Overview of this review: the quantum sensing landscape using spin defects, tracing the progression from fundamental research to technological developments aimed at enabling real-world applications. A directional bar is included to illustrate this trajectory—from foundational studies to advanced technological implementations—highlighting the increasing complexity and maturity of each stage.

Example of a fluorescence image of the color centers. When ensemble NV centers in diamond are illuminated by a green laser at 532 nm, they emit red fluorescence around 620 to 800 nm.

2.1.1.

Electronic structures of the diamond NV center

The NV center consists of three carbon atoms, a nitrogen substitutional atom, and a vacancy within a diamond lattice. A total of six electrons (three from the carbon atoms, two from the nitrogen atom, and one from the host materials’ electronic bands) occupy the ground energy levels of the NV center. Two unpaired electrons at the highest energy levels form an $S=1$ spin system, with sub-spin states of $m_{\mathrm{s}}=0$, $m_{\mathrm{s}}=-1$, and $m_{\mathrm{s}}=+1$. In the presence of an external magnetic field, $\overrightarrow{B}$, the NV Hamiltonian can be written as follows ($ħ=1$):

In the ground state, the ZFS between $m_{\mathrm{s}}=0$ and the degenerated $m_{\mathrm{s}}=\pm 1$ states is 2.87 GHz at room temperature.²⁷ The ZFS varies with temperature at a rate of 75.0(6) kHz/K,²² making the NV center suitable for sensitive thermometry. By contrast, the degenerated spin states $m_{\mathrm{s}}=\pm 1$ can be split by the Zeeman effect in the presence of an external magnetic field. Monitoring the Zeeman splitting is the underlying mechanism of the NV magnetometry. Typically, one of the $m_{\mathrm{s}}=\pm 1$ states is chosen to define a two-level system, and these states can act as a qubit. Therefore, developing techniques for properly reading out and manipulating these qubits is important in several quantum sensing protocols based on spin defects.

The energy levels and optical transitions associated with the NV center are illustrated in Fig. 3(a). Upon illumination with a green laser ($\lambda =532\mathrm{nm}$), an electron in the $\text{A}_2^3$ ground electronic state is excited to the $\text{E}_3$ doublet state, followed by a rapid photon decay on the order of tens of nanoseconds.^{79 ,80} The resulting fluorescence spans ∼620–800 nm, with the zero-phonon line (ZPL) at 637 nm, and its intensity varies between the $m_{\mathrm{s}}=0$ (bright) and $m_{\mathrm{s}}=\pm 1$ (dark) states due to the spin-selective intersystem crossing (ISC) and transition via metastable singlet states. During the optical transition, mixed ground spin states can also be initialized into the $m_{\mathrm{s}}=0$ state through the spin-flipping process. This unique property is the key to the spin-dependent optical readout and initialization of the spin qubit. Note that manipulation between the $m_{\mathrm{s}}=0$ and $m_{\mathrm{s}}=\pm 1$ states is achieved by applying MW fields with a frequency that matches the spin transition energies.

2.1.2.

Confocal fluorescence microscopy

As shown in Fig. 3(b), a confocal fluorescence microscope is commonly used to address single NV centers. The microscope consists of "confocal" optical components: an objective lens and an eyepiece lens (detector), with a pinhole acting as a spatial filter. In addition, a scanning system—either a scanning laser beam or a sample stage—is incorporated into the microscope to spatially map the fluorescence signals. The scanning confocal fluorescence microscope serves as a fundamental tool for characterizing the optical and spin properties of spin defects. An example confocal image of NV centers in Fig. 3(c) clearly shows individual NV centers. The identification of single NV centers can be further confirmed through ${\mathit{g}}^{(2)}(t)$ measurements.⁸¹

To improve the spatial resolution and photon collection efficiency of individual NV centers, an objective lens with a high numerical aperture (NA) is typically used. However, the high refractive index of diamond (2.42) results in reduced collection efficiency, which decreases the number of readout photons and sensitivity. In general, the shot-noise-limited sensitivity (${\eta }_{\text{shot}}$) is related to the average photon number ($N_{\text{photon}}$) as follows:⁸²

2.1.3.

Optically detected magnetic resonance

Once a single NV center is identified from the confocal image, the next step is to determine the spin energy levels through electron spin resonance (ESR) spectroscopy. This is achieved by continuously applying the green laser and MW fields while collecting fluorescence signals as a function of the MW frequency [Fig. 4(a)]. This technique is referred to as continuous wave ESR (CW-ESR) spectroscopy. Continuous laser excitation and fluorescence detection enable spin initialization to $m_{\mathrm{s}}=0$ and real-time readout of the spin states. When the MW frequency matches the NV center resonance, the NV center undergoes continuous Rabi oscillations. As the probability of the spin being in the $m_{\mathrm{s}}=\pm 1$ states increases, the fluorescence intensity decreases, resulting in dark dips in the spectrum [Figs. 4(b) and 4(c)]. Without an applied DC magnetic field, the CW-ESR spectrum shows a single dip at the frequency of ZFS, i.e., 2.87 GHz at room temperature. However, when a DC magnetic field is applied, two split resonances appear, corresponding to the two transitions $m_{\mathrm{s}}=0\leftrightarrow m_{\mathrm{s}}=-1$ and $m_{\mathrm{s}}=0\leftrightarrow m_{\mathrm{s}}=+1$, respectively. ESR spectroscopy through optical readout is referred to as ODMR.

2.2.1.

Relaxometry

Quantum coherence is a valuable resource in quantum sensing,^{85 ,86} playing a crucial role in sensitivity.⁸² Quantum coherence time can be categorized into two types: longitudinal (spin-lattice) relaxation time $T_1$ and transverse (spin-spin) relaxation time $T_2$. The $T_1$ time is governed by energy exchange processes between the spin and energy-matched levels in the surrounding environment, such as a thermal bath or spin bath.⁸⁷ Through these interactions, the spin dissipates energy, and the corresponding relaxation rate is determined by the strength of the interaction.

$T_1$ relaxometry utilizes changes in the $T_1$ time due to interactions with environmental noise, especially when the relevant energy scale matches the spin two levels, typically in the GHz range for spin defects. As the electron’s precession frequency falls within this range, $T_1$ relaxometry has been used to probe GHz stochastic signals from ionic samples such as ${\mathrm{Gd}}^{3+}$. Figure 5(a) illustrates the pulse sequences used for NV’s $T_1$ relaxometry. The NV center is initialized and prepared in a specific state (e.g., the excited state $|1⟩$) using laser and/or MW pulses. Note that we assume that after initialization, the qubit is prepared in its ground state, taking into account the depopulation from the metastable state. After a finite evolution time $\tau$, qubit readout is performed with a laser pulse. By repeating the procedure with varying $\tau$, the NV’s photoluminescence (or contrast) is plotted as a function of $\tau$, and the $T_1$ time is obtained from the decay constant.

2.2.2.

Rabi-type sensing protocols

Although $T_1$ relaxometry measures changes in the $T_1$ time due to interactions with stochastic signals or noise at the spin resonance frequency, the Rabi protocol measures changes in the rate of population inversion between the ground state $|0⟩$ and excited state $|1⟩$ due to coherent driving at the spin resonance frequency. Figure 5(b) illustrates the pulse sequence and the resulting Rabi oscillations. After initialization, the qubit undergoes coherent transitions between $|0⟩$ and $|1⟩$ due to continuous MW. The magnitude of the MW field determines the oscillation period, known as the Rabi frequency. Changes in the Rabi frequency due to external coherent signals are used to detect GHz AC signals.^{88 ,89}

The Rabi frequency ${\mathrm{\Omega }}_{\mathrm{AC}}$, which varies according to the amplitude of the AC signal, can be calculated using the following equation:

Sometimes, pulsed and continuous protocols are distinguished by whether the qubit undergoes free evolution between initialization and readout. Although the Rabi protocol uses pulsed MWs, it can be classified as a continuous driving protocol because the qubit is continuously driven during the evolution period. Continuous MW driving, as used in the Rabi-type protocol, offers several advantages over pulse-based protocols. For instance, Rabi-type magnetometry has been demonstrated using continuous dynamical decoupling (CDD) methods.^{90 –94} CDD enables tunable control over the spin transition energy by transforming the bare spin states into dressed states, serving as an effective noise filter to suppress background noise outside the dressed state’s frequency. This approach allows for wide-bandwidth AC magnetometry with improved sensitivity. Note that typical CDD protocols resemble Rabi-type protocols when detecting coherent signals, whereas for randomly phased, incoherent signals, they become more like relaxometry techniques.

Recently, more advanced CDD methods have been developed. For example, concatenated CDD has successfully demonstrated prolonged coherence time, reduced noise floors, and extended spectral ranges toward both low-frequency (sub-MHz)⁹⁵ and high-frequency (GHz) regimes.⁹⁶ In addition, a phase-modulated CDD technique, known as phase-modulated Hartmann–Hahn double resonance, has demonstrated a four-order power reduction, resulting in spectral narrowing.⁹⁷ Although pulse-based sensing protocols are still predominantly used in quantum sensing experiments, the Rabi-type protocol is emerging as a promising alternative methodology for quantum sensing.

2.2.3.

Ramsey-type sensing protocols

Ramsey interferometry is the most basic pulse-based sensing protocol for detecting DC and low-frequency magnetic signals (typically less than tens of kHz). Its sensitivity is fundamentally limited by the $T_2$ time; however, due to non-zero inhomogeneous low-frequency noise, the sensitivity is instead determined by the effective $T_2$ time, known as $T_2^{*}$. Figure 5(c) illustrates the pulse sequence and an example result of Ramsey interferometry. After initialization, the spin qubit is transformed into a maximally superposed state of $|0⟩$ and $|1⟩$, e.g., $(|0⟩+|1⟩)/\sqrt{2}$, with a $\pi /2$ pulse. During the free evolution period $\tau$, the qubit acquires a spin-dependent phase, resulting in a phase difference given by

Although Ramsey interferometry is sensitive to detecting phase differences induced by DC signals, adding additional $\pi$ pulses in between the Ramsey sequence enables the detection of AC signals up to several tens of MHz. These pulse-based sensing protocols are known as pulsed dynamical decoupling (PDD) protocols. Figure 5(d) shows an example of the PDD protocol with a single $\pi$ pulse,⁹⁸ known as the Hahn echo. The detection of AC magnetic signals from neighboring $\mathrm{C}_{13}$ nuclear spins has been clearly demonstrated using the method [Fig. 5(d)]. Besides the Hahn echo, various PDD protocols have been developed by incorporating additional $\pi$ pulses, including Carr–Purcell–Meiboom–Gill (CPMG), the XY family (XY4, XY8, etc.), and Knill dynamical decoupling. Each protocol is designed to address specific challenges, such as mitigating phase errors or pulse imperfections.^{99 –101} PDD protocols have become fundamental building blocks both for conventional and NV-based nuclear magnetic resonance (NMR) spectroscopy.^{44 –62}

The effects of $\pi$ pulses can be understood through filter functions in the toggling frame. Without $\pi$ pulses, as in Ramsey interferometry, the calculated filter function acts as a low-pass filter, allowing only DC and low-frequency signals to pass through. By contrast, $\pi$ pulses create a band-pass filter, where the central frequency shifts toward higher frequencies, and the filter’s bandwidth becomes narrower as the number of $\pi$ pulses, $N$, increases, as depicted in Fig. 6(a). A higher central frequency and a narrower bandwidth enable the detection of AC signals, protect the qubit from band-mismatched noise, enhance coherence time, and improve both spectral resolution and sensitivity [Fig. 6(b)].^{13 ,35 ,102 –104} However, the performance is limited by the Rabi frequency (or the pulse width) and the $T_2$ time (which itself is limited to $T_2\approx 2T_1$). As a result, the highest AC frequency detectable using PDD protocols is typically limited to several tens of MHz. For AC sensing at higher frequencies ($\sim 100\mathrm{MHz}$ to GHz), $T_1$ relaxometry and Rabi-type protocols can be used, as discussed previously.

Although PDD protocols are typically vulnerable to pulse errors, various mitigation methods have been developed over the decades. For instance, under the assumption of only 0th-order terms in average Hamiltonian theory,^{99 –101} the XY4 sequence is more robust to qubit rotation errors than the CPMG sequence. Pulse errors can also be mitigated by designing composite pulse sequences, such as the M. Levitt pulse and BB1, or by employing optimal control techniques.^{105 –107} To avoid inherent spurious effects caused by the symmetries of pulse sequences, appropriate pulse sequences can be selected, as each sequence exhibits a different spurious response. In addition, this issue can be fundamentally alleviated by utilizing correlation measurements or implementing pulse sequences with a correlated random phase scheme, which will be discussed in detail in Sec. 2.2.4.^{108 ,109}

2.2.4.

Correlation-type sensing protocols

In addition to sensitivity, spectral resolution is also an important metric, particularly for NMR spectroscopy. Spectral resolution is typically limited by the spin coherence, which is on the order of a few milliseconds. In this section, we discuss how this challenge has been addressed, focusing on two sensing protocols: correlation measurement and quantum heterodyne measurement.

The spectral resolution of a sensing protocol is closely related to sampling theory: the longer the sampling time, the better the spectral resolution. In basic PDD protocols, the sampling time is limited by the $T_2$ coherence time, which for NV centers is typically less than 1 ms, resulting in a spectral resolution worse than 1 kHz. In nano-NMR spectroscopy, this resolution is insufficient to resolve chemical shifts (which depend on the applied magnetic field and are on the order of Hz for hydrogen NMR under a few teslas) or J-couplings (also on the order of Hz). The primary approach to overcoming this limitation is to utilize the correlation of sequential measurements, which effectively extends the sampling time beyond the $T_2$ limit, enabling significantly higher spectral resolution.

For instance, time correlations between two sequential phase measurements have demonstrated extended spectral resolution limited by $T_1$,⁵⁴ which can be further enhanced with the quantum memory, as shown in Fig. 7. The phase information from the first sensing period is stored in the qubit population during the correlation time, which is related to $T_1$, and then correlated with the phase acquired from the second sensing period. The spectral resolution can be improved further by a few orders of magnitude through the use of a nuclear memory qubit, with $T_1$ time that is much longer than that of the electron sensing qubit.^{54 ,110 ,111}

The correlation measurement can be further improved by synchronizing the sequential measurements with an external clock, a technique known as quantum heterodyne (Qdyne) measurements, or, equivalently, coherently averaged synchronized readout (CASR). These methods have demonstrated sub-Hz spectral resolution, limited by stability of the clock and coherence time of the sample.^{52 ,112 ,113} Achieving sub-Hz resolution enables chemical shift-resolved NMR spectroscopy at relatively low magnetic fields ($<1\mathrm{T}$).

One of the challenges of conventional PDD techniques for NMR studies is their difficulty in operating under high magnetic fields ($>1\mathrm{T}$), as the nuclear Larmor frequency becomes too high for MW drives to track. This issue is critical in NMR spectroscopy because samples often require high magnetic fields to ensure long coherence time or high sample polarization. This challenge has been addressed by several methods, such as direct quantum memory access,^{110 ,111 ,114} electron nuclear double resonance quantum heterodyne,¹¹⁵ and amplitude-encoded radio-induced signal.¹¹⁶

Moreover, time correlation protocols, which have been used for a single NV (or a single group of ensemble NV centers), have recently been applied to spatial correlation measurements between more than two NV centers, demonstrating enhanced bandwidth and improved signal-to-noise ratio (SNR).^{37 –41}

2.3.1.

Weak measurement

Investigating the fast-varying dynamics of a sample is an important objective of quantum spin sensors. However, conventional sensing protocols based on strong measurements inherently limit such studies by significantly perturbing the sample due to measurement backaction. By contrast, weak measurements harness the sequential detection of target dynamics using a weak probe. These relatively unperturbed measurements preserve the quantum properties of the target and allow for the tracking of nuclear spin dynamics, as illustrated in Fig. 8.¹¹⁷ Similar to the Qdyne measurement, however, the spectral resolution of the weak measurement is also limited by the sampling time and the target sample’s lifetime, resulting in a spectral linewidth on the order of Hz.

2.3.2.

Quantum Fisher information scaling

Increasing the number of sensing qubits ($N_{\text{sensor}}$) or expanding the dimension of the Hilbert space is one of the most straightforward approaches to increasing sensitivity or quantum Fisher information.^{118 –123} Entangled sensing qubits can coherently accumulate signals, allowing the quantum Fisher information to scale proportionally to $N_{\text{sensor}}^2$, known as the Heisenberg scaling. By contrast, the Fisher information of classic sensors scales linearly with $N_{\text{sensor}}$, which is eventually limited by the SQL. With robust quantum memory,¹²⁴ it has been demonstrated that with an optimal phase mapping basis, the Fourier basis, the quantum Fisher information scales as $4^{N_{\text{sensor}}}$ (Fig. 9). This represents a state-of-the-art demonstration that combines quantum sensing and the quantum Fourier transform (QFT) technique, where the electron sensor detects the signal, and the nuclear memory spin stores and processes it. Note that additional energy levels in a larger spin system, known as a "qudit," can be advantageous, as demonstrated in Ref. 124. These extra levels enlarge the Hilbert space, which in principle leads to a more condensed quantum resource with a limited physical system. Furthermore, utilizing multiquantum coherence is expected to provide a quantum advantage,^{82 ,125 –130} benefiting from the large Hilbert space.

2.3.3.

Entanglement-enabled noise filtering

A recent study¹³¹ demonstrated that quantum entanglement can be leveraged to distinguish pure quantum signals from classical noise (Fig. 10). As previously discussed, typical sensing information is acquired in the form of the accumulated phase. In this study, the base protocol is a simple correlation measurement, where the phase information is lost due to the decoherence process. However, if the signal originates from a quantum target, the entanglement backaction maps information in the qubit population, allowing it to survive against the decoherence process and effectively filtering out the classical noises. Another example of a quantum filter has been demonstrated with higher-order correlation measurements,¹³² which distinguishes quantum signals from classical ones by exploring the noncommuting nature of quantum signals. Similarly, other types of quantum correlations or quantum discord could serve as potential resources for quantum sensing.

Fig. 11

Fig. 12

2.4.1.

Single-spin scanning magnetometry

Single-spin scanning magnetometry has been implemented for various magnetic materials and current transport devices, operating in environments from room temperature down to cryogenic temperatures. Figure 13 presents imaging examples of magnetic domain walls,³⁷ a single vortex in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ (YBCO) superconductor,¹⁴⁸ ${\mathrm{CrI}}_3$ 2D ferromagnets,³⁹ spin waves in yttrium iron garnet (YIG),¹³⁵ ${\mathrm{BiFeO}}_3$ multiferroic materials,¹⁵⁰ current flows in graphene devices,¹⁵⁵ and super current in Josephson junctions.¹⁵⁶ This method has proven to be a powerful tool for investigating interesting magnetic phenomena in exotic materials, spintronics, magnonics, and transport devices.

In addition to the image applications, ongoing efforts are focused on enhancing its performance. For example, the atomic force microscope (AFM) tip with a parabolically designed diamond nanopillar has demonstrated improved photon collection efficiency and directionality of emitted photons, achieving saturation photon count rate of 2.1 million counts per second (Mcps).¹⁵⁷ Recently, the gradiometry technique has been introduced, which converts DC to AC magnetic fields by utilizing periodic driving of the AFM tip and the strong gradient of magnetic fields from the sample. This method enables the detection of DC signals using more sensitive AC sensing protocols.¹⁵⁸ Furthermore, the use of multiple NV centers for imaging has been proposed to improve sensitivity through correlated signal measurements.¹⁵⁹ These advancements continue to pave the way for further improvements and applications of defect-based scanning magnetometry.

2.4.2.

Wide-field quantum microscopy

By replacing a single-point photodetector with a multipixel array camera, wide-field quantum microscopy enables the simultaneous acquisition of fluorescence signals from spatially distributed NV centers within the field of view. This provides the advantage of short imaging times (seconds to minutes) and large imaging areas ($100\mu {\mathrm{m}}^2$ to ${\mathrm{mm}}^2$).^{105 ,160 ,161}

Wide-field quantum microscopy has been applied to imaging magnetic materials, current transport devices, geological rocks, and biological samples. Particularly, the nontoxic and bio-friendly nature of diamond makes this imaging method a powerful tool for studying cells, tissues, neurons, etc. Figure 14 illustrates imaging examples of magnetotactic bacteria,¹⁶² Fe nanowires,¹³⁸ spin waves in YIG,¹⁶³ magnetized eucrite ALHA81001,¹⁶⁰ current flows in graphene ribbons,¹⁴⁹ and magnetic steganography.¹⁶⁴

To improve diffraction-limited spatial resolution, super-resolution imaging techniques have been introduced. These include stochastic reconstruction microscopy,¹⁶⁵ structured light illumination microscopy,^{139 ,166} aberration correction through image deconvolution with an applied point spread function,¹⁶⁷ and leveraging magnetic field phase changes under field gradients for super-resolution imaging.^{168 ,169}

In addition, nanofabricated pillar arrays have been implemented to improve optical collection efficiency and increase photon signals.¹⁷⁰ For example, an improvement in sensitivity by $\sim 5$ times with nano-pillar arrays has been demonstrated.¹⁷⁰ Double-etched multicone structures have also been studied, narrowing the far-field angle and enabling coupling with low-NA objective lenses. These structures also improve spatial resolution due to the sharp apex of the pillar tips.¹⁷¹ These advancements in photon collection over a wide imaging area promote increased SNR and faster imaging acquisition rates.

3.1.1.

Zero-dimensional nanoparticle

Nanoparticles, as shown in Fig. 15(a), are a widely used form of nanostructures. These nanocrystals exhibit high optical brightness by minimizing total internal reflection,^{69 ,235 ,236} and they offer an excellent integrability with various platforms. Figure 15(b) describes how nanodiamonds can be easily integrated with sensing targets using a drop-casting method, a technique also applicable for practical platforms such as optical fibers.²⁴⁰ Moreover, their nanoscale size allows for precise positioning using a mechanical tip^{243 ,248} or optical trapping technique,^{246 ,249} as depicted in Fig. 15(c). These advantages of enhanced light extraction, integrability, and controllability make nanocrystals ideal for implementing efficient, deterministic, and potentially scalable quantum sensing through the arrangement of nanocrystal arrays.²³⁵

One prominent example of such utility is a tip-attached quantum sensing probe capable of single-defect operation for nanoscale resolution. Various host materials support spin defect emission in nanoparticle structures, including diamond,^{69 ,250 ,251} SiC,²⁵² and hBN.^{235 ,253} For high-resolution sensing, sub-10 nm nanocrystals are preferred due to their proximity to the target samples.²³⁶ However, shallow defects in such small crystals often suffer from poor optical stability, including brightness fluctuation, bleaching, or quenching caused by surface-bound states.²⁵⁴

To mitigate these issues, several approaches have been explored, including highly purified nanocrystals using chemical vapor deposition (CVD), slow-growth techniques at high-pressure high-temperature,^{255 –257} or mechanical milling of high-quality bulk crystals.^{258 ,259} Another approach involves post-growth treatments, such as surface modification via aerobic oxidation^{250 ,260 ,261} or post-annealing process at prolonged high temperature at 1600°C.²⁶² In addition, scaling up the nanocrystal size, while keeping them below 100 nm, can help maintain optical and spin properties for nanoscale quantum sensing.⁶⁹

3.1.2.

One-dimensional nanowire

Recently, one-dimensional (1D) nanostructures, such as nanotubes and nanowires [Fig. 15(d)], have gained significant attention for their utility and compatibility with scanning probe systems and integrated photonic platforms for wide-field imaging. Fabricated diamond nanowires, for instance, exhibit enhanced photon extraction through optical routing along the nanowire axis, demonstrating up to a 10-fold improvement compared with unstructured NV and SiV centers.^{263 ,264} Figure 15(e) displays a corresponding numerical simulation highlighting the enhanced light extraction via waveguiding effects in nanowire geometries.

Owing to these properties, ultra-bright wide-field imaging has been demonstrated using nanowire, nanopillar, and nanocone arrays fabricated on bulk diamond substrates.^{170 ,171 ,265} In addition, the directional light guiding enabled by these nanostructures allows for efficient collection of PL, particularly from sharp-tip scanning probes. For example, PL collection from a single NV center has reached up to 2.1 Mcps due to improved optical coupling to the objective lens.^{16 ,157} Beyond brightness enhancement, these 1D nanostructures also improve spatial resolution. Figure 15(f) illustrates a single C-related spin in a BN nanotube ($\sim 50\mathrm{nm}$ diameter) attached to an AFM cantilever, demonstrating high spatial resolution and strong photon collection for nanoscale sensing.²³⁴

Moreover, 1D elongated nanostructures can efficiently interface with mechanical forces, enabling ultra-sensitive force-sensing applications.^{266 ,267} Mechanical modes induced by external forces can couple to spin sublevels, forming hybrid spin-nanomechanical sensing platforms.^{29 ,268 ,269} Mechanical strain coupling, in particular, can modulate the spin qubit coherence, thereby enhancing spin coherence time and, in turn, improving sensitivity.²⁷⁰

Although the growth and synthesis of diamond-based nanowire architectures remain challenging, promising strategies involve the use of carbon nanotube²⁷¹ or silicon nanowire templates.²⁷² In addition, spin defects embedded in different stacking interfaces have attracted interest due to their unique optical properties, such as nontrivial optical performances in brightness and phonon dynamics. For example, recent studies on SiC nanowires have explored point defect-stacking fault complexes.²⁷³ These systems exhibit enhanced brightness and suppressed electron-phonon coupling due to the proximity of stacking faults to point defects. As a result, strong ZPL emission from $V_{\mathrm{si}}$ is observed even at room temperature, with minimized phonon sidebands. Such features are highly advantageous for integration with photonic platforms, providing low-loss and low-dispersion optical interfaces.

3.1.3.

Two-dimensional thin film

The utilization of thin film structures [Fig. 15(g)] is also an attractive candidate for quantum sensing, offering excellent integrability with targets and engineerability through established fabrication techniques. Such thin films can be prepared by CVD growth,^{274 –276} epitaxial layer growth (especially for SiC or GaN),^{277 ,278} or by mechanical thinning and polishing of bulk crystals.^{279 –281} Among these, hBN, a van der Waals material, can be easily exfoliated down to atomic layers without requiring complex growth processes. This two-dimensional (2D) nature makes hBN compatible with nanofabrication, enabling precise engineering of the optical environment around spin defects.

These thin-film platforms are ideal for integrating photonic structures that tailor light–matter interactions. For example, Fig. 15(h) shows a photonic crystal cavity fabricated in an hBN membrane, where the coupling between spin defects and cavity photons enhances optical interfacing efficiency with external optical systems.²⁴⁷ In addition, the mechanical flexibility of thin film materials allows them to respond strongly to external strain, making them suitable for wide-area sensing applications. As illustrated in Fig. 15(i), hBN atomic layers have been used to probe nanobubble morphologies on deposited metal films through their strain sensitivity.¹⁹⁶

Beyond hBN, emerging spin-hosting materials such as Si and GaN offer significant advantages due to their compatibility with existing industrial infrastructure. Thin film growth of these materials supports scalable and wide-area sensor fabrication, which is essential for translating quantum sensing technologies into real-world applications. Thus, the development and integration of 2D thin films in spin-hosting materials are critical steps toward commercialization and practical deployment of quantum sensing platforms.

3.2.1.

Solid immersion lens

A solid immersion lens (SIL) is a widely used microlens for defect characterization, whose schematic is illustrated in Fig. 16(a). The first reported SIL was fabricated using focused ion beam milling (FIB) to create a hemispherical solid immersion lens centered around single diamond NV centers.²⁹⁵ In this configuration, the NV center is precisely positioned at the focal point of the hemispherical SIL, enhancing the NA of the optical interface and significantly increasing the collimation angle of the emitted light. This improvement leads to a substantial boost in photon collection efficiency, with reported brightness enhancements exceeding an order of magnitude, as illustrated in Fig. 16(b). State-of-the-art implementations have achieved a saturated photon collection rate of up to 2.4 Mcps from a single NV center.³⁰⁵

Recent advancements in SIL fabrication include the development of Fresnel-type SILs,³⁰⁶ which significantly reduce FIB milling time by more than 2/3 compared with hemispherical designs, thereby lowering fabrication costs. Beyond monolithic fabrication, alternative methods involve heterogeneous integration of commercially available SILs made from high-refractive-index materials such as gallium phosphide (GaP)²⁸⁵ and gallium nitride (GaN).²⁹⁸ These hybrid approaches preserve the structural integrity of the host crystal while still achieving considerable brightness enhancement without degrading the spin or optical properties of the defects. In addition, the lens material and dimensions can be tailored to optimize performance for specific types of spin defects and application requirements.

3.2.2.

Microsphere

Microsphere-type lenses, as depicted in Fig. 16(c), offer distinct advantages over hemispherical SILs, particularly due to their unique ability to generate a spatially localized excitation spot known as a photonic nanojet,^{289 ,307} and to efficiently convert evanescent waves into propagating waves.³⁰⁸ These advantages in excitation and collection enable highly magnified, super-resolution imaging that surpasses the diffraction limit, achieving spatial resolution as small as $\sim \lambda /17$ through a virtual image plane formed beneath the microsphere.^{288 ,309}

When applied to spin defect characterization, microsphere-assisted confocal scanning microscopy facilitates the optical addressing of individual photons and spin states from closely spaced defects that are otherwise indistinguishable in conventional confocal microscopy.²⁹⁶ As shown in Fig. 16(d), NV centers that appear spatially unresolved in standard confocal imaging (labeled D and T) can be distinctly resolved into subcomponents (D1-2 and T1-3) using a microsphere lens. In addition, this virtual imaging configuration significantly enhances SNR, with reported improvements of up to fourfold compared with direct imaging at the sample surface.

Microspheres are readily available as off-the-shelf micro-optics and can be easily deployed onto host materials via simple drop-casting methods. For precise alignment, microspheres can be positioned deterministically using micro-manipulation tools such as microtips³¹⁰ or optical tweezers.²⁹⁶ Importantly, this method requires no complex fabrication, making it a versatile and noninvasive strategy compatible with a wide range of bulk materials and standard optical microscopy setups. Therefore, microspheres provide a relocatable, low-cost, and effective optical interface for enhancing the performance of solid-state quantum emitters.

3.2.3.

Metalens

The introduction of meta-optics offers a powerful pathway to achieve near-unity NA in photon collection using thin membrane structures.³¹¹ As illustrated in Fig. 16(e), which shows a schematic of a metalens coupled to spin defects, flat meta-optical architectures such as monolithically fabricated diamond metalenses have been developed. These structures consist of nanopillar arrays engineered to mimic discretized Fresnel phase profiles.³⁰⁰ In this work, a brightness enhancement of 3.5 times was reported at a collimation NA of 0.75.

A key strength of this approach lies in the ability to fabricate these meta-optics architectures as ultra-thin membranes, as shown in Fig. 16(f). By harnessing this thin membrane design, hybrid integration of silicon dioxide (${\mathrm{SiO}}_2$) metalens membranes onto NV centers in bulk diamond has been demonstrated, achieving a 34-fold enhancement in photon collection compared to pristine bulk media.³¹² Moreover, their ultra-thin nature allows metalenses to interface with diverse photonic platforms, such as the facets of an optical fiber.^{313 ,314} In addition, metalenses offer tunable focusing depth³¹⁵ and exhibit chiral functionality,^{316 ,317} enabling applications across a wide range of use cases—from addressing spin defects at varying depths to generating distinct optical responses to circularly polarized light. Therefore, meta-lenses represent a lightweight, customizable, and high-performance optical interface with transformative potential for quantum sensing.

3.3.1.

Dielectric photonic cavity

The fabrication of dielectric photonic cavities on host media has been extensively developed across various fields of nanophotonics. Representative examples include photonic crystals [Fig. 17(a)]²⁸² and circular Bragg gratings [Fig. 17(b)],³²⁵ which have been successfully integrated with NV centers in diamond. These monolithic dielectric photonic cavity structures are also compatible with a wide range of host materials, including the aforementioned crystals. By leveraging such dielectric photonic crystal cavities, enhanced spin sensitivity has been demonstrated through cavity-coupled spin defects, as in the case of the L3 photonic crystal cavity in SiC [Fig. 17(e)].³²¹ The cavity-coupled emission enables enhanced optical signal readout, resulting in improved contrast in spin characterization.

Importantly, these dielectric photonic cavities can selectively enhance specific optical frequencies, allowing suppression of background fluorescence and the generation of low-dispersion modes in integrated photonic platforms. For instance, Fig. 17(f) shows selective enhancement of the NV center’s ZPL at 637 nm in diamond photonic crystal cavities.³²² Beyond enhancing the radiative decay rate, the cavity’s diffractive structure can also modulate the far-field propagation profile. As shown in Fig. 17(g), a monolithic circular Bragg grating cavity in hBN exhibits a strong vertical emission pattern.³²³

For coupling photons in optical fibers or photonic integrated circuits, tapered nanobeam cavities³²⁴ and photonic crystal waveguides^{326 ,327} are employed, as shown in Figs. 17(c) and 17(d). These lateral photon-routing configurations enable efficient coupling, for example, a tapered photonic crystal nanobeam achieves $>90\%$ coupling efficiency to the facet of a tapered silica fiber, as shown in Fig. 17(h).³²⁴ As a result, single-mode fibers can collect $\sim 38\text{kcps}$ of single-photon emission from the SiV in diamond. In summary, the integration of dielectric micro- and nano-cavities enables simultaneous enhancement of radiative decay rates, reduction of optical dispersion, and improved photon collection along desired optical pathways. These advantages collectively facilitate the realization of highly sensitive and efficiently integrated quantum sensing platforms.

3.3.2.

Plasmonic antenna and cavity

Plasmonic cavities offer significant advantages due to their broadband optical coupling modes and ultra-small mode volumes, making them highly compatible with randomly distributed, phonon-broadened emission from spin defects. Their extremely small mode volumes enable very high Purcell enhancements. One of the simplest methods to induce plasmonic interactions between spin defects and localized plasmons involves positioning metal nanoparticles, such as gold or silver, around the spin defects.^{328 –330} Figure 18(a) shows schematics of metal nanoparticles placed close to diamond NV centers.³²⁸ Due to the broadband spectral response of plasmonic enhancement, the brightness of NV center emission is enhanced across a wide wavelength range, unlike high-Q dielectric photonic cavities, as shown in Fig. 18(d).

Another approach involves coupling shallow-embedded spin defects to surface plasmons on a metal thin film [Fig. 18(b)]. As shown in Fig. 18(e), the enhancement in brightness due to surface plasmonic interactions is evident in confocal PL images comparing SiC regions on Au coplanar waveguides with pristine regions.³³¹ This configuration demonstrated a brightness enhancement of $\sim 7$-fold and a $\sim 14$-fold improvement in spin manipulation sensitivity, highlighting the strong performance of plasmonic cavity effects. Similarly, shallow ${\mathrm{V}}_{\mathrm{B}}^{-}$ centers in hBN, when coupled to surface plasmons, achieve magnetic field sensitivity of sub-$10\mu \mathrm{T}/{\mathrm{Hz}}^{1/2}$ even under continuous-wave ODMR measurement, a regime where typical sensitivities are around $100\mu \mathrm{T}/{\mathrm{Hz}}^{1/2}$.³³³

Recent developments have focused on low-loss plasmonic nanocavity structures to further optimize plasmonic enhancement. For example, Fig. 18(c) illustrates an advanced nanopatch antenna consisting of a metallic naocube sandwiched with a metal substrate, yielding an extremely confined optical mode. Spin defects in hBN embedded between such a nanocube and a thin metallic film have demonstrated PL enhancement of up to $\sim 250$-fold without compromising ODMR contrast. Figure 18(f) shows simulation results indicating the dependence of brightness enhancement on the distance between shallow spin defects and the metallic substrate, with optimal enhancement observed at distances below 10 nm.

Similar approaches have also been demonstrated using Au nanotrenches,³³⁴ metasurfaces,³³⁵ nanogaps,³³⁶ and other nanostructures.³³⁷ Importantly, plasmonic cavities are relatively simple to fabricate³³⁸ and can even be integrated with dielectric nanophotonic cavities,^{339 ,340} enabling hybrid metal-dielectric platforms. Such configurations offer promising pathways toward chip-integrated optical quantum sensing platforms with enhanced performance and scalability.

3.3.3.

External optical cavity

External optical cavities typically consist of two or more optical mirrors that trap light between them, enabling sustained photon confinement.³⁴¹ A key advantage of such cavities is the tunability of their resonant modes, which can be readily adjusted to match the emission spectrum of target spin defects. By varying the distance between the mirrors, the cavity’s optical path length can be controlled, thereby tuning the steady-state photonic density and enhancing light–matter interactions when spin defects are positioned within the cavity.

Figure 19(a) illustrates the schematic of an optical setup for optical cavity-enhanced room temperature magnetometry. It consists of a 532 nm pump laser, a 1042 nm probe laser, and two spherical mirrors forming a cavity around diamond NV centers. The probing laser is resonant with the metastable singlet transition of the NV center and is used to acquire the ODMR transmission spectrum.^{335 ,341} To enable miniaturization, one study implemented a packaged cavity structure by bonding a bulk diamond with an integrated planar mirror to a piezoelectric transducer, paired with a spherical mirror.³⁴² This configuration demonstrated magnetic field sensitivity near the photon shot-noise limit, confirming the sensitivity enhancement enabled by cavity-assisted detection [Fig. 19(b)].

A similar approach has been used to explore collective effects in cavity-coupled NV centers. Figure 19(c) shows a photograph of a device comprising a spherical micromirror aligned with a diamond membrane on a planar mirror.³⁴³ This configuration significantly enhances ZPL emission from the NV centers, as observed in the cavity mode spectrum [Fig. 19(d)]. Second-order correlation measurements at the filtered ZPL reveal photon bunching behavior, indicative of superradiance among the coupled emitters [Figs. 19(e) and 19(f)]. These collective emissions enable access to the quantum-enhanced sensing regime.^{343 ,344}

Importantly, cavity-mediated interactions among spin defects not only support enhanced sensing via superradiance but can also be used to explore subradiant states for potential quantum storage applications,³⁴⁵ which may further contribute to improving sensitivity.¹¹⁴ Therefore, photonic cavities provide a powerful platform for engineering photon-mediated interaction between spins, offering significant promise for advanced quantum sensing.

4.1.1.

NMR and MRI: nanometer, micrometer scales, and microfluidics

As discussed in Sec. 2, various sensing protocols have been developed to enhance sensitivity, spatial and spectral resolutions in NMR measurements. These protocols include pulsed and continuous dynamical decoupling, as well as coherently averaged synchronized readout, as discussed in Sec. 2. Individual proton spins have been detected within 1 s of integration using two-qubit quantum systems consisting of a single NV center and a neighboring $\mathrm{N}_{15}$ nuclear spin.⁵⁷ In addition, proton spins in a volume of ten picoliters have been detected using ensemble NV centers, with a sensitivity of $30\mathrm{pT}/{\mathrm{Hz}}^{1/2}$.⁵² Alongside efforts to increase sensitivity, various methods to amplify the signal itself have been explored, such as polarizing nuclear spins beyond their thermal populations using techniques such as dynamic nuclear polarization and parahydrogen-based signal amplification by reversible exchange.^{377 –379} High spectral resolution is crucial for identifying different nuclear spins and chemical shifts resulting from interactions with the local chemical environment. Using nuclear memory qubits or synchronized readout techniques, high spectral resolution on the order of one part per million or sub-mHz has been demonstrated.^{52 ,57}

For spatially resolved NMR, the imaging techniques described in Sec. 2.3 are combined with single or ensemble NV centers to achieve MRI at nanometer and micrometer scales. For nanometer-scale MRI, the NMR sample is mounted on a scanning probe microscope, e.g., an atomic force microscope, and scanned over a single NV center embedded in a diamond plate.^{50 ,374} This configuration is advantageous because the NV center is positioned in a fixed position, providing stability during the scanning process. As a result, sensitive NMR protocols, such as XY8-N, can be applied over prolonged periods. Early experiments have successfully demonstrated 2D NMR images of $\mathrm{H}_1$ nuclear spins in poly-methyl methacrylate⁵⁰ and $\mathrm{F}_{19}$ nuclear spins in Teflon microspheres [Fig. 20(a)],³⁷⁴ with spatial resolution of $\sim 12\mathrm{nm}$ and $29\pm 2\mathrm{nm}$, respectively. As the NV’s sensing volume is fixed, the scanned image provides a laterally resolved NMR image, limiting it to 2D MRI. To extend this technique to full three-dimensional (3D) MRI, large magnetic field gradients are required to differentiate nuclear spins along the vertical direction. Recent experiments have demonstrated 3D NMR spectroscopy of neighboring nuclear spins in diamond by utilizing strong local magnetic field gradients, either from the intrinsic magnetic dipole field of the NV center^{380 ,381} or from externally fabricated microwire structures.³⁸² Applying similar techniques to achieve 3D MRI on external samples remains an area for further research.

Although scanning probe-based MRI offers nanometer-scale resolution, its small imaging area and slow scanning process limit both the image size and acquisition time. This limitation can be overcome using wide-field quantum microscopy, which provides a larger field of view ($\sim 100\mu {\mathrm{m}}^2$ to $\mathrm{m}{\mathrm{m}}^2$) and spatial resolution on the order of several hundred nanometers. Early experiments using ensemble NV centers with wide-field quantum microscopy demonstrated 2D MRI of $\mathrm{F}_{19}$ nuclear spins in ${\mathrm{CaF}}_2$ islands³⁷⁵ and fluorinated samples,⁴⁹ achieving lateral resolutions of $\sim 40$ and $\sim 500\mathrm{nm}$, respectively. Recently, this technique has been integrated with microfluidic platforms.^{352 ,353 ,372 ,376 ,383} Microfluidic channels offer several advantages: biomolecular samples can be continuously supplied and maintained in a bio-friendly, in vivo environment with constant temperature and pH. In addition, samples can be polarized and detected at various points along the channels, providing flexibility in setup design and reducing the need for large-scale equipment.³⁵² Furthermore, microfluidic systems enable precise control over sample flow, volume, biochemical reactions, and real-time monitoring.^{352 ,353 ,372 ,376 ,383} As a result, combining micro-MRI with microfluidic channels enhances the technique’s suitability for biomedical applications.

4.1.2.

Cell detections and virus diagnostics

NV centers are also utilized for the detection of individual cells and viruses, offering a novel diagnostic tool for the quantitative analysis of various biological samples. These samples range from naturally existing magnetic species, such as magnetic chains in magnetotactic bacteria¹⁶² and malarial hemozoin paramagnetic crystals,³⁸⁴ to magnetically labeled samples, including SKBR3 cancer cells,³⁵¹ human lung tumor tissues,³⁴⁹ and mammalian cells and tissues.³⁷¹

MNPs are commonly used as magnetic tags, which are incorporated into living cells and tissues to generate localized magnetic fields.^{349 ,351 ,371} Figure 21(a) illustrates the concept of magnetic tagging: MNPs are selectively bound to SKBR3 cancer cells, allowing magnetic imaging to differentiate between MNP-labeled cancer cells and nonlabeled healthy cells, which are not identifiable under optical microscopy.³⁵¹ Figure 21(b) demonstrates immunomagnetic microscopy, a technique that quantitatively identifies the density of MNPs and reconstructs magnetic images using deep learning algorithms.³⁴⁹

Although MNPs serve as magnetic tags for DC field sensing, magnetic ions such as ${\mathrm{Gd}}^{3+}$, ${\mathrm{Cu}}^{2+}$, ${\mathrm{Mn}}^{2+}$, and ${\mathrm{Fe}}^{3+}$ are employed as spin labels for AC field sensing.^{353 ,372 ,373 ,385} These ions are commonly used as contrast agents in magnetic resonance imaging or naturally occur in ferritin protein cells. Their ESR occurs at GHz frequencies, influencing the NV $T_1$ relaxation time. $T_1$ relaxometry measures this GHz magnetic noise from the ions by tracking changes in $T_1$ time. Imaging ions has been demonstrated at both nanometer and micrometer scales using scanning magnetometry and wide-field quantum microscopy. For instance, ferritin proteins in Hep G2 cells [Fig. 21(c)] and ${\mathrm{Cu}}^{2+}$ ions [Fig. 21(d)] have been imaged with spatial resolutions of $\sim 10$ and $\sim 300\mathrm{nm}$, respectively.^{372 ,385}

Functionalized nanodiamonds have also been studied for rapid point-of-care diagnostic tests for contagious viruses, such as SARS-CoV-2 and HIV.^{353 ,354} Figure 21(e) illustrates the use of $T_1$ relaxometry to detect the SARS-CoV-2 virus with ${\mathrm{Gd}}^{3+}$ ions.³⁵³ In this setup, the surface of the nanodiamond is functionalized with $c$-DNA linked to ${\mathrm{Gd}}^{3+}$ complex molecules. When viral RNA binds to the $c$-DNA on the nanodiamond surface, the ${\mathrm{Gd}}^{3+}$ ions detach, reducing magnetic noise detected by the NV centers, which results in the recovery of the $T_1$ time. This method demonstrates sensitivity down to several hundred viral RNA copies, with a false-negative rate of less than 1%.³⁵³

In addition to $T_1$ relaxometry, a novel fluorescence detection method has been employed for in vitro biosensing.³⁵⁴ To separate the fluorescence signal from the NV center in nanodiamonds from background autofluorescence, an amplitude-modulated MW field is applied to the functionalized nanodiamonds, modulating the NV’s fluorescence emission, which is then detected by lock-in measurement. This technique provides a detection limit of $8.2<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-19}$ mol for a biotin–avidin model and enables single-copy detection of HIV-1 RNA [Fig. 21(f)].³⁵⁴

4.1.3.

Toward clinical applications: MCG and MEG

Two potential clinical applications of NV-based magnetic sensors are magnetocardiography (MCG) and magnetoencephalography (MEG), which are used to detect cardiac and neural signals.^{346 ,347 ,386 ,387} MCG measures stray magnetic fields induced by electrical currents flowing through the heart, whereas MEG detects magnetic signals generated by action potentials in neural networks. MEG requires sensitivity at the $\mathrm{fT}/{\mathrm{Hz}}^{1/2}$ level, but cardiac magnetic signals are typically several orders of magnitude stronger than neural signals, making them detectable by NV magnetic sensors. In addition, many heart diseases, such as atrial flutter and atrial fibrillation, are characterized by irregular current flow in the heart at the millimeter scale, necessitating miniaturized magnetic sensors. Diamond sensors based on ensemble NV centers can be compact ($\sim 10{\mathrm{cm}}^3$) and offer sensitivities in the 1 to $100\mathrm{pT}/{\mathrm{Hz}}^{1/2}$ range, making them well suited for MCG applications in diagnosing cardiovascular diseases. Recent experiments with diamond magnetic sensors have successfully measured magnetic cardiac signals from anesthetized and thoracotomized rats. For instance, Fig. 22(a) demonstrates the detection of magnetic cardiac signals from a living rat with a sensitivity of $9\mathrm{pT}/{\mathrm{Hz}}^{1/2}$ using a 5 mm-sized sensor.³⁴⁷ The results of NV-based MCG are consistent with conventional electrocardiography (ECG) measurements, underscoring the potential of NV-based MCG sensors for clinical use.

For MEG applications, NV centers still require further improvements in sensitivity. Early proof-of-principle experiments have demonstrated the ability to detect magnetic fields generated by ionic currents propagating through marine worm axons and mouse corpus callosum axons, as shown in Fig. 22(b). With a sensitivity range of 10 to $50\mathrm{pT}/{\mathrm{Hz}}^{1/2}$ and sub-microsecond temporal resolution, these experiments enable the measurement of magnetic field traces from transient action potentials occurring on the millisecond time scale.³⁴⁶

Recent theoretical calculations, shown in Fig. 22(c), suggest that neural network imaging will soon be possible with an improved wide-field quantum microscope setup. This setup could achieve an area-normalized sensitivity of $\sim 10\mathrm{nT}/\mu \mathrm{m}$, with $\sim 100\mu \mathrm{m}$ spatial resolution and $\sim 1\mathrm{ms}$ temporal resolution, averaged over ${10}^2$ to ${10}^3$ repetitions.³⁸⁷ However, single-shot imaging of neural activity still requires significant improvements in sensitivity by several orders of magnitude. This could be achieved by optimizing diamond treatment and enhancing $T_2^{*}$.

4.2.1.

Individual magnetic bits in MRAM

Diamond scanning magnetometry has proven to be an invaluable technique for characterizing MRAM devices at the nanometer scale.³⁵⁶ MRAM is a nonvolatile memory that stores information in magnetic bits. It relies on arrays of magnetic tunnel junctions (MTJs), which consist of two magnetic layers separated by a tunnel barrier. Magnetic information is stored based on the relative magnetization orientations of these layers: a parallel alignment represents "1," whereas an anti-parallel alignment represents "0." External magnetic fields can switch the magnetization direction of one of the layers, enabling data writing. As the MTJ pillars in MRAM devices are typically smaller than several tens of nanometers, analyzing device performance and identifying manufacturing flaws at the individual bit level requires both high field sensitivity and nanometer spatial resolution. Therefore, diamond scanning magnetometry serves as a novel tool for investigating MRAM devices.

For instance, Fig. 23 presents magnetic images of a spin transfer torque-MRAM (STT-MRAM) array consisting of $45<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>45$ bits.³⁵⁶ The NV center probes the distinct magnetic fields from the parallel and anti-parallel states of individual MTJ pillars, providing valuable insights into the switching behavior. These magnetic images offer a comprehensive overview of the bit switching process, allowing the characterization of key device properties such as magnetic characteristics, switching statistics, and uniformity across the magnetic bits. Figures 23(b) and 23(c) show the percentage of parallel bits as a function of the external magnetic field. By analyzing the bit-switching data, Borràs et al. were able to extract critical magnetic properties, including retention values ($\mathrm{\Delta }$) and anisotropy fields ($H_{\mathrm{k}}$).³⁵⁶ This novel nanoscale magnetic imaging technique holds significant potential for the MRAM industry, providing a method to characterize MRAM performance and screen device imperfections during the early stages of the manufacturing process.

4.2.2.

IC devices

In addition to characterizing magnetic devices such as MRAM, NV centers can also be used to image current profiles in electronic devices by measuring the Oersted magnetic fields generated by currents in the DC to GHz frequency range. For instance, wide-field diamond microscopy has been successfully applied to image current flow in printed circuit board traces, MW devices,¹⁴³ and photovoltaic devices.³⁸⁸ By measuring the Oersted stray field, a 2D current density map ($J_{2\mathrm{D}}$) can be reconstructed using the Biot–Savart law:

Recent research has also focused on expanding the capabilities of current flow imaging to more complex electronic circuits, such as ICs.^{357 ,358} Nanodiamond-based NV centers and wide-field quantum microscopy have been utilized to study IC microchips.^{357 ,358 ,389} For instance, nanodiamonds distributed over GaN HEMT junctions enable the simultaneous mapping of magnetic fields and temperature changes induced by current flows.³⁸⁹ On the other hand, wide-field quantum microscopy has been used to image magnetic fields generated by complex current paths in IC devices, facilitating the visualization of current profiles in multilayered devices and enabling the identification of malicious circuitry or Trojans.³⁵⁷

For example, Fig. 24(a) shows an experiment on a commercial FPGA designed to image current flow within ring oscillator clusters.³⁵⁷ These clusters, which consist of multiple ring oscillators, allow the control of current flow in different regions of the FPGA, switching between active and idle states. The magnetic field profiles measured by NV centers are overlaid on high-resolution optical images of the circuit die. Although most of the magnetic images align with the device’s designed structure, some features in the images reveal sub-surface structures or discontinuities in current flow. These hidden features, such as disconnected traces, are not easily detected in optical images, highlighting how magnetic imaging can offer a new approach to characterizing IC devices. In addition, quantitative analysis using machine learning techniques has been shown to identify the pre-programmed active states of the FPGA, paving the way for detecting hardware attacks such as malicious circuitry or Trojans and preventing counterfeiting during the manufacturing and distribution stages.

A key challenge in applying diamond microscopy at an industrial level is extending nearly 2D images into three dimensions. As the Oersted magnetic field induced by current flows in wires decays rapidly with distance (e.g., 1/distance), imaging sub-surface current flows at greater depths becomes challenging. Recent work, shown in Fig. 24(b), has demonstrated 3D current imaging up to $\sim 12\mu \mathrm{m}$ in the vertical direction within multilayered IC devices.³⁵⁸ By modeling magnetic field profiles in three dimensions $(x,y,z)$ using the Biot–Savart law, Garsi et al.³⁵⁸ were able to isolate Oersted fields from different layers of the device. However, this method requires prior knowledge of the device’s structure and layout. Extending this technique to deeper layers of a chip will require further improvements in sensitivity and spatial resolution. In addition, the practical application of diamond microscopy in the semiconductor industry faces challenges due to the complex architectures of multilayer IC devices, which consist of billions of transistors, metal layers, and wire networks. This complexity makes image analysis challenging and necessitates further research to refine these techniques for more efficient and accurate device characterization.

4.3.1.

Compact and portable sensors

In laboratory-scale experiments, diamond magnetometers based on ensembles of NV centers have demonstrated magnetic sensitivity, reaching as low as $\mathrm{pT}/{\mathrm{Hz}}^{1/2}$.¹⁴ However, there has been a growing effort to extend these advancements beyond the laboratory environment and develop compact magnetometers suitable for real-world industrial applications. Achieving this requires integrating essential sensor components, such as light sources, MW circuits, detectors, and the diamond sensor head, while ensuring that the high sensitivity observed in controlled lab environments is maintained.

Figure 25 showcases recent advancements in miniaturized diamond magnetometers, typically ranging in size from a few ${\mathrm{cm}}^3$ to several tens of $\mathrm{c}{\mathrm{m}}^3$.^{11 ,359 ,360 ,390 ,392 –394} The diamond plate itself is small, typically in the millimeter scale, and contains NV centers with densities ranging from ${10}^{13}$ to ${10}^{17}\mathrm{per}\mathrm{c}{\mathrm{m}}^3$. The diamond plate is then integrated with various commercial or custom-designed components, such as LEDs, MW circuit boards, optical elements (e.g., lenses and filters), and photodetectors.^{11 ,360 ,392 ,393} To further reduce the sensor’s overall size and minimize potential spurious coupling between MW sources and the detector, some designs use external lasers and photodetectors connected to the sensor head via optical fibers.^{12 ,359 ,390 ,391 ,394} The reported sensitivities from these compact sensors range from a few hundred $\mathrm{pT}/{\mathrm{Hz}}^{1/2}$ to $\mathrm{nT}/{\mathrm{Hz}}^{1/2}$, representing a significant step toward realizing practical, portable diamond magnetometers for industrial applications.

Figure 26 illustrates examples of portable diamond sensors for industrial applications, such as detecting elevator movement and measuring the thickness of steel sheets. The diamond sensor shown in Fig. 26(a) demonstrates real-time detection of magnetic fields induced by the movement of an elevator.³⁶¹ The sensor successfully monitors changes in magnetic fields as the elevator approaches the second level of the building where the sensor is located. In addition, the sensor detects the magnetic field change in real time when the elevator doors open. On the other hand, Fig. 26(b) presents a fiber-integrated hBN sensor used to measure the thickness of metal sheets by assessing the amount of magnetic field shielding caused by the sheet.¹⁹³ This compact fiber-based sensor is realized by integrating an hBN circular Bragg grating cavity, which contains ${\mathrm{V}}_{\mathrm{B}}^{-}$ spin defects, with an optical fiber. In this experiment, the steel sheet acts as a magnetic shield, and the sensor measures the suppression of the external magnetic field with varying sheet thicknesses, ranging from 0 to 9 mm.

For broader industrial applications of portable magnetometers, further development is necessary to make the entire sensor system even more compact, including the integration of external controllers and data analysis units, while maintaining high magnetic field sensitivity over a wide dynamic range. These advancements are crucial to enhancing the versatility and usability of portable sensors in diverse industrial environments.

4.3.2.

Battery monitor in electrical vehicles

Compact diamond sensors are also applied for monitoring EV battery systems. As the electric automobile market grows and concerns about battery safety become more prominent, accurate current sensors are critical for assessing the state of charge (SOC) of EV batteries. These sensors help predict remaining battery lifetime and drive range, and identify potential hazards in battery operations.

Figure 27 demonstrates how compact diamond sensors are used to monitor current flows in the busbar of an EV battery module.^{362 ,395} These sensors can detect magnetic fields generated by a busbar current of 20 mA, even when external magnetic noise is as high as $80\mu \mathrm{T}$. The system employs two separate diamond sensors placed on opposite sides of the battery busbar, measuring magnetic fields from opposing currents. By performing a differential measurement between the two sensors, the system effectively cancels out external magnetic noise, which is common in real driving situations. This approach significantly improves the accuracy of SOC estimation, suggesting that the EV’s driving range could be increased by 10%.^{362 ,395} Despite these advancements, further research and optimization are necessary to improve the sensor’s versatility for different charging situations. With ongoing development, these sensors will be better equipped to monitor EV battery performance reliably under real driving conditions.

4.3.3.

Harsh environment: high-voltage grids in power network systems

In high-voltage power grid systems, accurate current measurement is critical for minimizing energy losses and preventing hazards such as overheating of cables and equipment. As power network systems require current sensors capable of measuring large currents (several kA) within confined spaces, diamond sensors present a significant solution for monitoring currents in these environments.³⁶³

Figure 28 illustrates an example of how diamond sensors can be implemented in electrical power systems. Liu et al.³⁶³ proposed a design that separates the diamond sensor head from the control units for practical reasons, such as limited space near the high-voltage side and challenges associated with delivering MW signals through coaxial cables in high-voltage areas. The compact diamond sensor is placed on the high-voltage side, whereas the control units (including laser, MW circuits, detectors, and power supplies) are positioned on the low-voltage side. Optical excitation and readout of the NV centers are achieved using optical fibers, and MW drives are transmitted with directional antennas. In proof-of-principle experiments, this remote sensor system, positioned 11 m apart, successfully measured currents with high accuracy, showing only a 0.4% error over 1000 A.³⁶³ This demonstrates the potential of diamond sensors for high-voltage power grid applications. However, for practical implementation in the power grid, further improvements are needed to enhance the sensor’s sensitivity and the efficiency of the directional antenna system.

4.4.1.

Inertial sensors

Inertial sensing involves the measurement of force, acceleration, or displacement. Two primary methods for sensing inertial motion with diamond NV centers are detecting external magnetic fields or internal strain fields induced by the motion of mechanical oscillators. For example, the oscillatory motion of a magnetized AFM tip generates a time-varying magnetic field.³⁹⁶ This magnetic field drives spin transitions in the NV centers, which can be detected using NV’s AC sensing protocols. In this approach, both driven and Brownian motions of the AFM resonator have been detected with a precision of less than 6 picometers under ambient room temperature conditions.³⁹⁶

On the other hand, NV centers are also sensitive to strain fields through spin-lattice interaction. For instance, Ovartchaiyapong et al.³¹ demonstrated strain-mediated coupling between a diamond cantilever and an embedded NV center, quantitatively characterizing both axial and transverse strain sensitivities. With a strain sensitivity of $3<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-6}\text{strain}/{\mathrm{Hz}}^{1/2}$, they were able to detect $\sim 7\mathrm{nm}$ motion within a 1-s measurement. These measurements highlight the potential of inertial sensing, whether through spin coupling to time-varying external magnetic fields or by detecting internal strain fields.

Figure 29(a) illustrates a concept for force sensing using diamond nanopillars with NV centers positioned at two different locations: one at the tip of the nanopillar and the other at the base.²⁹ The NV center at the tip measures the spin-magnetic response caused by the nanopillar’s motion under a strong transverse magnetic field gradient, whereas the NV center at the base measures the stress and strain induced by the same mechanical motion. Each method offers distinct advantages and limitations. Magnetic sensitivity is significantly higher than strain sensitivity as evidenced by the measured displacements in the picometer and nanometer range.³¹ However, calculations from Ref. 29 suggest that spin-magnetic coupling requires large field gradients (e.g., $\gtrsim 30\mathrm{mT}/\mu \mathrm{m}$), presenting practical challenges, such as the need to position nanoferromagnetic structures close to the tip, which limits the range of mechanical motion. Consequently, both methods may need to be employed together to obtain complementary inertial data, depending on the scale of the motion.

Figure 29(b) presents a concept for force sensing based on arrays of diamond nanopillars, capable of detecting forces induced by biological cells through spin–strain coupling.²⁹ This approach can also be adapted for navigation applications. Recent advancements in diamond nanofabrication have enabled the development of various diamond nanomechanical structures with high mechanical quality factors. By combining nanofabrication techniques with advanced sensing protocols, the inertial sensitivity of these devices can be significantly enhanced.

4.4.2.

Rotation sensors: gyroscope

Alongside inertial sensing, rotation sensing or gyroscopes are essential components for navigation and IMU systems. The fundamental mechanisms of rotation sensing based on NV centers can be categorized into two main approaches: directly monitoring the rotational motion of the NV electron spin or sensing the effective magnetic field generated by rotating nuclear spins.^{64 –67 ,397} For instance, Fig. 30(a) demonstrates stroboscopic imaging of NV centers in a rotating diamond, where the laser and MW fields are relatively stationary.³⁶⁷ The fluorescence from a rotating NV center is collected by applying synchronized laser and MW pulses that match the rotational period of a spindle driven by an electric motor. This technique enables the detection of rapid rotations, e.g., 200,000 r/min, using a single NV center. Further experiments show that the phase shift of the NV center, induced by the relative motion between a fixed external MW field and the rotating NV spin, can be precisely measured.³⁹⁷ The external MW field, in combination with the rotating NV center, generates geometric or Berry phases in the NV’s Bloch sphere. The accumulated phase can then be detected using spin Hahn echo protocols.

Another approach leverages nuclear spins near the NV center. This method exploits nuclear spins as effective generators of magnetic fields due to rotation, known as rotational pseudo-fields.³⁶⁸ The NV center serves as a sensing qubit to probe these magnetic fields using AC sensing protocols. In a rotating frame, a precessing nuclear spin generates an effective magnetic field, $B$, which is determined by the precession frequency, $\mathrm{\Omega }$, and the spin gyromagnetic ratio, $\gamma$, such that $B=\mathrm{\Omega }/\gamma$. As the gyromagnetic ratio of nuclear spins is $\sim 1000$ times smaller than that of electron spins, nuclear spins can generate significantly larger rotational pseudo-fields.³⁶⁸ The $\mathrm{N}_{14}$ and $\mathrm{N}_{15}$ nuclear spins associated with NV centers are utilized for nuclear spin gyroscopes, with an example system illustrated in Fig. 30(b).⁶⁵ In this system, both the diamond sensor and control components (including laser, MW source, and detector) are mounted on the spindle and rotate together. Double quantum (DQ) pulse protocols, such as DQ Ramsey and DQ echo, are employed to monitor nuclear spin precession while minimizing the effects of temperature fluctuations. Recent experiments have demonstrated sensitivities in the range of 1 to $1000\mathrm{deg}/(\mathrm{s}{\mathrm{Hz}}^{1/2})$, with the best sensitivity being $4.7\mathrm{deg}/(\mathrm{s}{\mathrm{Hz}}^{1/2})$ and a bias stability of $0.4\mathrm{deg}/\mathrm{s}$.⁶⁵

4.4.3.

Mapping magnetic fields and detecting hidden targets

One important military application of diamond quantum sensors is the mapping of Earth’s magnetic fields, which is vital for navigation, geophysical surveillance, and detecting concealed targets.³⁶⁴ Mapping the Earth’s magnetic field or identifying magnetic anomalies caused by underground magnetic compositions or hidden objects holds significant potential for military use. Miniaturized, portable sensors with high magnetic field sensitivity can be integrated into unmanned aerial vehicles for remote detection of magnetic fields from both the Earth’s surface and hidden magnetic objects. This provides a highly effective tool for surveillance and reconnaissance.

Figure 31 illustrates examples of using diamond magnetometers to map magnetic fields from road surfaces and detect concealed magnetic objects.^{365 ,366} The sensor systems are mounted on a vehicle to probe changes in magnetic fields in both indoor and outdoor environments [Fig. 31(a)].³⁶⁵ In outdoor environments, several factors contribute to the total magnetic field, including Earth’s magnetic field, as well as permanent or induced fields from the vehicle and its control units. Using two magnetometers fixed to the vehicle and measuring the vector magnetic fields along 3D coordinates, Graham et al. effectively canceled out the permanent magnetic fields from the vehicle itself, enabling them to measure variations in the Earth’s core magnetic field along the vehicle’s track.³⁶⁵

On the other hand, miniaturized diamond sensors, integrated with a 2D scanning stage, are used to image stray magnetic fields from various sizes of magnets embedded in a toy diorama, as shown in Fig. 31(b).³⁶⁶ This setup emulates a scenario in which remote detection of landmines on a battlefield or concealed objects at construction sites is required. By comparing the measured data with simulations, Choi et al.³⁶⁶ were able to accurately estimate the location and size of the magnets. In addition, vector magnetometry was proposed to compensate for any relative tilt between the sensor and the target, reducing the uncertainty in determining the target’s location from several centimeters to tenths of millimeters.³⁶⁶