Skip to content
6CCVD Research

Wideband Covariance Magnetometry below the Diffraction Limit

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2025-09-25
JournalPhysical Review Letters
AuthorsXuan Hoang Le, Pavel E. Dolgirev, Piotr Put, Emilee Anne Peterson, Arjun Pillai
InstitutionsETH Zurich, Harvard University
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”
  • Core Achievement: Experimental demonstration of wideband covariance magnetometry, achieving spatial resolution below the optical diffraction limit (nm to ”m scale).
  • Sensor Technology: Utilizes two spectrally resolved Nitrogen-Vacancy (NV) centers in 12C-enriched diamond as nanoscale magnetometers.
  • MHz Sensitivity: Probed correlated MHz-range noise with a high magnetic sensitivity of 15 nT Hz-1/4 using a Ramsey-based protocol.
  • GHz Correlation: Extended sensing capability to GHz frequencies using Correlated T1 spectroscopy, observing complex superradiant-like coherent and incoherent dynamics.
  • Readout Fidelity: Achieved low readout noise (σR = 3-4) necessary for correlation measurements by employing optical super-resolution and Resonantly-Assisted Spin-to-Charge Conversion (RA-SCC).
  • Application Scope: Provides a powerful, scalable tool for investigating nonlocal collective phenomena, critical fluctuations, and correlated transport in 2D condensed matter systems across the DC to GHz spectrum.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Sensor TypeTwo NV CentersN/ASpectrally resolved, independently controlled.
Spatial ResolutionBelow diffraction limitN/AProbes correlations from nm to ”m lengthscales.
Frequency RangeDC to GHzN/AWideband correlation measurement capability.
MHz Noise Sensitivity15 nT Hz-1/4N/AAchieved via Ramsey-based protocol (Correlated T2).
Detectable Noise Strength (MHz)4 nT2/HzN/AMeasured in 40 minutes total experimental time.
Projected Sensitivity (T2, 1 ms sensing)less than 1 nT2/HzN/AProjected correlated noise detection capability.
Readout Noise (RA-SCC)3-4N/AσR achieved for both NV defects using RA-SCC.
Spin Coherence Time (T2)greater than 1 msN/ALong coherence time enables extended sensing periods.
NV Center Depth~50 nmN/ABelow the diamond surface, distribution across ~20 nm.
Diamond MaterialElectronic Grade CVDN/A12C isotopically enriched (Element Six).
14N Implantation Dose1011 cm-2N/AUsed to create NV centers at 25 keV.
Annealing Temperature (Step 2)1200 °CN/AUsed to remove divacancies and multi-vacancy centers.
Resonant Excitation Wavelength637.22 nmN/AProvided by actively stabilized External Cavity Diode Lasers (ECDLs).

Key Methodologies

Section titled “Key Methodologies”
  1. Diamond Sample Fabrication: Used electronic grade CVD diamond isotopically enriched in 12C. NV centers were created via 14N ion implantation (25 keV, 1011 cm-2 dose).
  2. Defect Engineering: A two-step vacuum annealing process (800 °C for 8 hours, then 1200 °C for 2 hours) was used to form NV centers and minimize magnetic noise from defects.
  3. Surface Preparation: The diamond was cleaned using a triacid mixture (sulfuric:nitric:perchloric 1:1:1) to remove graphitic carbon, resulting in NV centers approximately 50 nm below the surface.
  4. Optical Setup: Experiments were conducted at ~11 K using a home-built single-path scanning confocal microscope. Individual NV centers were addressed using optical super-resolution based on inhomogeneous optical transitions.
  5. MW Delivery: Microwave (MW) control pulses and test signals were delivered via a photolithographically defined omega-loop stripline (225 nm Au) fabricated directly on the diamond surface.
  6. High-Fidelity Readout (RA-SCC): The Resonantly-Assisted Spin-to-Charge Conversion (RA-SCC) protocol was used for low-noise spin readout. This involves selective resonant excitation (637 nm) of the |ms = 0> state, simultaneous high-power 660 nm ionization, and sequential charge readout.
  7. Correlated T2 Spectroscopy (MHz Range): Used the XY8-N sensing sequence on each NV independently, tuned by interpulse spacing τ, to probe phase-modulated AC magnetic fields (e.g., 2.5 MHz signal with 25 kHz Gaussian noise bandwidth).
  8. Correlated T1 Spectroscopy (GHz Range): Probed GHz noise by applying amplitude-modulated MW signals (10 MHz bandwidth Gaussian noise) close to the NV transition frequencies (Δi), enabling the study of spin-spin co-relaxation dynamics.

Commercial Applications

Section titled “Commercial Applications”
  • Quantum Sensing and Metrology: Provides a foundational technique for next-generation quantum sensors, particularly for integration into scanning NV tips and nanopillar arrays, enabling high-resolution magnetic field mapping.
  • 2D Materials Research: Essential tool for characterizing correlated dynamics, critical fluctuations, and magnetic order in emerging 2D magnets (e.g., CrCl3) and thin-film superconductors (e.g., Bi2Sr2CaCu2O8+ÎŽ).
  • High-Frequency Device Diagnostics: Capable of detecting and imaging high-frequency (GHz) magnetic noise, crucial for understanding dissipation and dynamics in spintronic devices and high-speed electronics.
  • Nonlocal Transport Studies: Applicable to investigating nonlocal collective phenomena, such as hydrodynamic electron transport and correlated current fluctuations in materials like graphene.
  • Solid-State Qubit Development: The technique for studying correlated dephasing and superradiant dynamics provides insight into managing noise and enhancing coherence in solid-state quantum registers.
View Original Abstract

We experimentally demonstrate a method for measuring correlations of wideband magnetic signals with spatial resolution below the optical diffraction limit. Our technique employs two nitrogen-vacancy (NV) centers in diamond as nanoscale magnetometers, spectrally resolved by inhomogeneous optical transitions. Using high-fidelity optical readout and long spin coherence time, we probe correlated megahertz-range noise with sensitivity of 15 nTHz^{-1/4}. In addition, we use this system for correlated T_{1} relaxometry, enabling correlation measurements of gigahertz-range noise. Under such externally applied noise, while individual NV centers exhibit featureless relaxation, their correlation displays rich coherent and incoherent dynamics reminiscent of superradiance physics. This capability to probe high-frequency correlations provides a powerful tool for investigating a variety of condensed-matter phenomena characterized by nonlocal correlations.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 1997 - Quantum Optics [Crossref]
  2. 2004 - Introduction to Superconductivity

Reads Similar to This

Symmetrically-Cooled Ti -sapphire Thin-Disk Laser Using Single-Crystal Diamond Heat Spreaders Pressure-Induced Conductivity and Yellow-to-Black Piezochromism in a Layered Cu–Cl Hybrid Perovskite From Semiconducting to Metallic - Jahn–Teller-Induced Phase Transformation in Skyrmion Host GaV₄S₈