Miniaturized magnetic-field sensor based on nitrogen-vacancy centers

TL;DR

This work delivers a compact, fiber-integrated NV-center magnetometer head that couples a single $15~\mu\mathrm{m}$ diamond to a MW antenna via direct laser writing, achieving a shot-noise-limited sensitivity of $5.9~\mathrm{nT}/\sqrt{\mathrm{Hz}}$ and enabling vector magnetic-field measurements in a three-dimensional environment. The authors introduce a dual-fiber architecture that dramatically suppresses autofluorescence, potentially permitting the use of substantially smaller diamonds and improving operation in light-sensitive settings. Through detailed fluorescence, sensitivity, and noise analyses, they show the platform's robustness, long-term stability, and practical utility for vector field mapping, while outlining clear paths toward further miniaturization and pulsed magnetometry for enhanced performance. Overall, the study advances portable, high-sensitivity NV-based sensing by integrating photonic and microwave components at the sensor tip, paving the way for endoscopic and in vivo applications.

Abstract

The nitrogen-vacancy (NV) center in diamond is a prime candidate for quantum sensing technologies. Here, we present a fully integrated and mechanically robust fiber-based endoscopic sensor with a tip diameter of $1.25 \mathrm{mm}$. On its tip, a direct laser writing process is used to fixate a diamond containing NV centers above the fiber's core inside a polymer structure. Additionally, a metallic direct laser-written antenna structure next to the fiber facet allows efficient microwave manipulation of NV center spins. The sensor achieves a shot-noise-limited magnetic-field sensitivity of $5.9 \mathrm{nT}/\sqrt{\mathrm{Hz}}$ using a $15 \mathrm{μm}$-sized microdiamond at a microwave power of $50 \mathrm{mW}$ and optical power of $2.15 \mathrm{mW}$. Using lock-in techniques, we measure a sensitivity of $51.8 \mathrm{nT}/\sqrt{\mathrm{Hz}}$. Furthermore, we introduce a dual-fiber concept that enables, in combination with a direct laser-written structure, independent guiding of excitation and fluorescence light and thus reduces background autofluorescence. Moreover, controlled guiding of excitation light to the diamond while avoiding sample illumination may enable operation in light-sensitive environments such as biological tissue. While the demonstrated sensitivity is achieved using a single-fiber configuration, the dual-fiber approach provides a path towards integrating smaller diamonds, where autofluorescence would otherwise limit performance. We demonstrate the capability of vector magnetic field measurements in a magnetic field as used in state-of-the-art ultracold quantum gas experiments, opening a potential field in which high resolution and high sensitivity are necessary.

Figures (12)

• Figure 1: (A) Rendered images of the assembled sensor platform. Two silver wires and two optical fibers are inserted into a ceramic ferrule. As a reinforcement between the ferrule and the protective tubing of the fibers a mating sleeve is added. (B) Tip of a sensor after fabrication. (C) and (D) rendered images of the sensor tip with the antenna and a polymer waveguide structure and the light path for green excitation light and red fluorescence in a single-fiber and dual-fiber configuration.
• Figure 2: Mounted sensor platform during the direct laser writing process. The resist used is either a water-based silver solution or a polymer resist with suspended diamonds.
• Figure 3: Images of the assembled sensor with a total length of $\approx 35cm$. (A) Close view of the sensor tip encased in a brass rod with an additional protective cap. The fluorescence of the diamond during excitation through the multi-mode fiber is shown through a longpass filter ($>600nm$). (B) Entire sensor, including the additional optional cap for a constant magnetic bias field, the optical fibers, and the SMA connector for the MW signal connection at the back of the sensor.
• Figure 4: Schematics of the electrical and optical setup, specifically in a configuration for LIA measurements. (A) The MW signal is frequency modulated and directly connected to the sensor. The fluorescence signal, measured either with a photodiode (PD) or an avalanche PD (APD), is demodulated and filtered by a LIA. (B) Optical fiber coupling is done in a custom anodised enclosure, milled from aluminium. A first half-wave plate and a polarizing beam cube ensure a linear polarization of the in-coupled laser light and allow monitoring changes in laser power caused by fluctuations in the polarization via a PD. A second non-polarizing beam cube allows monitoring of the optical excitation power and can be used for further improvements using balanced detection schemes. ND and laser clean-up filters can be inserted, if needed, before the laser light is coupled into the sensor head fiber. The collected fluorescence is either separated by a dichroic mirror for single fiber measurements or, for dual fiber measurements, fluorescence collected from the sensor head is guided back through the second fiber into the optical setup and then filtered with a longpass filter. Finally, the fluorescence intensity is either measured by a PD or an APD or coupled to a MM fiber which guides the fluorescence light to a SPCM. (C) The sensor head inside of the coil system. A bias magnetic field can be applied with a screw-on cap containing an integrated magnet.
• Figure 5: Left: Lock-in amplified ODMR signal while a bias magnetic field is applied (solid blue). Resonances are fitted with derivatives of Lorentzian functions (red), where the zero-crossing marks the resonance frequency. Right: The magnetic-field sensitivity is dependent on the calculated slope at the zero-crossing point that is visualized as a dashed green line.