Towards high-sensitivity magnetometry with nitrogen-vacancy centers in diamond using the singlet infrared absorption

At a Glance

Metadata	Details
Publication Date	2025-05-07
Journal	Physical Review Applied
Authors	Ali Tayefeh Younesi, Muhib Omar, Arne Wickenbrock, Dmitry Budker, Ronald Ulbricht
Institutions	University of California, Berkeley, Johannes Gutenberg University Mainz
Citations	4
Analysis	Full AI Review Included

Executive Summary

This research demonstrates a significant advancement in Nitrogen-Vacancy (N-V^-) magnetometry by achieving high sensitivity at room temperature without requiring complex resonant cavities.

Record Sensitivity: Achieved a magnetic sensitivity of 18 pT/√Hz (DC to 900 Hz) using cavity-free, room-temperature N-V^- magnetometry, surpassing previous cavity-enhanced or cryogenic results.
Methodology: The technique relies on Optically Detected Magnetic Resonance (ODMR) detected via the Infrared (IR) singlet absorption (1042 nm probe, ¹E → ¹A₁ transition), which offers inherently high spin contrast.
Operational Simplicity: The system operates using continuous-wave (CW) 532 nm pump and 1042 nm probe lasers, eliminating the need for elaborate cavity stabilization schemes or cryogenic cooling.
Optimization Strategy: Sensitivity was maximized through a long beam propagation path (2.6 mm), balanced probe beam photodetection, and multifrequency microwave (MW) excitation to address all hyperfine transitions simultaneously.
Performance Limit: The calculated shot-noise limit for the current setup is 5 pT/√Hz, indicating that further sensitivity improvements are possible by increasing the IR probe power.
Material Finding: An additional defect native to Chemical Vapor Deposition (CVD) diamond was identified (Zero-Phonon Line at 1358 nm) that absorbs the IR probe, suggesting that High-Pressure High-Temperature (HPHT) diamond may be a superior material choice.

Technical Specifications

Parameter	Value	Unit	Context
Achieved Magnetic Sensitivity	18 pT/√Hz	pT/√Hz	Room temperature, cavity-free operation
Calculated Shot-Noise Limit	5 pT/√Hz	pT/√Hz	Theoretical limit based on detected power
Electronic Noise Floor	1.5 pT/√Hz	pT/√Hz	Measured with light sources blocked
Diamond Material	¹²C-enriched CVD	N/A	[100] orientation, 2.6 mm interaction length
Substitutional Nitrogen Concentration	13 ppm	ppm	Concentration before N-V creation
N-V Concentration (Total)	~4 ppm	ppm	Negatively and neutrally charged
Pump Laser Wavelength	532 nm	nm	CW excitation (³A₂ → ³E)
Probe Laser Wavelength	1042 nm	nm	CW probing (¹E → ¹A₁ singlet transition)
Probe Spot Diameter	~24 µm	µm	At the center of the sample
Bias Magnetic Field Strength	~1 mT	mT	Applied along [110] axis
Effective Spin Contrast (C_cw)	1.6 %	%	Achieved using mixed MW signal
Spin Transition Linewidth (Δν)	700 kHz	kHz	FWHM of the central resonance peak
MW Modulation Frequency (f_mod)	5.6 kHz	kHz	Used for LIA demodulation
Detected IR Power (R)	~12 mW	mW	Power incident on the photodiode
¹⁴N Hyperfine Splitting	2.16 MHz	MHz	Due to nitrogen nuclear spin

Key Methodologies

The high-sensitivity magnetometry was achieved using optimized CW absorption ODMR techniques on a bulk CVD diamond sample:

Sample Configuration: A ¹²C-enriched CVD diamond (2.6 mm path length) was used to maximize IR absorption without a cavity. The sample was mounted on a larger diamond substrate for enhanced thermal dissipation.
Optical Setup: A 532 nm CW laser (Pump) and a 1042 nm CW laser (Probe) were combined collinearly using a dichroic mirror and focused onto the sample. Probe polarization was optimized using a halfwave plate (λ/2) to maximize interaction with specific N-V center orientations.
Magnetic Bias: A permanent magnet was oriented along the [110] axis to ensure equal magnetic field projection on two pairs of N-V axes, simplifying the ODMR spectrum into two overlapping pairs of peaks.
Multifrequency MW Excitation: A customized MW signal was generated by mixing two signal generator outputs (SG1 fixed at ZFS, SG2 customized sine wave) to produce six components. This signal simultaneously drove all hyperfine transitions (ms = 0 ↔ ms = ±1) for the two relevant N-V orientations, maximizing the ODMR contrast.
Balanced Photodetection: The transmitted 1042 nm probe beam was filtered to remove the 532 nm pump and detected using a balanced photodiode (BPD). This technique significantly reduced common-mode noise, such as laser intensity fluctuations.
Lock-in Detection: The MW signal was frequency-modulated (f_mod = 5.6 kHz) around the central resonance peak. The BPD output was demodulated by a Lock-in Amplifier (LIA) to obtain a dispersive signal, linearizing the magnetic field measurement near the zero-crossing.
Defect Mitigation: Transient absorption spectroscopy was employed to quantify and correct for the competing effect of pump-induced bleaching of a native CVD defect (ZPL at 1358 nm) that otherwise masked the pure N-V singlet absorption signal.

Commercial Applications

The development of robust, high-sensitivity, room-temperature magnetometers based on N-V singlet absorption is critical for several high-value engineering and scientific fields:

Quantum Sensing: Enabling the deployment of diamond-based quantum sensors in industrial and field environments where cryogenic cooling or complex cavity stabilization is impractical.
Biomagnetism and Neuroscience: High-resolution, non-invasive detection of extremely weak magnetic fields generated by biological processes, such as neuronal activity and cellular metabolism.
Microscale NMR/EPR: Providing enhanced sensitivity for Nuclear Magnetic Resonance (NMR) and Electron Paramagnetic Resonance (EPR) spectroscopy on microscopic samples, crucial for chemical analysis and drug discovery.
Materials Science Research: Used for high-spatial-resolution magnetic imaging of condensed matter systems, including magnetic domains, current flows, and phase transitions in novel materials.
Integrated Photonics: The cavity-free approach simplifies integration, paving the way for miniature, diamond-on-chip magnetometers compatible with waveguide geometries for compact device manufacturing.
Geophysical Mapping: Development of portable, robust sensors for high-resolution magnetic field measurements in geological and planetary science applications.

View Original Abstract

The negatively charged nitrogen vacancy center in diamond is widely used for quantum sensing due to the sensitivity of the spin triplet in the electronic ground state to external perturbations such as strain and electromagnetic fields, which makes it an excellent probe for changes in these perturbations. The spin state can be measured through optically detected magnetic resonance, which is most commonly achieved by detecting the photoluminescence after exciting the spin-triplet transition. Recently, methods have been proposed and demonstrated that use the absorption of the infrared singlet transition at 1042 nm instead. These methods, however, require cryogenic temperatures or external cavities to enhance the absorption signal. Here, we report on our efforts to optimize the magnetometer sensitivity at room temperature and without cavities. We reach a sensitivity of <a:math xmlns:a=“http://www.w3.org/1998/Math/MathML” display=“inline”> <a:mn>18</a:mn> <a:mspace width=“0.2em”/> <a:mi>pT</a:mi> <a:mo>/</a:mo> <a:msqrt> <a:mi>Hz</a:mi> </a:msqrt> </a:math> , surpassing previously reported values, and a calculated shot-noise limit of <d:math xmlns:d=“http://www.w3.org/1998/Math/MathML” display=“inline”> <d:mn>5</d:mn> <d:mspace width=“0.2em”/> <d:mi>pT</d:mi> <d:mo>/</d:mo> <d:msqrt> <d:mi>Hz</d:mi> </d:msqrt> </d:math> . We also report on a defect that is native to diamond grown by chemical vapor deposition and thus absent in high-pressure high-temperature diamond, the excitation of which impacts the measured singlet absorption signal.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.23.054019