Beyond Silicon: 2D Materials and the Quest for Quantum Sensors

Author: Denis Avetisyan

A new generation of nanoscale quantum sensors is emerging, leveraging the unique properties of two-dimensional materials to push the boundaries of precision measurement.

This review details the progress and challenges in utilizing spin defects in 2D materials-particularly hexagonal boron nitride-for advancing quantum sensing technologies and exploring coherence enhancement strategies.

While realizing scalable quantum technologies demands materials exhibiting both quantum coherence and facile integration, two-dimensional (2D) materials are rapidly emerging as a uniquely versatile platform, as comprehensively detailed in this Roadmap: 2D Materials for Quantum Technologies. This review highlights recent progress in exploiting atomic-scale control and novel quantum phenomena-from spin defects in hexagonal boron nitride to correlated states in Moiré superlattices-for advances in sensing, computation, and communication. By outlining key challenges in defect control and scalable integration alongside opportunities driven by machine learning, this Roadmap positions 2D materials as foundational building blocks-but can these materials truly bridge the gap between microscopic quantum states and macroscopic, functional devices?

Unveiling Quantum Potential: The Delicate Art of Spin Control

Quantum sensing’s potential for revolutionary measurement hinges on the precise control and detection of spin states – the intrinsic angular momentum of particles. Achieving unprecedented sensitivity isn’t simply about detecting a signal, but about isolating and maintaining the delicate quantum properties of these spins for as long as possible. A spin’s quantum state, representing either ‘up’ or ‘down’, is incredibly susceptible to environmental noise, leading to decoherence – the loss of quantum information. Therefore, the development of robust quantum sensors demands materials and techniques that can shield these spin states from external disturbances, allowing for prolonged observation and highly accurate measurements of even the faintest signals. This requires not only minimizing interactions with the surroundings but also actively manipulating and reading out the spin state without collapsing its quantum properties, a significant challenge driving current research in the field.

Conventional magnetic field sensors, such as those based on the Hall effect or giant magnetoresistance, struggle to detect magnetic signals at the nanoscale. Their operational principles rely on measuring averaged magnetic properties over relatively large areas, inherently blurring the ability to resolve features smaller than a few micrometers. This limitation stems from a fundamental trade-off: increasing sensitivity often compromises spatial resolution, and vice versa. Consequently, applications demanding high-resolution magnetic imaging-like analyzing individual magnetic domains in materials, detecting weak magnetic fields emitted by biological samples, or reading data stored on high-density magnetic storage media-remain largely inaccessible with these established technologies. The inability to pinpoint magnetic sources with sufficient precision restricts their effectiveness in emerging fields and underscores the need for alternative sensing modalities capable of operating at the nanoscale with both high sensitivity and resolution.

The pursuit of increasingly sensitive quantum sensors has led researchers to deliberately exploit imperfections within otherwise perfect materials. These imperfections, known as spin defects, arise from missing or misplaced atoms in a crystal lattice, creating localized electron spins that are exquisitely sensitive to external fields. Unlike traditional magnetic sensors, which rely on collective material properties, these defects offer a nanoscale sensing volume, promising significantly enhanced spatial resolution. The quantum state of the electron spin within the defect can be precisely controlled and read out using techniques like optical or microwave spectroscopy, allowing for the detection of incredibly weak magnetic, electric, or thermal signals. This approach opens doors to applications ranging from biological imaging and materials science to fundamental tests of physics, effectively turning atomic-scale flaws into powerful tools for measurement and discovery.

The realization of practical quantum sensors hinges critically on the meticulous selection and characterization of spin defects within host materials. These atomic-scale imperfections, while offering the potential for exceptionally sensitive measurements, are inherently fragile; maintaining quantum coherence – the ability of a quantum system to exist in multiple states simultaneously – is paramount, yet easily disrupted by environmental noise. Researchers are actively investigating a diverse range of materials, from diamond and silicon carbide to hexagonal boron nitride, seeking defects that exhibit both long coherence times and strong optical or microwave signals. Detailed characterization, employing techniques like electron spin resonance and optical spectroscopy, is essential to identify defects with favorable properties and to understand the factors limiting their performance. Optimizing material purity, defect density, and surrounding crystal structure allows for the engineering of quantum sensors with enhanced sensitivity and stability, paving the way for breakthroughs in fields ranging from medical imaging to materials science.

Two-Dimensional Materials: A Platform for Quantum Precision

The quantum properties exhibited by two-dimensional (2D) materials, including hexagonal Boron Nitride (hBN) and Transition Metal Dichalcogenides (TMDs), are directly attributable to their atomic thickness. This extreme confinement of electrons leads to enhanced quantum effects, such as increased electron-hole interactions and modified electronic band structures. Specifically, the reduced dimensionality alters the density of states, resulting in stronger excitonic effects in TMDs and the creation of localized states suitable for quantum information processing. Furthermore, the van der Waals interaction between layers in these materials allows for the creation of heterostructures with tailored electronic and optical properties, offering a platform for engineering novel quantum devices. The limited number of atoms in the vertical direction also reduces the scattering volume for charge carriers, contributing to higher mobilities and longer coherence times.

The atomic-scale thickness of two-dimensional materials facilitates the spatial confinement of spin defects, such as nitrogen-vacancy (NV) centers in hexagonal Boron Nitride (hBN). This confinement increases defect stability by reducing the probability of defect migration and annihilation. Furthermore, restricting the defect’s environment minimizes interactions with fluctuating external fields and reduces the influence of lattice vibrations – primary sources of environmental noise that contribute to decoherence. By isolating the spin from external disturbances, confinement effectively extends spin coherence times, a critical parameter for applications in quantum sensing and information processing.

Hexagonal Boron Nitride (hBN) functions as an effective platform for quantum spins due to its insulating properties. This characteristic significantly reduces charge-related decoherence, a primary limitation in many quantum systems. Decoherence arises from interactions between spin states and fluctuating electric fields caused by mobile charge carriers; as hBN lacks mobile charge carriers, these interactions are minimized. Consequently, spin coherence times – the duration for which a quantum state remains stable – are substantially extended in hBN-based systems compared to materials with higher conductivity. This extended coherence is critical for performing complex quantum operations and improving the sensitivity of quantum sensors.

The creation of controlled defects within two-dimensional materials is fundamental to customizing their capabilities as quantum sensors. These defects, such as vacancies, interstitials, or substitutional impurities, act as localized quantum spins. The sensing properties, including sensitivity and operating frequency, are directly linked to the type, density, and spatial distribution of these engineered defects. By precisely controlling these parameters during material growth or through post-growth techniques like ion irradiation or focused electron beam manipulation, researchers can tailor the material’s response to external stimuli – such as magnetic fields, electric fields, or strain – and optimize performance for specific quantum sensing applications. Furthermore, the concentration of defects influences the signal strength and detection limit of the resulting quantum sensor.

Engineering Spin Defects in hBN: Precision at the Atomic Scale

The Boron Vacancy (VB-) center in hexagonal boron nitride (hBN) represents a point defect where a boron atom is missing from the lattice structure. This vacancy creates a spin-1 ground state, meaning the defect possesses three energy levels corresponding to different spin orientations. This spin-1 property is crucial for quantum sensing because it allows for highly sensitive detection of external fields – electric, magnetic, and strain – through manipulation and readout of the spin state. The relatively simple electronic structure of the VB- center, combined with its optical addressability, facilitates coherent control and measurement, making it a leading candidate for nanoscale sensing platforms and quantum information processing. Furthermore, the defect’s neutrality minimizes charge-related noise, enhancing sensor stability and performance.

Carbon-related defects within the hexagonal boron nitride (hBN) lattice are emerging as viable alternatives to boron vacancy (VB-) centers for quantum sensing applications due to their notable spin characteristics. These defects exhibit hyperfine coupling, a measurable interaction between the electron spin of the defect and the nuclear spins of nearby carbon or boron isotopes, with observed values reaching up to 300 MHz. This strong hyperfine coupling indicates significant electron-nuclear interaction, which enhances the sensitivity of these defects to external magnetic fields and electric fields, and provides a mechanism for spin readout and control. The degree of hyperfine coupling is dependent on the specific carbon defect configuration and isotopic composition of the hBN material, influencing the coherence properties and potential applications of these spin probes.

The Spin-Pair Model is a theoretical construct used to describe the interactions between two spin defects within the hexagonal boron nitride (hBN) lattice. This model accounts for the dipole-dipole coupling and hyperfine interactions between the electron spins and nuclear spins of the defect pair, allowing for the prediction of energy level splittings and coherence properties. Specifically, the model utilizes parameters such as the distance between defects, the orientation of their spin axes, and the strength of hyperfine coupling to simulate the system’s behavior. Accurate modeling with the Spin-Pair Model is critical for optimizing sensor designs by enabling the prediction of sensitivity, dynamic range, and optimal operating frequencies based on the characteristics of the chosen defect pair configuration.

Comprehensive characterization of spin defects in hBN necessitates the employment of multiple advanced spectroscopic and measurement techniques. Electron paramagnetic resonance (EPR) spectroscopy is used to determine energy level structure and identify defect species, while optically detected magnetic resonance (ODMR) provides sensitivity enhancements and allows for investigation of spin dynamics. Coherence times, crucial for quantum sensing applications, are typically measured via Ramsey or Hahn echo experiments. Furthermore, assessing sensitivity to external stimuli – such as electric and magnetic fields, strain, and temperature – requires precise control of the sample environment coupled with sensitive detection methods, often utilizing diamond nitrogen-vacancy (NV) centers as a reference magnetometer to correlate defect behavior with external conditions. These combined approaches allow for a detailed understanding of defect properties and optimization for specific sensing applications.

Boosting Coherence: Refinements in Material and Control

Isotopic engineering in hexagonal boron nitride (hBN) leverages the principle that naturally occurring boron has two stable isotopes, ¹⁰B and ¹¹B, both possessing nuclear spin. These nuclear spins act as fluctuating magnetic fields, contributing to decoherence of quantum states in nearby defects. By enriching hBN with ¹¹B, which has a smaller nuclear spin, or utilizing isotopically pure ¹¹B, the overall nuclear spin noise is substantially reduced. This suppression of fluctuating magnetic fields directly extends the coherence times of electron or hole spins associated with defects in hBN, enabling more prolonged observation and highly accurate manipulation of quantum information.

Dynamical decoupling is a technique used to mitigate decoherence caused by fluctuating magnetic fields. It operates by applying a sequence of precisely timed radiofrequency or microwave pulses to the quantum system. These pulses effectively average out the effects of the fluctuating fields, suppressing transitions between quantum states induced by magnetic noise. The pulse sequence is designed such that the accumulated phase shift due to the fluctuating fields is cancelled or minimized, thereby extending the coherence time. The effectiveness of dynamical decoupling is dependent on the specific pulse sequence employed and the frequency spectrum of the noise, with sequences tailored to address specific noise environments.

Recent advances in hexagonal boron nitride (hBN) quantum sensing have demonstrated coherence times exceeding hundreds of nanoseconds at room temperature. This represents a substantial improvement over typical coherence times of tens of nanoseconds observed in standard hBN materials. This enhancement is achieved through the combined application of isotopic purification – specifically, reducing the concentration of ¹¹B and ¹³C isotopes with nuclear spin – and dynamical decoupling techniques. Dynamical decoupling utilizes a sequence of precisely timed microwave pulses to mitigate the effects of fluctuating magnetic fields, thereby suppressing decoherence. The synergistic effect of these methods significantly extends the period for which quantum information can be reliably stored and manipulated within hBN-based sensors.

Carbon-related defects within hexagonal boron nitride (hBN) demonstrate remarkably long nuclear spin coherence times, exceeding one hundred microseconds at room temperature. This characteristic is notable as most nuclear spins in naturally occurring materials exhibit coherence times on the order of seconds at cryogenic temperatures, or milliseconds at room temperature. The extended coherence observed in carbon-related defects is attributed to the low density of other nuclear spin species in close proximity and the relative isolation of ¹³C nuclear spins within the hBN lattice. This extended coherence makes these defects promising candidates for use as quantum sensors and for the storage of quantum information.

The realization of practical quantum sensors utilizing defects in hexagonal boron nitride (hBN) necessitates the combined application of isotopic engineering, dynamical decoupling, and meticulous material preparation. Isotopic purification of hBN minimizes decoherence caused by naturally occurring nuclear spins, while dynamical decoupling sequences further suppress fluctuations in the magnetic environment. These techniques extend spin coherence times from tens to hundreds of nanoseconds at room temperature, a critical improvement for enhancing sensor sensitivity and measurement fidelity. Careful control over material growth and processing is also essential to minimize the density of defects that can introduce noise and limit performance, ultimately enabling the development of robust and reliable quantum sensing devices.

Beyond hBN: Expanding the Horizon of Quantum Sensing

Boron Nitride Nanotubes (BNNTs) present a compelling alternative to traditional materials in scanning probe magnetometry due to their distinct structural and electronic properties. Unlike their carbon nanotube counterparts, BNNTs possess an intrinsic piezoelectricity and a wide band gap, offering enhanced sensitivity and reduced noise in magnetic field detection. Their hollow cylindrical geometry allows for efficient spin manipulation and readout, while nitrogen vacancies within the lattice act as sensitive spin centers. These defects exhibit coherent spin states that can be precisely controlled using electromagnetic radiation, enabling nanoscale resolution in magnetic imaging. Researchers are actively exploring methods to integrate BNNTs into scanning probes, aiming to create devices capable of mapping magnetic fields at the single-molecule level and characterizing materials with unprecedented precision. The unique combination of geometry and spin response positions BNNTs as a key component in the next generation of quantum sensors.

Research initially focused on hexagonal boron nitride (hBN) has revealed fundamental principles applicable to a much wider range of two-dimensional materials. The sensitivity of quantum sensors doesn’t solely rely on the specific material, but rather on the creation and control of defects within its structure – these imperfections act as localized quantum systems. Consequently, scientists are now actively exploring analogous defects in materials like transition metal dichalcogenides and other layered compounds, leveraging established techniques for defect engineering and spin manipulation. This expansion beyond hBN dramatically broadens the landscape of potential quantum sensors, allowing for tailored designs optimized for specific applications and sensing modalities, and ultimately promising sensors capable of detecting increasingly subtle signals with greater precision.

The recent progress in quantum sensing, driven by materials like hexagonal boron nitride, is poised to revolutionize several fields through dramatically improved imaging capabilities. Researchers anticipate the development of sensors capable of mapping magnetic fields with unprecedented resolution – moving beyond current limitations to visualize phenomena at the nanoscale. This extends beyond magnetism; these advancements also promise highly localized temperature measurements, allowing for the study of heat dissipation in nanoscale devices and biological processes. Perhaps most significantly, the heightened sensitivity offered by these sensors opens the door to detecting and characterizing single molecules, with implications for early disease diagnosis, drug discovery, and materials science – essentially allowing scientists to ‘see’ the building blocks of life and matter in entirely new ways.

The trajectory of quantum sensing is increasingly defined by a synergistic relationship between materials science and control engineering. Researchers are actively pursuing novel materials beyond established platforms like hexagonal boron nitride, exploring two-dimensional systems and defects engineered at the nanoscale to maximize sensitivity and tailor functionality. However, material innovation alone is insufficient; advancements in control techniques – including precise optical and microwave manipulation, sophisticated readout schemes, and the implementation of dynamic decoupling sequences – are crucial to harness the full potential of these new sensors. This combined approach promises to not only enhance existing capabilities, such as magnetic field imaging with sub-nanometer resolution and single-molecule detection, but also to unlock entirely new sensing modalities, potentially revolutionizing fields ranging from materials science and biology to medicine and fundamental physics. The ultimate goal is to create quantum sensors that are not merely more sensitive, but also more versatile, robust, and integrated into practical devices.

The pursuit of quantum coherence in 2D materials, as detailed in the roadmap, echoes a fundamental principle of elegant design. Achieving extended coherence times within spin defects – particularly those involving vanadium-boron in hexagonal boron nitride – isn’t merely about technical advancement; it’s about refining a system to its essential state. As Werner Heisenberg observed, “The very act of observing changes an object.” This resonates deeply with the challenges outlined in the research; manipulating and measuring quantum states necessitates minimizing disturbance, striving for a harmonious interaction between observer and observed. Beauty scales – clutter doesn’t – and in this context, ‘clutter’ represents any decoherence mechanism that obscures the delicate quantum signal.

The Horizon Beckons

The pursuit of robust, scalable quantum sensing with two-dimensional materials has revealed a fundamental truth: defect engineering is not merely a technical challenge, but an exercise in material conversation. Vanadium-boron defects in hexagonal boron nitride represent a promising dialect, yet true fluency demands far greater control over defect density, spatial distribution, and – crucially – the surrounding nuclear spin environment. Current methods, while advancing, still feel akin to sculpting with fog; precision is elusive, and reproducibility, a persistent ghost.

The field now requires a shift in emphasis. Simply finding new defects is insufficient; the focus must turn to understanding how these defects interact – not just with electromagnetic fields, but with each other. A lone spin is a curiosity; a networked ensemble, a potential computational substrate. This necessitates exploration beyond hBN, venturing into other 2D platforms – materials where native defects are fewer, or where tailored defects can be reliably introduced. The elegance lies not in brute-force creation, but in harmonious integration.

Ultimately, the success of this endeavor hinges on a willingness to embrace complexity, and to abandon the illusion of perfect control. Nature rarely offers simplicity; it prefers nuance, and demands respect. The path forward is not paved with flawless crystals, but with a deep understanding of imperfection – a recognition that sometimes, the most profound signals arise from the subtle whispers within the noise.

Original article: https://arxiv.org/pdf/2512.14973.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/