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Creative Education

All-Optical, Qualitative Diamond Magnetometry

Arius Seow, Luna Xu

Published Jan 1, 2025• 0 citations
Publisher

Abstract

The nitrogen-vacancy center in diamond offers unique possibilities for room-temperature quantum magnetometry. Here, we demonstrate and explore diamond magnetometry with a portable, educational toolkit from Quantum Education Initiative (QEI). A green (532 nm) laser diode and optical elements are used to electronically excite NV− electrons, while a transverse magnetic field induces spin mixing. Through all-optical initialization and readout (fluorescence), we gained a conceptual understanding of diamond-based magnetometry. We aligned several optical elements and measured intensity data to create an intensity versus magnetic field relationship. The employed magnetometry scheme does not attain state of the art sensitivities, but it is purely employed for educational purposes.

DOI
10.4236/ce.2025.1610103

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The nitrogen-vacancy center in diamond offers unique possibilities for room-temperature quantum magnetometry. Here, we demonstrate and explore diamond magnetometry with a portable, educational toolkit from Quantum Education Initiative (QEI). A green (532 nm) laser diode and optical elements are used to electronically excite NV− electrons, while a transverse magnetic field induces spin mixing. Through all-optical initialization and readout (fluorescence), we gained a conceptual understanding of diamond-based magnetometry. We aligned several optical elements and measured intensity data to create an intensity versus magnetic field relationship. The employed magnetometry scheme does not attain state of the art sensitivities, but it is purely employed for educational purposes.
Point defects in diamond, also known as color centers, offer unique possibilities for solid-state quantum magnetometry. Here, we demonstrate and explore quantum spin mixing and optical readout of nitrogen vacancy diamond quantum states with a portable, educational toolkit from Quantum Education Initiative. Visible light photons electronically excite NV centers, upon which their integrated photoluminescence is measured through a free-space photodetector. With household magnets, changes in photoluminescence are correlated with changes in the NV electrons magnetic spin state to intuitively understand optical initialization and readout of an NV ensemble. As a continuous wave laser is used for excitation, we do not demonstrate the trivial procedure for optical readout for quantum gates. Rather, we employ a magnet with a non-zero transverse field component to induce steady-state spin mixing, allowing us to observe decreases in photoluminescence due to occupation of the m s =±1 sublevels.
The nitrogen vacancy (NV) center in diamond is a versatile color center used for magnetometry, quantum computing, and quantum communications. In this article, using a single laser beam as a pump and probe, we measure the spin states of the NV center using the Faraday effect and use such measurements to develop a novel magnetic field sensor. Using the spin-state-dependent effect on the left and right circularly polarized light, we probe and confirm the existence of spin-orbit coupling in the NV center at room temperature. The sensitivity of our magnetometer is 350 350~350 nT/Hz\sqrt{Hz}Hz​, limited by the background produced by the laser trapped inside the diamond. We argue that by using an optical cavity and a high-purity diamond, sensitivities in the femtotesla level can be achieved.
High precision magnetometry is important for a range of applications from the monitoring of biologically generated magnetic fields (e.g. magnetoencephalography and magnetocardiography), to navigation in GPS denied environments, to the detection of gravitational waves. Diamond containing the negatively-charged nitrogen vacancy colour centre (NV-) has emerged as a powerful room-temperature sensing solution. Here we explore NV- centres as a laser medium for a new form of magnetometry: laser threshold magnetometry (LTM). LTM works by placing NV- inside an optical cavity and uses the coherent laser output as a potentially more sensitive readout channel than is possible using conventional (incoherent) optically detected magnetic resonance. Here we show progress towards LTM with diamond. We show twolaser excitation and stimulated emission in free space, and report progress towards diamond-cavity experiments. Our studies highlight the need for different NV- optimisation for laser applications, rather than those conventionally used for quantum information applications
So far most well-established quantum sensing techniques based on the negatively charged nitrogen-vacancy (NV) center in diamond utilize the recorded fluorescence intensity to detect the electronic spin states. However, the fluorescence intensity of a NV center is not only dependent on its spin state, but also affected by measurement issues, such as fluctuations of the excitation laser power and charge state transformation of the NV center. Instabilities in terms of output power or polarization changes in the laser source as well as sample drifts during a measurement are common factors that weaken the precision. Here, we demonstrate proof-of-principle of a sensing method making use of the excited-state lifetimes of a NV center for magnetometry experiments.
We propose a new type of sensor, which uses diamond containing the optically active nitrogen-vacancy (NV − ) centres as a laser medium. The magnetometer can be operated at room-temperature and generates light that can be readily fibre coupled, thereby permitting use in industrial applications and remote sensing. By combining laser pumping with a radio-frequency Rabi-drive field, an external magnetic field changes the fluorescence of the NV − centres. We use this change in fluorescence level to push the laser above threshold, turning it on with an intensity controlled by the external magnetic field, which provides a coherent amplification of the readout signal with very high contrast. This mechanism is qualitatively different from conventional NV − –based magnetometers which use fluorescence measurements, based on incoherent photon emission. We term our approach laser threshold magnetometer (LTM). We predict that an NV − –based LTM with a volume of 1 mm 3 can achieve shot-noise limited dc sensitivity of 1.86 fT and ac sensitivity of 3.97 fT .
Quantum sensors based on nitrogen vacancy (NV) centers in diamond have been a central topic in the sensing community for more than a decade. The extraordinary properties at room temperature of the spin system in diamond have made it one of the most prominent quantum platforms for the development of commercial quantum sensors. In particular, the sensitivity of the electronic spin in NV centers has made diamond-based magnetic sensors of special interest for their potential application in medical, industrial or navigation solutions. However, the use of these sensors for universal vector magnetometry was constrained by the need for previous knowledge on the field being measured to fully exploit their benefits. In this work, we show a method to perform unconditional vector magnetometry without the need of external information on the magnetic field, based only on the spatial arrangement of the diamond and the microwave antenna combination. While previous NV-based vector magnetometry methods require partial knowledge of the magnetic field (e.g. a calibrated bias field), we exploit the possibilities of selecting particular directions of the spins in the diamond with elliptically polarized microwave fields. We prove that our method allows to estimate both magnitude and direction of external magnetic fields without further assumptions or constraints.