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Metrology

A Highly Efficient, Low-Cost Microwave Resonator for Exciting a Diamond Sample from a Miniaturized Quantum Magnetometer

André Bülau, Daniela Walter, Magnus Kofoed, Florian Janek, Volker Kible, Karl-Peter Fritz

Published Nov 17, 2025• 0 citations

Abstract

Optically detected magnetic resonance (ODMR) of nitrogen-vacancy centers in diamonds, in addition to optical excitation with green light, requires microwave excitation and thus a microwave structure. While many different microwave structures including microwave resonators have been presented in the past, none of them fulfilled the need to fit inside the miniaturized quantum magnetometer with limited space used in this work. This is why a novel microwave resonator design using commercially available printed circuit board technology is proposed. It is demonstrated that this design is of small form factor, highly power efficient and low-cost, with very good reproducibility, and in addition, it can be fabricated as a flexible printed circuit board to be bent and thus fit into the miniaturized sensor used in this work. The design choices made for the resonator and the way in which it was trimmed and optimized geometrically are presented and ODMR spectra made with a miniaturized quantum sensor in combination with such a resonator, which was fed by a microwave generator set to different microwave powers, are shown. These measurements revealed that a microwave power of −4 dBm is sufficient to excite the ms = ±1 states of the nitrogen-vacancy centers, while exceeding −1 dBm already introduces sidebands in the ODMR spectrum. This underlines the efficiency of the resonator in exciting the nitrogen-vacancies of the diamond in the sensor platform used and can lead to development of low-power quantum sensors in the future.

DOI: 10.3390/metrology5040069

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Optically detected magnetic resonance (ODMR) of nitrogen-vacancy centers in diamonds, in addition to optical excitation with green light, requires microwave excitation and thus a microwave structure. While many different microwave structures including microwave resonators have been presented in the past, none of them fulfilled the need to fit inside the miniaturized quantum magnetometer with limited space used in this work. This is why a novel microwave resonator design using commercially available printed circuit board technology is proposed. It is demonstrated that this design is of small form factor, highly power efficient and low-cost, with very good reproducibility, and in addition, it can be fabricated as a flexible printed circuit board to be bent and thus fit into the miniaturized sensor used in this work. The design choices made for the resonator and the way in which it was trimmed and optimized geometrically are presented and ODMR spectra made with a miniaturized quantum sensor in combination with such a resonator, which was fed by a microwave generator set to different microwave powers, are shown. These measurements revealed that a microwave power of −4 dBm is sufficient to excite the ms = ±1 states of the nitrogen-vacancy centers, while exceeding −1 dBm already introduces sidebands in the ODMR spectrum. This underlines the efficiency of the resonator in exciting the nitrogen-vacancies of the diamond in the sensor platform used and can lead to development of low-power quantum sensors in the future.

Nitrogen-vacancy (NV) centers in diamond are solid-state quantum sensors that operate at room temperature and allow optical readout of spin states through optically detected magnetic resonance (ODMR). Reliable ODMR measurements need controlled microwave excitation to manipulate NV spin states and modulate their fluorescence. In this paper, we report on ensemble NV centers’ interaction with microwave fields transmitted by printed-circuit-board (PCB) radio-frequency (RF) devices. Two microwave structures, a coplanar waveguide transmission line (CPW-TL) and a single-finger interdigitated capacitor (IDC), are designed and impedance-matched at the NV zero-field splitting frequency. Nanodiamond solutions containing NV ensembles are deposited as few-microliter droplets onto the PCB devices and dried, forming localized NV layers on the RF structures. Two-dimensional fluorescence scans are used to link spatial changes in NV fluorescence to the microwave field distributions created by each device geometry. CPW-TLs enable interaction over extended regions along the transmission line, while IDCs produce localized excitation near the capacitive gaps.

In this work we present a compact and portable tabletop magnetometer that utilizes negatively charged nitrogen-vacancy (NV) centers in diamond. The magnetometer is operated using a dual microwave resonance detection approach in combination with an optically detected magnetic resonance (ODMR) technique (mitigating drifts in results due to changes of the diamond temperature), capable of simultaneously exciting and registering two ODMR transitions. The experimentally measured magnetic field noise-floor is

\approx 2.3~\textrm{nT}\sqrt{\textrm{Hz}}

while the calculated shot-noise-limited magnetic field sensitivity is

\approx 585~\textrm{pT}\sqrt{\textrm{Hz}}

when excited with a continuous wave laser at 0.5~W. These results pave the way for realizing a simple set-up magnetometer for precise single axis magnetic field measurements for example for accurate electric current measurements for stabilization purposes and magnetic communication applications.

The nitrogen-vacancy (NV) color center in diamond has found applications in magnetometry across various geometrical scales. In particular, vector magnetometry using hand-held devices with low power consumption is of interest for mobile and geological applications. However, such a device requires efficient driving of the four different orientations of the NV center, thus placing demands on a broadband microwave resonator that provides a uniform and highly insensitive microwave magnetic field at the resonance frequencies of the NV centers. Here, we propose a unique planar resonator design that enables efficient driving of all four orientations of NV color centers over a 100 MHz range with only 0.5 W of microwave input power and 250 mW of laser power, allowing vector magnetic field measurements. Using the planar resonator to drive NV centers in diamond, a compact vector magnetometer was fabricated and experimentally studied. The sensitivity of the device is about

{1.2\text{ nT}}/{\sqrt{\text{Hz}}}\;

. While this is not a record sensitivity for diamond-based magnetometers, this device occupies its own niche as a compact hand-held magnetometer.

Abstract We developed a compact, portable quantum sensor module for pulsed measurements using diamond nitrogen-vacancy (NV) centers, an excitation laser diode of 3 mW and a microwave source of 15 dBm. This module allowed the evaluation of ensemble diamond NV centers for measuring magnetic fields by continuous wave— optically detected magnetic resonance (ODMR), Rabi oscillation, pulsed ODMR, Ramsey measurement and spin-echo measurement. The sensitivity through pulsed measurements reached 40 nT/ Hz at 1 Hz, which achieved higher sensitivity by one order, especially at a frequency around 1 Hz compared with previously reported compact modules using diamond NV centers.

Optically detected magnetic resonance (ODMR) spectroscopy of defect-rich semiconductors is being increasingly exploited for realizing a variety of practical quantum sensing devices. A prime example is the on-going development of compact magnetometers based on the nitrogen-vacancy (N-

V

) defect in diamond for the remote sensing of magnetic signals with high accuracy and sensitivity. In these applications, the ODMR-active material is integrated into the overall apparatus to form a self-contained sensor. However, some emerging applications require the sensing material to be in physical contact with an external object of interest, thus requiring an independent readout device. Here we present an ODMR meter, a compact device specially designed to allow convenient, contactless monitoring of ODMR in a target object, and demonstrate its application to temperature monitoring with N-

V

defects. Our prototype is composed of a handheld readout head (integrating all the necessary optical components and a microwave antenna) and a control box connected to a laptop computer, all made primarily from commercial off-the-shelf components. We test our device using a N-

V

-rich bulk diamond as the object, demonstrate a temperature sensitivity of

10\phantom{\rule{0.2em}{0ex}}\mathrm{mK}/\sqrt{\mathrm{Hz}}

in static conditions, and demonstrate the feasibility of handheld operation. The limitations to measurement speed, sensitivity, and accuracy are discussed. The presented device may find immediate use in medical and industrial applications where accurate thermometry is required, and can be extended to magnetic field measurements.

As promising quantum sensors, nitrogen-vacancy (N-

V

) centers in diamond have been widely used in frontier studies in condensed-matter physics, material science, and life sciences. In practical applications, weak laser excitation is favorable as it reduces the side effects of laser irradiation, such as phototoxicity and heating. Here we report a combined theoretical and experimental study of (near) zero-field optically detected magnetic resonance (ODMR) of N-

V

-center ensembles under weak 532-nm laser excitation. In this region, both the width and splitting of the ODMR spectra decrease with increasing laser power. This power dependence is reproduced with a model that accounts for the laser-induced charge neutralization of $\mathrm{N}\text{\ensuremath{-}}{V}^{\ensuremath{-}}$--

{\mathrm{N}}^{+}

pairs, which alters the local electric field environment. These results are useful for the understanding and development of N-

V

-based quantum sensing in light-sensitive applications.