A portable LED-based diamond magnetometer for outreach and teaching labs

We present a compact, low-cost version of an NV center diamond magnetometer which replaces the standard green laser with a high-power LED. This modification improves safety, reduces cost, and allows the green excitation and red photoluminescence to be viewed directly during demonstrations. The device is simple to assemble and suitable for outreach activities and undergraduate laboratories. We show that it can produce ODMR spectra and respond to nearby magnetic objects, with a sensitivity on the order of 1 $μ$T/$\sqrt{\text{Hz}}$. Supplementary material provides details of the construction and suggestions for student investigations to support use in teaching laboratories.

💡 Research Summary

The authors present a compact, low‑cost nitrogen‑vacancy (NV) diamond magnetometer designed specifically for outreach demonstrations and undergraduate laboratory courses. The key innovation is the replacement of the conventional 532 nm laser pump with a high‑power green LED (≈370 mW optical output). This substitution eliminates the need for laser safety protocols and precise beam alignment, dramatically improving safety and reducing the overall cost of the system while still providing sufficient excitation power to generate a visible green beam and a bright red photoluminescence (PL) that can be seen with the naked eye.

The optical layout is deliberately simple: the LED output is coupled into a short hexagonal light‑mixing rod that homogenizes the illumination. A right‑angle prism directs the green light onto a millimetre‑scale diamond mounted on a printed circuit board (PCB) and simultaneously collects the red PL, sending it back through the rod, a long‑pass filter, and onto an amplified photodiode. No lenses are required, which keeps the alignment tolerances loose and the assembly straightforward.

Electrically, the PCB incorporates a single‑turn copper loop antenna fabricated directly on the board. The loop is driven by a microwave source via an SMA connector and a short transmission line, placing the antenna directly beneath the diamond to achieve strong near‑field coupling. Microwave excitation is applied either as a continuous wave or with frequency modulation (FM). The PL signal is demodulated using a Red Pitaya‑based digital lock‑in amplifier. The lock‑in extracts the in‑phase (X) and quadrature (Y) components, and a graphical user interface provides real‑time plots of the demodulated signal, allowing students to explore how parameters such as microwave frequency range, modulation depth, lock‑in time constant, and LED drive current affect ODMR contrast, linewidth, and signal‑to‑noise ratio.

Performance testing shows clear optically detected magnetic resonance (ODMR) spectra with well‑resolved Zeeman‑split peaks when a small bias field is applied using permanent magnets. The device operates with a sensitivity on the order of 1 µT · Hz⁻¹ᐟ², which is adequate for educational demonstrations. In a “magnetometer mode,” the microwave frequency is fixed at a point of maximum slope on the ODMR curve, and the lock‑in output is monitored as a function of time. Bringing a steel Allen key close to the sensor produces a distinct spike in the in‑phase channel, providing a compelling visual demonstration of magnetic field detection. Calibration with a Helmholtz coil confirms a linear relationship between the lock‑in magnitude and the applied magnetic field over the range relevant for classroom experiments.

The authors provide extensive supplementary material: complete Gerber files for the PCB, a detailed parts list with current market prices, CAD files for 3D‑printed components, and a set of suggested student investigations. These include studies of microwave power dependence, LED current effects on ODMR contrast, distance‑dependent magnetic field measurements, and quantitative field mapping using the Helmholtz coil. The design emphasizes robustness, ease of assembly (the entire system fits into two small plastic boxes), and rapid re‑assembly (a few minutes).

In the discussion, the authors acknowledge that while the LED‑based system is less sensitive than laser‑driven counterparts and that the diamond itself constitutes a significant portion of the total cost, the trade‑off is justified for teaching environments where visual clarity, safety, and simplicity are paramount. They note that cheaper alternatives using ensembles of micro‑diamonds are possible, but such approaches would broaden the ODMR linewidth and reduce sensitivity, compromising the pedagogical goals of clear, interpretable spectra.

Overall, the paper demonstrates that a LED‑pumped NV diamond magnetometer can serve as an effective, low‑budget platform for introducing students to quantum sensing, magnetic resonance, and experimental techniques such as lock‑in detection. Its ability to produce visible fluorescence, generate real‑time ODMR data, and respond measurably to nearby magnetic objects makes it a valuable tool for both outreach events and structured laboratory curricula.