arXiv subjects

R. L. Walsworth

Publications and source records attributed to R. L. Walsworth.

At least 19 recordsLinked to original sources

arXiv2022

Snowmass2021 Cosmic Frontier Dark Matter Direct Detection to the Neutrino Fog

We present a summary of future prospects for direct detection of dark matter within the GeV/c2 to TeV/c2 mass range. This is paired with a new definition of the neutrino fog in order to better quantify the rate of diminishing returns on sensitivity due to irreducible neutrino backgrounds. A survey of dark matter candidates predicted to fall within this mass range demonstrates that fully testing multiple well-motivated theo-ries will require expanding the currently-funded generation of experiments down to and past the neutrino fog. We end with the status and plans for next-generation exper-iments and novel R&D concepts which will get us there.

hep-ex↗

arXiv2021

Estimating Magnetic Filling Factors From Simultaneous Spectroscopy and Photometry: Disentangling Spots, Plage, and Network

State of the art radial velocity (RV) exoplanet searches are limited by the effects of stellar magnetic activity. Magnetically active spots, plage, and network regions each have different impacts on the observed spectral lines, and therefore on the apparent stellar RV. Differentiating the relative coverage, or filling factors, of these active regions is thus necessary to differentiate between activity-driven RV signatures and Doppler shifts due to planetary orbits. In this work, we develop a technique to estimate feature-specific magnetic filling factors on stellar targets using only spectroscopic and photometric observations. We demonstrate linear and neural network implementations of our technique using observations from the solar telescope at HARPS-N, the HK Project at the Mt. Wilson Observatory, and the Total Irradiance Monitor onboard SORCE. We then compare the results of each technique to direct observations by the Solar Dynamics Observatory (SDO). Both implementations yield filling factor estimates that are highly correlated with the observed values. Modeling the solar RVs using these filling factors reproduces the expected contributions of the suppression of convective blueshift and rotational imbalance due to brightness inhomogeneities. Both implementations of this technique reduce the overall activity-driven RMS RVs from 1.64 m/s to 1.02 m/s, corresponding to a 1.28 m/s reduction in the RMS variation. The technique provides an additional 0.41 m/s reduction in the RMS variation compared to traditional activity indicators.

astro-ph.SR↗

arXiv2021

Detection Limits of Low-mass, Long-period Exoplanets Using Gaussian Processes Applied to HARPS-N Solar RVs

Radial velocity (RV) searches for Earth-mass exoplanets in the habitable zone around Sun-like stars are limited by the effects of stellar variability on the host star. In particular, suppression of convective blueshift and brightness inhomogeneities due to photospheric faculae/plage and starspots are the dominant contribution to the variability of such stellar RVs. Gaussian process (GP) regression is a powerful tool for statistically modeling these quasi-periodic variations. We investigate the limits of this technique using 800 days of RVs from the solar telescope on the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N) spectrograph. These data provide a well-sampled time series of stellar RV variations. Into this data set, we inject Keplerian signals with periods between 100 and 500 days and amplitudes between 0.6 and 2.4 m s$^{-1}$. We use GP regression to fit the resulting RVs and determine the statistical significance of recovered periods and amplitudes. We then generate synthetic RVs with the same covariance properties as the solar data to determine a lower bound on the observational baseline necessary to detect low-mass planets in Venus-like orbits around a Sun-like star. Our simulations show that discovering planets with a larger mass ($\sim$ 0.5 m s$^{-1}$) using current-generation spectrographs and GP regression will require more than 12 yr of densely sampled RV observations. Furthermore, even with a perfect model of stellar variability, discovering a true exo-Venus ($\sim$ 0.1 m s$^{-1}$) with current instruments would take over 15 yr. Therefore, next-generation spectrographs and better models of stellar variability are required for detection of such planets.

astro-ph.EP↗

arXiv2020

Testing the Spectroscopic Extraction of Suppression of Convective Blueshift

Efforts to detect low-mass exoplanets using stellar radial velocities (RVs) are currently limited by magnetic photospheric activity. Suppression of convective blueshift is the dominant magnetic contribution to RV variability in low-activity Sun-like stars. Due to convective plasma motions, the magnitude of RV contributions from the suppression of convective blueshift is related to the depth of formation of photospheric spectral lines of a given species used to compute the RV time series. Meunier et al. (2017), used this relation to demonstrate a method for spectroscopic extraction of the suppression of convective blueshift in order to isolate RV contributions, including planetary RVs, that contribute equally to the timeseries for each spectral line. Here, we extract disk-integrated solar RVs from observations over a 2.5 year time span made with the solar telescope integrated with the HARPS-N spectrograph at the Telescopio Nazionale Galileo (La Palma, Canary Islands, Spain). We apply the methods outlined by Meunier et al. (2017). We are not, however, able to isolate physically meaningful contributions of the suppression of convective blueshift from this solar dataset, potentially because our dataset is from solar minimum when the suppression of convective blueshift may not sufficiently dominate activity contributions to RVs. This result indicates that, for low-activity Sun-like stars, one must include additional RV contributions from activity sources not considered in the Meunier et al. (2017) model at different timescales as well as instrumental variation in order to reach the sub-meter per second RV sensitivity necessary to detect low-mass planets in orbit around Sun-like stars.

astro-ph.SR↗

arXiv2019

Microwave-assisted spectroscopy technique for studying charge state in nitrogen-vacancy ensembles in diamond

We introduce a microwave-assisted spectroscopy technique to determine the relative concentrations of nitrogen vacancy (NV) centers in diamond that are negatively-charged (NV${}^-$) and neutrally-charged (NV${}^0$), and present its application to studying spin-dependent ionization in NV ensembles and enhancing NV-magnetometer sensitivity. Our technique is based on selectively modulating the NV${}^-$ fluorescence with a spin-state-resonant microwave drive to isolate, in-situ, the spectral shape of the NV${}^-$ and NV${}^0$ contributions to an NV-ensemble sample's fluorescence. As well as serving as a reliable means to characterize charge state ratio, the method can be used as a tool to study spin-dependent ionization in NV ensembles. As an example, we applied the microwave technique to a high-NV-density diamond sample and found evidence for a new spin-dependent ionization pathway, which we present here alongside a rate-equation model of the data. We further show that our method can be used to enhance the contrast of optically-detected magnetic resonance (ODMR) on NV ensembles and may lead to significant sensitivity gains in NV magnetometers dominated by technical noise sources, especially where the NV${}^0$ population is large. With the high-NV-density diamond sample investigated here, we demonstrate up to a 4.8-fold enhancement in ODMR contrast. The techniques presented here may also be applied to other solid-state defects whose fluorescence can be selectively modulated by means of a microwave drive. We demonstrate this utility by applying our method to isolate room-temperature spectral signatures of the V2-type silicon vacancy from an ensemble of V1 and V2 silicon vacancies in 4H silicon carbide.

cond-mat.mes-hall↗

arXiv2019

Imaging crystal stress in diamond using ensembles of nitrogen-vacancy centers

We present a micrometer-resolution millimeter-field-of-view stress imaging method for diamonds containing a thin surface layer of nitrogen vacancy (NV) centers. In this method, we reconstruct stress tensor elements over a two-dimensional field of view from NV optically-detected magnetic resonance (ODMR) spectra. We use this technique to study how stress inhomogeneity affects NV magnetometry performance, and show how NV stress imaging is a useful and direct way to assess these effects. This new tool for mapping stress in diamond will aid optimization of NV-diamond sensing, with wide-ranging applications in the physical and life sciences.

physics.optics↗

arXiv2019

HARPS-N Solar Radial-Velocity Variations Are Dominated By Large, Bright Magnetic Regions

State of the art radial-velocity (RV) exoplanet searches are currently limited by RV signals arising from stellar magnetic activity. We analyze solar observations acquired over a 3-year period during the decline of Carrington Cycle 24 to test models of RV variation of Sun-like stars. A purpose-built solar telescope at the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N) provides disk-integrated solar spectra, from which we extract RVs and $\log{R'_{\rm HK}}$. The Solar Dynamics Observatory (SDO) provides disk-resolved images of magnetic activity. The Solar Radiation and Climate Experiment (SORCE) provides near-continuous solar photometry, analogous to a Kepler light curve. We verify that the SORCE photometry and HARPS-N $\log{R'_{\rm HK}}$ correlate strongly with the SDO-derived magnetic filling factor, while the HARPS-N RV variations do not. To explain this discrepancy, we test existing models of RV variations. We estimate the contributions of the suppression of convective blueshift and the rotational imbalance due to brightness inhomogeneities to the observed HARPS-N RVs. We investigate the time variation of these contributions over several rotation periods, and how these contributions depend on the area of active regions. We find that magnetic active regions smaller than $60 \ \rm Mm^2$ do not significantly suppress convective blueshift. Our area-dependent model reduces the amplitude of activity-induced RV variations by a factor of two. The present study highlights the need to identify a proxy that correlates specifically with large, bright magnetic regions on the surfaces of exoplanet-hosting stars.

astro-ph.SR↗

arXiv2018

Spin-torque oscillation in a magnetic insulator probed by a single-spin sensor

Coherent, self-sustained oscillation of magnetization in spin-torque oscillators (STOs) is a promising source for on-chip, nanoscale generation of microwave magnetic fields. Such fields could be used for local excitation of spin-wave resonances, control of spin qubits, and studies of paramagnetic resonance. However, local characterization of fields emitted by an STO has remained an outstanding challenge. Here, we use the spin of a single nitrogen-vacancy (NV) defect in diamond to probe the magnetic fields generated by an STO in a microbar of ferromagnetic insulator yttrium-iron-garnet (YIG). The combined spectral resolution and sensitivity of the NV sensor allows us to resolve multiple spin-wave modes and characterize their damping. When damping is decreased sufficiently via spin injection, the modes auto-oscillate, as indicated by a strongly reduced linewidth, a diverging magnetic power spectral density, and synchronization of the STO frequency to an external microwave source. These results open the way for quantitative, nanoscale mapping of the microwave signals generated by STOs, as well as harnessing STOs as local probes of mesoscopic spin systems.

cond-mat.mes-hall↗

arXiv2016

Optical magnetic detection of single-neuron action potentials using quantum defects in diamond

A key challenge for neuroscience is noninvasive, label-free sensing of action potential (AP) dynamics in whole organisms with single-neuron resolution. Here, we present a new approach to this problem: using nitrogen-vacancy (NV) quantum defects in diamond to measure the time-dependent magnetic fields produced by single-neuron APs. Our technique has a unique combination of features: (i) it is noninvasive, as the light that probes the NV sensors stays within the biocompatible diamond chip and does not enter the organism, enabling activity monitoring over extended periods; (ii) it is label-free and should be widely applicable to most organisms; (iii) it provides high spatial and temporal resolution, allowing precise measurement of the AP waveforms and conduction velocities of individual neurons; (iv) it directly determines AP propagation direction through the inherent sensitivity of NVs to the associated AP magnetic field vector; (v) it is applicable to neurons located within optically opaque tissue or whole organisms, through which magnetic fields pass largely unperturbed; and (vi) it is easy-to-use, scalable, and can be integrated with existing techniques such as wide-field and superresolution imaging. We demonstrate our method using excised single neurons from two invertebrate species, marine worm and squid; and then by single-neuron AP magnetic sensing exterior to whole, live, opaque marine worms for extended periods with no adverse effect. The results lay the groundwork for real-time, noninvasive 3D magnetic mapping of functional neuronal networks, ultimately with synapse-scale (~10 nm) resolution and circuit-scale (~1 cm) field-of-view.

quant-ph↗

arXiv2014

Fourier Magnetic Imaging with Nanoscale Resolution and Compressed Sensing Speed-up using Electronic Spins in Diamond

Optically-detected magnetic resonance using Nitrogen Vacancy (NV) color centres in diamond is a leading modality for nanoscale magnetic field imaging, as it provides single electron spin sensitivity, three-dimensional resolution better than 1 nm, and applicability to a wide range of physical and biological samples under ambient conditions. To date, however, NV-diamond magnetic imaging has been performed using real space techniques, which are either limited by optical diffraction to 250 nm resolution or require slow, point-by-point scanning for nanoscale resolution, e.g., using an atomic force microscope, magnetic tip, or super-resolution optical imaging. Here we introduce an alternative technique of Fourier magnetic imaging using NV-diamond. In analogy with conventional magnetic resonance imaging (MRI), we employ pulsed magnetic field gradients to phase-encode spatial information on NV electronic spins in wavenumber or k-space followed by a fast Fourier transform to yield real-space images with nanoscale resolution, wide field-of-view (FOV), and compressed sensing speed-up.

cond-mat.mes-hall↗

arXiv2014

Sub-nanometer resolution in three-dimensional magnetic-resonance imaging of individual dark spins

Magnetic resonance imaging (MRI) has revolutionized biomedical science by providing non-invasive, three-dimensional biological imaging. However, spatial resolution in conventional MRI systems is limited to tens of microns, which is insufficient for imaging on molecular and atomic scales. Here we demonstrate an MRI technique that provides sub-nanometer spatial resolution in three dimensions, with single electron-spin sensitivity. Our imaging method works under ambient conditions and can measure ubiquitous 'dark' spins, which constitute nearly all spin targets of interest and cannot otherwise be individually detected. In this technique, the magnetic quantum-projection noise of dark spins is measured using a single nitrogen-vacancy (NV) magnetometer located near the surface of a diamond chip. The spatial distribution of spins surrounding the NV magnetometer is imaged with a scanning magnetic-field gradient. To evaluate the performance of the NV-MRI technique, we image the three-dimensional landscape of dark electronic spins at and just below the diamond surface and achieve an unprecedented combination of resolution (0.8 nm laterally and 1.5 nm vertically) and single-spin sensitivity. Our measurements uncover previously unidentified electronic spins on the diamond surface, which can potentially be used as resources for improved magnetic imaging of samples proximal to the NV-diamond sensor. This three-dimensional NV-MRI technique is immediately applicable to diverse systems including imaging spin chains, readout of individual spin-based quantum bits, and determining the precise location of spin labels in biological systems.

cond-mat.mes-hall↗

arXiv2013

All-optical sensing of a single-molecule electron spin

We demonstrate an all-optical method for magnetic sensing of individual molecules in ambient conditions at room temperature. Our approach is based on shallow nitrogen-vacancy (NV) centers near the surface of a diamond crystal, which we use to detect single paramagnetic molecules covalently attached to the diamond surface. The manipulation and readout of the NV centers is all-optical and provides a sensitive probe of the magnetic field fluctuations stemming from the dynamics of the electronic spins of the attached molecules. As a specific example, we demonstrate detection of a single paramagnetic molecule containing a gadolinium (Gd$^{3+}$) ion. We confirm single-molecule resolution using optical fluorescence and atomic force microscopy to co-localize one NV center and one Gd$^{3+}$-containing molecule. Possible applications include nanoscale and in vivo magnetic spectroscopy and imaging of individual molecules.

cond-mat.mes-hall↗

arXiv2012

Dressed-State Polarization Transfer between Bright & Dark Spins in Diamond

Under ambient conditions, spin impurities in solid-state systems are found in thermally-mixed states and are optically "dark", i.e., the spin states cannot be optically controlled. Nitrogen-vacancy (NV) centers in diamond are an exception in that the electronic spin states are "bright", i.e., they can be polarized by optical pumping, coherently manipulated with spin-resonance techniques, and read out optically, all at room temperature. Here we demonstrate a dressed-state, double-resonance scheme to transfer polarization from bright NV electronic spins to dark substitutional-Nitrogen (P1) electronic spins in diamond. This polarization-transfer mechanism could be used to cool a mesoscopic bath of dark spins to near-zero temperature, thus providing a resource for quantum information and sensing, and aiding studies of quantum effects in many-body spin systems.

quant-ph↗

arXiv2012

Nanoscale magnetic imaging of a single electron spin under ambient conditions

The detection of ensembles of spins under ambient conditions has revolutionized the biological, chemical, and physical sciences through magnetic resonance imaging and nuclear magnetic resonance. Pushing sensing capabilities to the individual-spin level would enable unprecedented applications such as single molecule structural imaging; however, the weak magnetic fields from single spins are undetectable by conventional far-field resonance techniques. In recent years, there has been a considerable effort to develop nanoscale scanning magnetometers, which are able to measure fewer spins by bringing the sensor in close proximity to its target. The most sensitive of these magnetometers generally require low temperatures for operation, but measuring under ambient conditions (standard temperature and pressure) is critical for many imaging applications, particularly in biological systems. Here we demonstrate detection and nanoscale imaging of the magnetic field from a single electron spin under ambient conditions using a scanning nitrogen-vacancy (NV) magnetometer. Real-space, quantitative magnetic-field images are obtained by deterministically scanning our NV magnetometer 50 nanometers above a target electron spin, while measuring the local magnetic field using dynamically decoupled magnetometry protocols. This single-spin detection capability could enable single-spin magnetic resonance imaging of electron spins on the nano- and atomic scales and opens the door for unique applications such as mechanical quantum state transfer.

cond-mat.mes-hall↗

arXiv2012

Magnetic field imaging with NV ensembles

We demonstrate a method of imaging spatially varying magnetic fields using a thin layer of nitrogen-vacancy (NV) centers at the surface of a diamond chip. Fluorescence emitted by the two-dimensional NV ensemble is detected by a CCD array, from which a vector magnetic field pattern is reconstructed. As a demonstration, AC current is passed through wires placed on the diamond chip surface, and the resulting AC magnetic field patterns are imaged using an echo-based technique with sub-micron resolution over a 140 μm x 140 μm field of view, giving single-pixel sensitivity ~100 nT/\sqrt{Hz}. We discuss ongoing efforts to further improve sensitivity and potential bioimaging applications such as real-time imaging of activity in functional, cultured networks of neurons.

cond-mat.mes-hall↗

arXiv2012

Enhanced metrology using preferential orientation of nitrogen-vacancy centers in diamond

We demonstrate preferential orientation of nitrogen-vacancy (NV) color centers along two of four possible crystallographic axes in diamonds grown by chemical vapor deposition on the {100} face. We identify the relevant growth regime and present a possible explanation of this effect. We show that preferential orientation provides increased optical read-out contrast for NV multi-spin measurements, including enhanced AC magnetic field sensitivity, thus providing an important step towards high fidelity multi-spin-qubit quantum information processing, sensing and metrology.

cond-mat.mes-hall↗

arXiv2012

Enhanced solid-state multi-spin metrology using dynamical decoupling

We use multi-pulse dynamical decoupling to increase the coherence lifetime (T2) of large numbers of nitrogen-vacancy (NV) electronic spins in room temperature diamond, thus enabling scalable applications of multi-spin quantum information processing and metrology. We realize an order-of-magnitude extension of the NV multi-spin T2 for diamond samples with widely differing spin environments. For samples with nitrogen impurity concentration <~1 ppm, we find T2 > 2 ms, comparable to the longest coherence time reported for single NV centers, and demonstrate a ten-fold enhancement in NV multi-spin sensing of AC magnetic fields.

cond-mat.mes-hall↗

arXiv2012

Efficient photon detection from color centers in a diamond optical waveguide

A common limitation of experiments using color centers in diamond is the poor photon collection efficiency of microscope objectives due to refraction at the diamond interface. We present a simple and effective technique to detect a large fraction of photons emitted by color centers within a planar diamond sample by detecting light that is guided to the edges of the diamond via total internal reflection. We describe a prototype device using this "side-collection" technique, which provides a photon collection efficiency of approximately 47% and a photon detection efficiency of approximately 39%. We apply the enhanced signal-to-noise ratio gained from side-collection to AC magnetometry using ensembles of nitrogen-vacancy (NV) color centers, and demonstrate an AC magnetic field sensitivity of 100 pT/\sqrt{Hz}, limited by added noise in the prototype side-collection device. Technical optimization should allow significant further improvements in photon collection and detection efficiency as well as sub-picotesla NV-diamond magnetic field sensitivity using the side-collection technique.

cond-mat.mes-hall↗