Components for an Inexpensive CW-ODMR NV-Based Magnetometer

At a Glance

Metadata	Details
Publication Date	2025-08-01
Journal	Magnetism
Authors	André Bülau, Daniela Walter, Karl-Peter Fritz
Institutions	Hahn-Schickard-Gesellschaft für angewandte Forschung
Citations	1
Analysis	Full AI Review Included

Executive Summary

This research details the development and characterization of components for an exceptionally inexpensive (<EUR 100) Continuous Wave Optically Detected Magnetic Resonance (CW-ODMR) Nitrogen-Vacancy (NV) based magnetometer, primarily targeting educational use.

Cost-Effective Design: A functional CW-ODMR setup was realized using Commercial Off-The-Shelf (COTS) components, achieving a total hardware cost of less than EUR 100 (excluding power supply and microcontroller).
Laser Replacement: Expensive lasers were successfully replaced by high-intensity green Light-Emitting Diodes (LEDs). Mechanical modification of the LED lens (LED2 shortened) significantly increased optical coupling efficiency to the diamond.
Optimal Filtering: Inexpensive dye-coated PET foil filters (LEE FILTERS 027/787) were identified as the best choice, offering efficient green excitation light suppression while exhibiting minimal autofluorescence compared to color glass filters.
High-Q Microwave Resonator: A simple, discrete LC resonator (C=1 pF, L≈3 nH) was designed and tuned to 2.86 GHz, achieving a high return loss (S11 = -50.8 dB). This high quality factor allows direct driving from a cheap ADF4351 board (+8 dBm output) without an external gain block.
Performance Demonstrated: The setup successfully demonstrated fluorescence, quenching, ODMR, and Zeeman splitting. The highest CW-ODMR dip height (300 mV) was achieved using LED3 (60 mA current) combined with a large High-Pressure, High-Temperature (HPHT) diamond.
Diamond Quality Metrics: Zero-Field Splitting (ZFS) values ranged from 9 to 31 MHz, and Full Width at Half Maximum (FWHM) ranged from 13 to 23 MHz, consistent with the expected internal strain of HPHT diamonds.

Technical Specifications

Parameter	Value	Unit	Context
Total Component Cost	<100	EUR	Excluding µC platform and power supply
MW Resonance Frequency (f)	2.85976	GHz	Discrete LC Resonator
MW Return Loss (S11)	-50.8	dB	Measured by VNA
MW Excitation Power (Max)	+8	dBm	ADF4351 output (no external gain block)
Resonator Capacitance (C)	1	pF	Ceramic NP0 type
Resonator Inductance (L)	~3	nH	Calculated for 2 mm loop diameter (0.2 mm wire)
TIA Bandwidth	~200	Hz	Limited for microcontroller ADC digitization
Max LED Current Tested	70	mA	LED3
Max CW-ODMR Dip Height	300	mV	LED3 (60 mA) + Large HPHT Diamond
ZFS (Microdiamond)	31	MHz	LED3 + Microdiamond (No external field)
ZFS (Large HPHT Diamond)	9 to 11	MHz	HPHT Diamond builds
FWHM (Microdiamond)	13	MHz	LED3 + Microdiamond
FWHM (Max, HPHT)	23	MHz	LED2 Shortened + Large HPHT Diamond
LED Operating Temperature	-40 to +100	°C	Typical range for tested LEDs
Filter Operating Temperature	+180	°C	LEE PET foil filters (027/787)

Key Methodologies

The inexpensive NV magnetometer was developed and characterized through systematic component testing and integration:

LED Characterization:
- Three green LEDs (LED1, LED2, LED3) were compared based on calculated optical power (up to 13.2 mW for LED3 at 50 mA).
- LED2 was mechanically modified (lens removed, shortened) to minimize the distance between the LED chip and the diamond, maximizing optical coupling.
Diamond Integration:
- High-Pressure, High-Temperature (HPHT) microdiamonds (150 µm) and larger HPHT slabs were used.
- Diamonds were attached to the LED chip and the photodiode filter using NOA61 optical adhesive to minimize optical losses.
Color Filter Selection:
- Various color glass, gel, and coated PET foil filters were tested using a spectrometer.
- The primary selection criteria were efficient blocking of the green excitation light (515-528 nm) and minimal yellow/orange autofluorescence when excited by green or UV light.
- Inexpensive dye-coated PET foil filters (LEE 027/787) were selected for superior performance.
Microwave Resonator Fabrication:
- A discrete LC parallel circuit was built using two SMA connectors, a 1 pF ceramic capacitor, and a 0.2 mm copper wire loop (D=2 mm).
- The resonator was tuned to 2.87 GHz by adjusting the loop shape, achieving high return loss (S11 = -50.8 dB) for efficient MW coupling.
Signal Detection and Amplification:
- A low-cost silicon photodiode (BPW34) was used to collect fluorescence.
- A single-stage Transimpedance Amplifier (TIA) based on OPA140 was implemented, limiting the bandwidth to ~200 Hz to ensure the output voltage remained below 3.3 V for the microcontroller’s Analog-to-Digital Converter (ADC).
CW-ODMR Measurement:
- The microwave frequency was swept from 2.67 GHz to 3.07 GHz.
- Measurements were captured either by a high-speed oscilloscope (HDO6054-MS, 36 averages) or directly by the STM32 microcontroller’s built-in ADC (100 accumulations per step).
- Magnetic field variation was achieved by moving a permanent bar magnet using a linear stage, with field strength monitored by a reference 3D Hall sensor.

Commercial Applications

The technology developed for this inexpensive NV magnetometer platform has direct relevance across several fields requiring low-cost, robust quantum sensing solutions:

Digital Education and STEM: The primary application is the QOOOL Kit Magneto platform, designed for integration into the senseBox ecosystem, enabling schools and makerspaces to teach quantum physics (fluorescence, ODMR, Zeeman splitting) using COTS components.
Miniaturized Magnetometry: Provides a blueprint for developing compact, room-temperature magnetic field sensors for industrial monitoring or simple laboratory applications where ultra-high sensitivity (pT range) is not required.
Low-Cost Sensor Prototyping: The methodology of using modified LEDs and PET filters offers a cost-effective alternative to traditional laser and dichroic mirror setups, accelerating prototyping in quantum sensor development.
Non-Cryogenic Quantum Systems: Reinforces the viability of NV centers as robust quantum systems operating conveniently at room temperature, contrasting with systems requiring cryogenic cooling (e.g., SQUIDs).
Integrated Photonics: The ongoing work to miniaturize the platform (π-Mk1 platform, 7 cm³) and integrate fluorescence waveguide excitation/collection devices points toward future integrated photonic quantum sensors.

View Original Abstract

Quantum sensing based on NV-centers in diamonds has been demonstrated many times in multiple publications. The majority of publications use lasers in free space or lasers with fiber optics, expensive optical components such as dichroic mirrors, or beam splitters with dichroic filters and expensive detectors, such as Avalanche photodiodes or single photon detectors, overall, leading to custom and expensive setups. In order to provide an inexpensive NV-based magnetometer setup for educational use in schools, to teach the three topics, fluorescence, optically detected magnetic resonance, and Zeeman splitting, inexpensive, miniaturized, off-the-shelf components with high reliability have to be used. The cheaper such a setup, the more setups a school can afford. Hence, in this work, we investigated LEDs as light sources, considered different diamonds for our setup, tested different color filters, proposed an inexpensive microwave resonator, and used a cheap photodiode with an appropriate transimpedance amplifier as the basis for our quantum magnetometer. As a result, we identified cheap and functional components and present a setup and show that it can demonstrate the three topics mentioned at a hardware cost <EUR 100.

Tech Support

Original Source

DOI: https://doi.org/10.3390/magnetism5030018

References

2018 - Little bits of diamond: Optically detected magnetic resonance of nitrogen-vacancy centers [Crossref]
2020 - A hand-held magnetometer based on an ensemble of nitrogen-vacancy centers in diamond [Crossref]
2023 - Modular low-cost 3D printed setup for experiments with NV centers in diamond [Crossref]
2021 - Integrated and Portable Magnetometer Based on Nitrogen-Vacancy Ensembles in Diamond [Crossref]
2010 - Temperature dependence of the nitrogen-vacancy magnetic resonance in diamond [Crossref]
2023 - Temperature sensing with RF-dressed states of nitrogen-vacancy centers in diamond [Crossref]
2011 - Electric-field sensing using single diamond spins [Crossref]
2019 - Robust and Accurate Electric Field Sensing with Solid State Spin Ensembles [Crossref]