David A. Broadway

Quantum imaging of current flow in graphene

We demonstrate diamond-based quantum imaging of the current flow in graphene structures with submicrometer resolution. Since its first discovery 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. Typically, electronic transport in graphene has been investigated via resistivity measurements; however, these measurements are generally blind to spatial information critical to observing and studying landmark transport phenomena in real space and in realistic imperfect devices. We apply quantum imaging to the problem and demonstrate noninvasive, high-resolution imaging of current flow in monolayer graphene structures. Our method uses 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 monoribbons 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 submicrometer 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.

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

We demonstrate an all-optical approach of nanoscale magnetic resonance (MR) spectroscopy whereby quantum relaxation (T1) of a single probe spin in diamond is monitored during a precise static magnetic field sweep to construct a spectrum of the surrounding spin environment. The method is inherently noninvasive as it involves no driving fields, and instead relies on the natural resonance between the quantum probe and target spins. As a proof of concept, we measure the T1-MR spectra across a wide band [megahertz (MHz) to gigahertz (GHz)] of a small ensemble of N14 impurities surrounding a single probe spin, providing information on both electron spin transitions (in the GHz range) and nuclear spin transitions (in the MHz range) of the N14 spin targets. Analysis of the T1-MR spectrum reveals that the electron spin transitions are probed via dipole interactions with the probe, while the relatively weak nuclear spin resonances are dramatically enhanced by hyperfine coupling in an electron-mediated process. With a projected sensitivity to external single-proton spins, this work establishes T1-MR as a powerful noninvasive wide-band technique for nanoscale MR spectroscopy.

Microwave-free nuclear magnetic resonance at molecular scales

The implementation of nuclear magnetic resonance (NMR) at the nanoscale is a major challenge, as the resolution of conventional methods is limited to mesoscopic scales. Approaches based on quantum spin probes, such as the nitrogen-vacancy (NV) centre in diamond, have 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 NMR on a nanoscale organic environment of proton spins using the NV centre while eliminating the need for microwave manipulation of either the NV or the environmental spin states. We also show that the sensitivity of our significantly simplified approach matches that of existing techniques using the NV centre. Removing the requirement for coherent manipulation while maintaining measurement sensitivity represents a significant step towards the development of robust, non-invasive nanoscale NMR probes.

Anticrossing Spin Dynamics of Diamond Nitrogen-Vacancy Centers and All-Optical Low-Frequency Magnetometry

David A. Broadway, James D. A. Wood, Liam T. Hall, Alastair Stacey, 3 Matthew Markham, David A. Simpson, Jean-Philippe Tetienne, ∗ and Lloyd C. L. Hollenberg 2 Centre for Quantum Computation and Communication Technology, School of Physics, The University of Melbourne, VIC 3010, Australia School of Physics, The University of Melbourne, VIC 3010, Australia Element Six Innovation, Fermi Avenue, Harwell Oxford, Didcot, Oxfordshire OX110QR, United Kingdom (Dated: July 15, 2016)

Quantum probe hyperpolarisation of molecular nuclear spins

Hyperpolarisation of nuclear spins is important in overcoming sensitivity and resolution limitations of magnetic resonance imaging and nuclear magnetic resonance spectroscopy. Current hyperpolarisation techniques require high magnetic fields, low temperatures, or catalysts. Alternatively, the emergence of room temperature spin qubits has opened new pathways to achieve direct nuclear spin hyperpolarisation. Employing a microwave-free cross-relaxation induced polarisation protocol applied to a nitrogen vacancy qubit, we demonstrate quantum probe hyperpolarisation of external molecular nuclear spins to ~50% under ambient conditions, showing a single qubit increasing the polarisation of ~106 nuclear spins by six orders of magnitude over the thermal background. Results are verified against a detailed theoretical treatment, which also describes how the system can be scaled up to a universal quantum hyperpolarisation platform for macroscopic samples. Our results demonstrate the prospects for this approach to nuclear spin hyperpolarisation for molecular imaging and spectroscopy and its potential to extend beyond into other scientific areas.Molecules with ‘hyperpolarised’ nuclear spins can be used to improve MRI performance but require an efficient polarisation method. Broadway et al. demonstrate a quantum control protocol using a nitrogen vacancy centre inside a diamond to hyperpolarise protons within molecules deposited on the surface.

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 localized 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) center 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 from single NV centers in various settings.

Spatial mapping of band bending in semiconductor devices using in situ quantum sensors

Local variations in the charge distribution at semiconductor interfaces can lead to energy level band bending in the structure’s band diagram. Measuring this band bending is important in semiconductor electronics and quantum technologies, but current methods are typically only surface sensitive and are unable to probe the extent of band bending at a depth within the semiconductor. Here, we show that nitrogen–vacancy centres in diamond can be used as in situ sensors to spatially map band bending in a semiconductor device. These nitrogen–vacancy quantum sensors probe the electric field associated with surface band bending, and we map the electric field at different depths under various surface terminations. Using a two-terminal device based on the conductive two-dimensional hole gas formed at a hydrogen-terminated diamond surface, we also observe an unexpected spatial modulation of the electric field, which is attributed to the interplay between charge injection and photo-ionization effects (from the laser used in the experiments). Our method offers a route to the three-dimensional mapping of band bending in diamond and other semiconductors that host suitable quantum sensors.Nitrogen–vacancy centres can be used as in situ quantum sensors to map the electric field in an electrical device based on a hydrogen-terminated diamond surface.

Proximity-Induced Artefacts in Magnetic Imaging with Nitrogen-Vacancy Ensembles in Diamond

Magnetic imaging with ensembles of nitrogen-vacancy (NV) centres in diamond is a recently developed technique that allows for quantitative vector field mapping. Here we uncover a source of artefacts in the measured magnetic field in situations where the magnetic sample is placed in close proximity (a few tens of nm) to the NV sensing layer. Using magnetic nanoparticles as a test sample, we find that the measured field deviates significantly from the calculated field, in shape, amplitude and even in sign. By modelling the full measurement process, we show that these discrepancies are caused by the limited measurement range of NV sensors combined with the finite spatial resolution of the optical readout. We numerically investigate the role of the stand-off distance to identify an artefact-free regime, and discuss an application to ultrathin materials. This work provides a guide to predict and mitigate proximity-induced artefacts that can arise in NV-based wide-field magnetic imaging, and also demonstrates that the sensitivity of these artefacts to the sample can make them a useful tool for magnetic characterisation.

High precision single qubit tuning via thermo-magnetic field control

Precise control of the resonant frequency of a spin qubit is of fundamental importance to quantum sensing protocols. We demonstrate a control technique on a single nitrogen-vacancy (NV) centre in diamond where the applied magnetic field is modified by fine-tuning a permanent magnets magnetisation via temperature control. Through this control mechanism, nanoscale cross-relaxation spectroscopy of both electron and nuclear spins in the vicinity of the NV centre are performed. We then show that through maintaining the magnet at a constant temperature an order of magnitude improvement in the stability of the NV qubit frequency can be achieved. This improved stability is tested in the polarisation of a small ensemble of nearby

Spin properties of dense near-surface ensembles of nitrogen-vacancy centers in diamond

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