on hyperpolarized ¹³C nuclear spins in a thin diamond membrane is proposed, offering high spin density (> 1 nm^-3) while minimizing stray magnetic fields.
Systematic Mitigation: Key systematic errors, including shear stress from air friction and magnetic fields from moving surface charges, require mitigation via vacuum operation (10^-4 bar or lower) or phase-sensitive detection techniques.

Technical Specifications

Parameter	Value	Unit	Context
NV Gyromagnetic Ratio (γ_nv)	28.03	GHz/T	Conversion factor for effective magnetic field.
Laser Wavelength	532	nm	Initialization and readout of NV spin state.
Fluorescence Wavelength	650-750	nm	Detected signal range (Far Red).
Bias Magnetic Field (B₀)	~10	mT	Applied to lift m_s = ±1 degeneracy.
Modulation Frequency (f_m)	1	MHz	Target oscillation frequency of MEMS resonator.
Peak Displacement (d₁)	0.75	µm	Peak amplitude of test mass oscillation.
Peak Velocity (v)	4.7	m/s	Peak velocity of test mass oscillation.
Time/Volume Normalized Sensitivity (Target)	3.7 x 10^-10	T s^1/2 µm^3/2	Optimistic sensitivity target.
Minimum Detectable Field (B_min, t=1s)	1 to 10	pT	Range depending on interaction length (λ=50 µm to 0.5 µm).
Test Mass Nucleon Density (Unpolarized, SiO₂)	1.6 x 10³⁰	m^-3	Assumed density for unpolarized mass calculations.
Test Mass Radius (R_tm)	150	µm	Radius used for optimization across all geometries.
Test Mass Thickness (d_tm)	100	µm	Thickness for unpolarized mass (V₁₂₊₁₃, V₄₊₅).
Test Mass Thickness (d_tm)	2	µm	Thickness for polarized mass (V₆₊₇, V₁₄, V₁₅) to reduce stray fields.
Minimum Gap (d_gap)	0.2 to 5	µm	Gap between test mass and diamond surface (optimized per λ).
Excess Polarized Nuclear Spin Density (¹³C)	5 x 10²⁵	m^-3	Density used for polarized mass calculations.
¹³C Nuclear Spin Relaxation Time (T_1,13C)	~10	s	Repolarization time required for the polarized test mass.
Worst-Case Shear Stress (λ=0.5 µm)	~400	Pa	Equivalent to ~300 pT magnetic field shift (requires vacuum mitigation).
Surface Charge Density (Mitigation Target)	10	µC/m²	Target density using low-affinity triboelectric coating.

Key Methodologies

NV Center Preparation and Initialization:
- A diamond chip containing a near-surface layer of NV centers is used.
- NV centers are initialized into the m_s = 0 ground state using continuous 532 nm green laser illumination, leveraging the spin-dependent non-radiative relaxation pathway.
Mechanical Oscillation and Actuation:
- The test mass (SiO₂ or hyperpolarized ¹³C diamond membrane) is attached to a Microelectromechanical System (MEMS) resonator.
- The resonator is actuated laterally using electrostatic comb drives to achieve a target peak displacement of 0.75 µm and a high modulation frequency (f_m) of 1 MHz.
Quantum Sensing Protocol (Synchronized Readout):
- The XY8-N multi-pulse sequence is applied to the NV centers, acting as a phase-sensitive bandpass filter centered at the oscillation frequency f_m.
- The inter-pulse spacing (2τ) is set to approximately 1/(2f_m) to maximize sensitivity to the oscillating effective magnetic field produced by the exotic interaction.
- Fluorescence is measured sequentially after each XY8-N sequence to obtain an aliased trace of the effective field, which is then analyzed via Fourier analysis.
Spin Polarization (Hyperpolarization):
- For velocity-dependent spin-spin interactions (V₆₊₇, V₁₄, V₁₅), a thin diamond membrane containing ¹³C nuclear spins is hyperpolarized using the PulsePol sequence.
- This sequence transfers polarization from the NV electron spins to the ¹³C nuclei, repeated over the ¹³C nuclear spin relaxation time (T_1,13C ≈ 10 s).
Systematic Error Mitigation:
- Air Friction/Shear Stress: Mitigation involves operating the MEMS resonator under high vacuum (10^-4 bar or lower) or using differential measurements by alternately addressing the m_s = 0 ↔ m_s = +1 and m_s = 0 ↔ m_s = -1 NV spin transitions.
- Stray Magnetic Fields (Polarized Mass): The test mass thickness (d_tm) is reduced to 2 µm. Residual stray fields are suppressed by exploiting the 90° phase difference between the displacement (stray field source) and the velocity (exotic interaction signal).

Commercial Applications

Quantum Sensing and Metrology: NV centers in diamond are a leading platform for high-sensitivity magnetic field detection, crucial for fundamental physics tests and advanced sensor development.
MEMS/NEMS Technology: Development of high-frequency (MHz range) lateral actuation MEMS resonators with nano-scale precision for integration into quantum experiments and micro-robotics.
Nuclear Magnetic Resonance (NMR) and Hyperpolarization: Techniques like PulsePol developed for ¹³C hyperpolarization are directly applicable to enhancing signal strength in NMR spectroscopy and medical imaging (MRI).
Materials Science (Diamond): Requirement for high-purity, near-surface NV layers and thin, high-quality diamond membranes (for ¹³C polarization) drives advancements in diamond growth and processing technology.
Fundamental Physics Research: Provides a new experimental window for constraining hypothetical particles (e.g., axions, dark photons) that mediate exotic forces, impacting particle physics and cosmology.

View Original Abstract

For decades, searches for exotic spin interactions have used increasingly precise laboratory measurements to test various theoretical models of particle physics. However, most searches have focused on interaction length scales of ≳ 1 mm, corresponding to hypothetical boson masses of ≲ 0.2 meV. Recently, quantum sensors based on nitrogen-vacancy (NV) centers in diamond have emerged as a promising platform to probe spin interactions at the micrometer scale, opening the door to explore new physics at this length scale. Here, we propose experiments to search for several hypothetical interactions between NV electron spins and moving masses. We focus on potential interactions involving the coupling of NV spin ensembles to both spin-polarized and unpolarized masses attached to vibrating mechanical oscillators. For each interaction, we estimate the sensitivity, identify optimal experimental conditions, and analyze potential systematic errors. Using multipulse quantum sensing protocols with NV spin ensembles to improve sensitivity, we project constraints that are a 5-orders-of-magnitude improvement over previous constraints at the micrometer scale. We also identify a spin-polarized test mass, based on hyperpolarized <sup>13</sup>C nuclear spins in a thin diamond membrane, which offers a favorable combination of high spin density and low stray magnetic fields. Our analysis is timely in light of a recent preprint by Rong et al. (arXiv:2010.15667) reporting a surprising nonzero result of micrometer-scale spin-velocity interactions.

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Original Source

DOI: https://doi.org/10.1103/physrevresearch.4.023162