arXiv subjects

David A. Simpson

Publications and source records attributed to David A. Simpson.

At least 19 recordsLinked to original sources

arXiv2026

Method for real-time monitoring of paramagnetic reactions using spin relaxometry with fluorescent nanodiamonds

Spin relaxometry using fluorescent nanodiamonds (FNDs) has been applied successfully to sense numerous paramagnetic target molecules such as free radicals and metalloproteins. However, despite their high sensitivity, T1 spin relaxation measurements are often hampered by their slow acquisition speed. Here, we demonstrate a method that allows for real-time monitoring of paramagnetic chemical reactions. We demonstrate T1 spin relaxometry from thousands of FNDs using an optimised cuvette-based system integrating an avalanche photodiode operated in linear mode, and a fast, fieldprogrammable gate array (FPGA) for data collation. We demonstrate chemical monitoring of the reduction of Cu(II) to Cu(I) ions in-solution with a 15 second integration using an optimised T1 sensing protocol. Our method achieves more than two orders of magnitude speed up with an order of magnitude reduction in cost when compared with traditional techniques. With further technical improvements, we believe this in-solution method could be extended to sense the sub-second chemical kinetics of paramagnetic molecules in solution.

cond-mat.mes-hall↗

arXiv2026

Rotating fluorescent nanodiamond assemblies with focused Laguerre-Gaussian beams

Optical tweezers which utilize structured light fields enable the rotation of trapped nanoparticles through the transfer of orbital angular momentum (OAM) from holographically generated Laguerre-Gaussian (LG) modes. In this research we use OAM transfer to demonstrate controlled rotation of bright fluorescent nanodiamond clusters assembled in a focused higher-order LG beam. We find that the assemblies can be effectively rotated in a two-dimensional optical trap with orbital frequencies of up to 5 Hz. We use video tracking to explore the Brownian dynamics of such a trapping arrangement and look at the impact of orientation stability on measurements of optically detected magnetic resonance (ODMR) with an applied weak external magnetic field. By collecting ODMR spectra at multiple points along the orbit, we show that the constrained two-dimensional motion can provide additional insights for vector magnetic field reconstruction.

physics.optics↗

arXiv2025

A method for robust spin relaxometry in the presence of imperfect state preparation

Spin relaxometry based on quantum spin systems has developed as a valuable tool in medical and condensed matter systems, offering the advantage of operating without the need for external DC or RF fields. Spin relaxometry with nitrogen-vacancy (NV) centers has been applied to paramagnetic sensing using both single crystal diamond and nanodiamond materials. However, these methods often suffer from artifacts and systematic uncertainties, particularly due to imperfect spin state preparation, leading to artificially fast T$_1$ relaxation times. Current analysis techniques fail to adequately account for these issues, limiting the precision of parameter estimation. In this work, we introduce a minimal fitting procedure that enables more robust parameter estimation in the presence of imperfect spin polarization. Our model improves upon existing approaches by offering more accurate fits and provides a framework for efficiently parallelizing single-spin dynamics studies.

quant-ph↗

arXiv2025

Machine learning assisted tracking of magnetic objects using quantum diamond magnetometry

Remote magnetic sensing can be used to monitor the position of objects in real-time, enabling ground transport monitoring, underground infrastructure mapping and hazardous detection. However, magnetic signals are typically weak and complex, requiring sophisticated physical models to analyze them and a detailed knowledge of the system under study, factors that are frequently unavailable. In this work, we provide a solution to these limitations by demonstrating a Machine Learning (ML) method that can be trained exclusively on experimental data, without the need of any physical model, to predict the position of a magnetic target in real-time. The target can be any object with a magnetic signal above the floor noise, and in this case we use a quantum diamond magnetometer to track variations of few hundreds of nanoteslas produced by an elevator moving along a single axis. The one-dimensional movement is a simple yet challenging scenario, resembling realistic environments such as high buildings, tunnels or train circuits, and is the first step towards building broader applications. Our ML algorithm can be trained in approximately 40 min, achieving over 80% accuracy in predicting the target's position at a rate of 10 Hz, for a positional error tolerance of 30 cm, which is a precise distance compared to the 4-meter spacing between parking levels. Our results open up the possibility to apply this ML method more generally for real-time monitoring of magnetic objects, which will broaden the scope of magnetic detection applications.

physics.app-ph↗

arXiv2024

Methods for color center preserving hydrogen-termination of diamond

Chemical functionalization of diamond surfaces by hydrogen is an important method for controlling the charge state of near-surface fluorescent color centers, an essential process in fabricating devices such as diamond field-effect transistors and chemical sensors, and a required first step for realizing families of more complex terminations through subsequent chemical processing. In all these cases, termination is typically achieved using hydrogen plasma sources which can etch or damage the diamond as well as deposited materials or embedded colour centers. This work explores alternative methods for lower-damage hydrogenation of diamond surfaces, specifically the annealing of diamond samples in high-purity, non-explosive mixtures of nitrogen and hydrogen gas, and the exposure of samples to microwave hydrogen plasmas in the absence of intentional stage heating. The effectiveness of these methods are characterized by x-ray photoelectron spectroscopy, and comparison of the results to density-functional modelling of the surface hydrogenation energetics implicates surface oxygen ligands as the primary factor limiting the termination quality of annealed samples. Finally, photoluminescence spectroscopy is used to verify that both the annealing and reduced sample temperature plasma methods are non-destructive to near-surface ensembles of nitrogen-vacancy centers, in stark contrast to plasma treatments which use heated sample stages.

cond-mat.mtrl-sci↗

arXiv2022

Method for in-solution, high-throughput T1 relaxometry using fluorescent nanodiamonds

Fluorescent nanodiamonds (FNDs) have been exploited as sensitive quantum probes for nanoscale chemical and biological sensing applications, with the majority of demonstrations to date relying on the detection of single FNDs. This places significant limits on the measurement time, throughput and statistical significance of a measured result as there is usually marked inhomogeneity within FND samples. Here we have developed a measurement platform that can report the T1 spin relaxation time from a large ensemble of FNDs in solution. We first describe a refined sensing protocol for this modality and then use it to identify the optimal FND size for the detection of paramagnetic targets. Our approach is simple to set up, robust and can be used for rapid material characterisation or a variety of in-situ quantum sensing applications.

quant-ph↗

arXiv2021

Proximal nitrogen reduces the fluorescence quantum yield of nitrogen-vacancy centres in diamond

The nitrogen-vacancy colour centre in diamond is emerging as one of the most important solid-state quantum systems. It has applications to fields including high-precision sensing, quantum computing, single photon communication, metrology, nanoscale magnetic imaging and biosensing. For all of these applications, a high quantum yield of emitted photons is desirable. However, diamond samples engineered to have high densities of nitrogen-vacancy centres show levels of brightness varying significantly within single batches, or even within the same sample. Here we show that nearby nitrogen impurities quench emission of nitrogen-vacancy centres via non-radiative transitions, resulting in a reduced fluorescence quantum yield. We monitored the emission properties of nitrogen-vacancy centre ensembles from synthetic diamond samples with different concentrations of nitrogen impurities. While at low nitrogen densities of 1.81 ppm we measured a lifetime of 13.9 ns, we observed a strong reduction in lifetime with increasing nitrogen density. We measure a lifetime as low as 4.4 ns at a nitrogen density of 380 ppm. The change in lifetime matches a reduction in relative fluorescence quantum yield from 77.4 % to 32 % with an increase in nitrogen density from 88 ppm to 380 ppm, respectively. These results will inform the conditions required to optimise the properties of diamond crystals devices based on the fluorescence of nitrogen-vacancy centres. Furthermore, this work provides insights into the origin of inhomogeneities observed in high-density nitrogen-vacancy ensembles within diamonds and nanodiamonds.

quant-ph↗

arXiv2020

Fluorescent diamond microparticles doped glass fiber for magnetic field sensing

Diamond containing the negatively charged nitrogen-vacancy (NV) center is emerging as a significant new system for magnetometry. However, most NV sensors require microscopes to collect the fluorescence signals and are therefore limited to laboratory settings. By incorporating micron-scale diamond particles at an annular interface within the cross section of a silicate glass fiber, a high-sensitivity and robust fiber platform for magnetic field sensing is demonstrated here. The fluorescence and spin properties of NV centers embedded in the diamond crystals are well preserved during the fiber drawing process, leading to enhanced continuous-wave diamond-magnetometry in fiber-transmitted sensing configurations. The interface doping of diamond particles also leads to reduced fiber propagation loss and benefits the guidance of NV-fluorescence in the hybrid fiber. Using the diamond-fiber system, magnetic field readout through 50 cm of fiber is achieved. This study paves the way for novel fiber-based diamond sensors for field-deployable quantum metrology applications.

physics.app-ph↗

arXiv2019

Quantum magnetic imaging of iron biomineralisation in teeth of the chiton Acanthopleura hirtosa

Iron biomineralisation is critical for life. Nature capitalises on the physical attributes of iron biominerals for a variety of functional, structural and sensory applications. Although magnetism is an integral property of iron biominerals, the role it plays in their nano-assembly remains a fundamental, unanswered question. This is well exemplified by the magnetite-bearing radula of chitons. Chitons, a class of marine mollusc, create the hardest biomineral of any animal in their abrasion-resistant, self-sharpening teeth4. Despite this system being subjected to a range of high resolution imaging studies, the mechanisms that drive mineral assembly remain unresolved. However, the advent of quantum imaging technology provides a new avenue to probe magnetic structures directly. Here we use quantum magnetic microscopy, based on nitrogen-vacancy centres in diamond, to attain the first subcellular magnetic profiling of a eukaryotic system. Using complementary magnetic imaging protocols, we spatially map the principal mineral phases (ferrihydrite and magnetite) in the developing teeth of Acanthopleura hirtosa with submicron resolution. The images reveal previously undiscovered long-range magnetic order, established at the onset of magnetite mineralisation. This is in contrast to electron microscopy studies that show no strong common crystallographic orientation. The quantum-based magnetic profiling techniques presented in this work have broad application in biology, earth science, chemistry and materials engineering and can be applied across the range of systems for which iron is vital.

physics.bio-ph↗

arXiv2018

Non-neurotoxic Nanodiamond Probes for Intraneuronal Temperature Mapping

Optical bio-markers have been used extensively for intracellular imaging with high spatial and temporal resolution. Extending the modality of these probes is a key driver in cell biology. In recent years, the NV centre in nanodiamond has emerged as a promising candidate for bio-imaging and bio-sensing with low cytotoxicity and stable photoluminescence. Here we study the electrophysiological effects of this quantum probe in primary cortical neurons. Multi-electrode array (MEA) recordings across five replicate studies showed no statistically significant difference in 25 network parameters when nanodiamonds are added at varying concentrations over various time periods 12-36 hr. The physiological validation motivates the second part of the study which demonstrates how the quantum properties of these biomarkers can be used to report intracellular information beyond their location and movement. Using the optically detected magnetic resonance from the nitrogen vacancy defects within the nanodiamonds we demonstrate enhanced signal-to-noise imaging and temperature mapping from thousands of nanodiamond probes simultaneously. This work establishes nanodiamonds as viable multi-functional intraneuronal sensors with nanoscale resolution, that may ultimately be used to detect magnetic and electrical activity at the membrane level in excitable cellular systems.

physics.bio-ph↗

arXiv2017

Quantum probe hyperpolarisation of molecular nuclear spins

The hyperpolarisation of nuclear spins within target molecules is a critical and complex challenge in magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) spectroscopy. Hyperpolarisation offers enormous gains in signal and spatial resolution which may ultimately lead to the development of molecular MRI and NMR. At present, techniques used to polarise nuclear spins generally require low temperatures and/or high magnetic fields, radio-frequency control fields, or the introduction of catalysts or free-radical mediators. The emergence of room temperature solid-state spin qubits has opened exciting new pathways to circumvent these requirements to achieve direct nuclear spin hyperpolarisation using quantum control. Employing a novel cross-relaxation induced polarisation (CRIP) protocol using a single nitrogen-vacancy (NV) centre in diamond, we demonstrate the first external nuclear spin hyperpolarisation achieved by a quantum probe, in this case of $^1$H molecular spins in poly(methyl methacrylate). In doing so, we show that a single qubit is capable of increasing the thermal polarisation of $\sim 10^6$ nuclear spins by six orders of magnitude, equivalent to an applied magnetic field of $10^5$\,T. The technique can also be tuned to multiple spin species, which we demonstrate using both \C{13} and $^1$H nuclear spin ensembles. Our results are analysed and interpreted via a detailed theoretical treatment, which is also used to describe how the system can be scaled up to a universal quantum hyperpolarisation platform for the production of macroscopic quantities of contrast agents at high polarisation levels for clinical applications. These results represent a new paradigm for nuclear spin hyperpolarisation for molecular imaging and spectroscopy, and beyond into areas such as materials science and quantum information processing.

quant-ph↗

arXiv2017

Nanomechanical sensing using spins in diamond

Nanomechanical sensors and quantum nanosensors are two rapidly developing technologies that have diverse interdisciplinary applications in biological and chemical analysis and microscopy. For example, nanomechanical sensors based upon nanoelectromechanical systems (NEMS) have demonstrated chip-scale mass spectrometry capable of detecting single macromolecules, such as proteins. Quantum nanosensors based upon electron spins of negatively-charged nitrogen-vacancy (NV) centers in diamond have demonstrated diverse modes of nanometrology, including single molecule magnetic resonance spectroscopy. Here, we report the first step towards combining these two complementary technologies in the form of diamond nanomechanical structures containing NV centers. We establish the principles for nanomechanical sensing using such nano-spin-mechanical sensors (NSMS) and assess their potential for mass spectrometry and force microscopy. We predict that NSMS are able to provide unprecedented AC force images of cellular biomechanics and to, not only detect the mass of a single macromolecule, but also image its distribution. When combined with the other nanometrology modes of the NV center, NSMS potentially offer unparalleled analytical power at the nanoscale.

quant-ph↗

arXiv2017

Environmentally mediated coherent control of a spin qubit in diamond

The coherent control of spin qubits forms the basis of many applications in quantum information processing and nanoscale sensing, imaging and spectroscopy. Such control is conventionally achieved by direct driving of the qubit transition with a resonant global field, typically at microwave frequencies. Here we introduce an approach that relies on the resonant driving of nearby environment spins, whose localised magnetic field in turn drives the qubit when the environmental spin Rabi frequency matches the qubit resonance. This concept of environmentally mediated resonance (EMR) is explored experimentally using a qubit based on a single nitrogen-vacancy (NV) centre in diamond, with nearby electronic spins serving as the environmental mediators. We demonstrate EMR driven coherent control of the NV spin-state, including the observation of Rabi oscillations, free induction decay, and spin-echo. This technique also provides a way to probe the nanoscale environment of spin qubits, which we illustrate by acquisition of electron spin resonance spectra of single NV centres in various settings.

cond-mat.mes-hall↗

arXiv2017

Quantum magnetic resonance microscopy

Magnetic resonance spectroscopy is universally regarded as one of the most important tools in chemical and bio-medical research. However, sensitivity limitations typically restrict imaging resolution to length scales greater than 10 μm. Here we bring quantum control to the detection of chemical systems to demonstrate high resolution electron spin imaging using the quantum properties of an array of nitrogen-vacancy (NV) centres in diamond. Our quantum magnetic resonance microscope selectively images electronic spin species by precisely tuning a magnetic field to bring the quantum probes into resonance with the external target spins. This provides diffraction limited spatial resolution of the target spin species over a field of view of ~50x50 μm^2. We demonstrate imaging and spectroscopy on aqueous Cu2+ ions over microscopic volumes (0.025 μm^3), with detection sensitivity at resonance of 104 spins/voxel, ~100 zeptomol (10^-19 mol). The ability to image, perform spectroscopy and dynamically monitor spin-dependent redox reactions and transition metal biochemistry at these scales opens up a new realm of nanoscopic electron spin resonance and zepto-chemistry in the physical and life sciences.

physics.bio-ph↗

arXiv2016

Microwave-Free Nuclear Magnetic Resonance at Molecular Scales

The implementation of nuclear magnetic resonance (NMR) at the nanoscale is a major challenge, as conventional systems require relatively large ensembles of spins and limit resolution to mesoscopic scales. New approaches based on quantum spin probes, such as the nitrogen-vacancy (NV) centre in diamond, have recently achieved nano-NMR under ambient conditions. However, the measurement protocols require application of complex microwave pulse sequences of high precision and relatively high power, placing limitations on the design and scalability of these techniques. Here we demonstrate a microwave-free method for nanoscale NMR using the NV centre, which is a far less invasive, and vastly simpler measurement protocol. By utilising a carefully tuned magnetic cross-relaxation interaction between a subsurface NV spin and an external, organic environment of proton spins, we demonstrate NMR spectroscopy of $^1$H within a $\approx(10~{\rm nm})^3$ sensing volume. We also theoretically and experimentally show that the sensitivity of our approach matches that of existing microwave control-based techniques using the NV centre. Removing the requirement for coherent manipulation of either the NV or the environmental spin quantum states represents a significant step towards the development of robust, non-invasive nanoscale NMR probes.

cond-mat.mes-hall↗

arXiv2016

Quantum imaging of current flow in graphene

Since its first isolation in 2004, graphene has been found to host a plethora of unusual electronic transport phenomena, making it a fascinating system for fundamental studies in condensed-matter physics as well as offering tremendous opportunities for future electronic and sensing devices. However, to fully realise these goals a major challenge is the ability to non-invasively image charge currents in monolayer graphene structures and devices. Typically, electronic transport in graphene has been investigated via resistivity measurements, however, such measurements are generally blind to spatial information critical to observing and studying landmark transport phenomena such as electron guiding and focusing, topological currents and viscous electron backflow in real space, and in realistic imperfect devices. Here we bring quantum imaging to bear on the problem and demonstrate high-resolution imaging of current flow in graphene structures. Our method utilises an engineered array of near-surface, atomic-sized quantum sensors in diamond, to map the vector magnetic field and reconstruct the vector current density over graphene geometries of varying complexity, from mono-ribbons to junctions, with spatial resolution at the diffraction limit and a projected sensitivity to currents as small as 1 μA. The measured current maps reveal strong spatial variations corresponding to physical defects at the sub-μm scale. The demonstrated method opens up an important new avenue to investigate fundamental electronic and spin transport in graphene structures and devices, and more generally in emerging two-dimensional materials and thin film systems.

cond-mat.mes-hall↗

arXiv2016

Spin dynamics of diamond nitrogen-vacancy centres at the ground state level anti-crossing and all-optical low frequency magnetic field sensing

We investigate the photo-induced spin dynamics of single nitrogen-vacancy (NV) centres in diamond near the electronic ground state level anti-crossing (GSLAC), which occurs at an axial magnetic field around 1024 G. Using optically detected magnetic resonance spectroscopy, we first find that the electron spin transition frequency can be tuned down to 100 kHz for the \NV{14} centre, while for the \NV{15} centre the transition strength vanishes for frequencies below about 2 MHz owing to the GSLAC level structure. Using optical pulses to prepare and readout the spin state, we observe coherent spin oscillations at 1024 G for the \NV{14}, which originate from spin mixing induced by residual transverse magnetic fields. This effect is responsible for limiting the smallest observable transition frequency, which can span two orders of magnitude from 100 kHz to tens of MHz depending on the local magnetic noise. A similar feature is observed for the \NV{15} centre at 1024 G. As an application of these findings, we demonstrate all-optical detection and spectroscopy of externally-generated fluctuating magnetic fields at frequencies from 8 MHz down to 500 kHz, using a \NV{14} centre. Since the Larmor frequency of most nuclear spin species lies within this frequency range near the GSLAC, these results pave the way towards all-optical, nanoscale nuclear magnetic resonance spectroscopy, using longitudinal spin cross-relaxation.

cond-mat.mes-hall↗

arXiv2016

Wide-band, nanoscale magnetic resonance spectroscopy using quantum relaxation of a single spin in diamond

We demonstrate a wide-band all-optical method of nanoscale magnetic resonance (MR) spectroscopy under ambient conditions. Our method relies on cross-relaxation between a probe spin, the electronic spin of a nitrogen-vacancy centre in diamond, and target spins as the two systems are tuned into resonance. By optically monitoring the spin relaxation time ($T_1$) of the probe spin while varying the amplitude of an applied static magnetic field, a frequency spectrum of the target spin resonances, a $T_1$-MR spectrum, is obtained. As a proof of concept, we measure $T_1$-MR spectra of a small ensemble of $^{14}$N impurities surrounding the probe spin within the diamond, with each impurity comprising an electron spin 1/2 and a nuclear spin 1. The intrinsically large bandwidth of the technique and probe properties allows us to detect both electron spin transitions -- in the GHz range -- and nuclear spin transitions -- in the MHz range -- of the $^{14}$N spin targets. The measured frequencies are found to be in excellent agreement with theoretical expectations, and allow us to infer the hyperfine, quadrupole and gyromagnetic constants of the target spins. Analysis of the strength of the resonances obtained in the $T_1$-MR spectrum reveals that the electron spin transitions are probed via dipole interactions, while the nuclear spin resonances are dramatically enhanced by hyperfine coupling and an electron-mediated process. Finally, we investigate theoretically the possibility of performing $T_1$-MR spectroscopy on nuclear spins without hyperfine interaction and predict single-proton sensitivity using current technology. This work establishes $T_1$-MR as a simple yet powerful technique for nanoscale MR spectroscopy, with broadband capability and a projected sensitivity down to the single nuclear spin level.

quant-ph↗