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High-finesse microcavities offer a platform for compact, high-precision sensing by employing high-reflectivity, low-loss mirrors to create effective optical path lengths that are orders of magnitude larger than the device geometry. Here, we investigate the radiation hardness of Fabry-Perot microcavities formed from dielectric mirrors deposited on the tips of optical fibers. The microcavities are irradiated under both conventional (~0.1 Gy/s) and ultrahigh (FLASH, ~20 Gy/s) radiotherapy dose rates. Within our measurement sensitivity of ~40 ppm loss, we observe no degradation in the mirror absorption after irradiation with over 300 Gy accumulated dose. This result highlights the excellent radiation hardness of the dielectric mirrors forming the cavities, enabling new optics-based, real-time, in-vivo, tissue-equivalent radiation dosimeters with ~10 micron spatial resolution (our motivation), as well as other applications in high-radiation environments., Comment: 8 pages, 4 figures

Optically active spin defects in solids offer promising platforms to investigate nuclear spin clusters with high sensitivity and atomic-site resolution. To leverage near-surface defects for molecular structure analysis in chemical and biological contexts using nuclear magnetic resonance (NMR), further advances in spectroscopic characterization of nuclear environments are essential. Here, we report Fourier spectroscopy techniques to improve localization and mapping of the testbed $^{13}\mathrm{C}$ nuclear spin environment of individual, shallow nitrogen-vacancy centers at room temperature. We use multidimensional spectroscopy, well-known from classical NMR, in combination with weak measurements of single-nuclear-spin precession. We demonstrate two examples of multidimensional NMR: (i) improved nuclear spin localization by separate encoding of the two hyperfine components along spectral dimensions and (ii) spectral editing of nuclear-spin pairs, including measurement of internuclear coupling constants. Our work adds important tools for the spectroscopic analysis of molecular structures by single-spin probes., Comment: 7 pages, 3 figures

Coupling between a single quantum emitter and an optical cavity presents a key capability for future quantum networking applications. Here, we explore interactions between individual germanium-vacancy (GeV) defects in diamond and an open microcavity at cryogenic temperatures. Exploiting the tunability of our microcavity system to characterize and select emitters, we observe a Purcell-effect-induced lifetime reduction of up to $4.5\pm0.3$, and extract coherent coupling rates up to $360\pm20$ MHz. Our results indicate that the GeV defect has favorable optical properties for cavity coupling, with a quantum efficiency of at least $0.34\pm0.05$ and likely much higher., Comment: 14 pages, 6 figures, journal accepted version after initial review

The long-lived electronic spin of the nitrogen-vacancy (NV) center in diamond is a promising quantum sensor for detecting nanoscopic magnetic and electric fields in a variety of experimental conditions. Nevertheless, an outstanding challenge in improving measurement sensitivity is the poor signal-to-noise ratio (SNR) of prevalent optical spin-readout techniques. Here, we address this limitation by coupling individual NV centers to optimized diamond nanopillar structures, thereby improving optical collection efficiency of fluorescence. First, we optimize the structure in simulation, observing an increase in collection efficiency for tall ($\geq$ 5 $\mu$m) pillars with tapered sidewalls. We subsequently verify these predictions by fabricating and characterizing a representative set of structures using a reliable and reproducible nanofabrication process. An optimized device yields increased SNR, owing to improvements in collimation and directionality of emission. Promisingly, these devices are compatible with low-numerical-aperture, long-working-distance collection optics, as well as reduced tip radius, facilitating improved spatial resolution for scanning applications., Comment: 22 pages, five figures

Photoexcitable donor-bridge-acceptor (D-B-A) molecules that support intramolecular charge transfer are ideal platforms to probe the influence of chiral-induced spin selectivity (CISS) in electron transfer and resulting radical pairs. In particular, the extent to which CISS influences spin polarization or spin coherence in the initial state of spin-correlated radical pairs following charge transfer through a chiral bridge remains an open question. Here, we introduce a quantum sensing scheme to measure directly the hypothesized spin polarization in radical pairs using shallow nitrogen-vacancy (NV) centers in diamond at the single- to few-molecule level. Importantly, we highlight the perturbative nature of the electron spin-spin dipolar coupling within the radical pair, and demonstrate how Lee-Goldburg decoupling can preserve spin polarization in D-B-A molecules for enantioselective detection by a single NV center. The proposed measurements will provide fresh insight into spin selectivity in electron transfer reactions., Comment: 7 pages and 4 pages appendix including an extensive description of the initial spin state of photo-generated radical pairs

Quantum sensing using optically addressable atomic-scale defects, such as the nitrogen--vacancy (NV) center in diamond, provides new opportunities for sensitive and highly localized characterization of chemical functionality. Notably, near-surface defects facilitate detection of the minute magnetic fields generated by nuclear or electron spins outside of the diamond crystal, such as those in chemisorbed and physisorbed molecules. However, the promise of NV centers is hindered by a severe degradation of critical sensor properties, namely charge stability and spin coherence, near surfaces (< ca. 10 nm deep). Moreover, applications in the chemical sciences require methods for covalent bonding of target molecules to diamond with robust control over density, orientation, and binding configuration. This forward-looking Review provides a survey of the rapidly converging fields of diamond surface science and NV-center physics, highlighting their combined potential for quantum sensing of molecules. We outline the diamond surface properties that are advantageous for NV-sensing applications, and discuss strategies to mitigate deleterious effects while simultaneously providing avenues for chemical attachment. Finally, we present an outlook on emerging applications in which the unprecedented sensitivity and spatial resolution of NV-based sensing could provide unique insight into chemically functionalized surfaces at the single-molecule level., Comment: Review paper, 36 pages

Nuclear magnetic resonance (NMR) imaging with shallow nitrogen-vacancy (NV) centers in diamond offers an exciting route toward sensitive and localized chemical characterization at the nanoscale. Remarkable progress has been made to combat the degradation in coherence time and stability suffered by near-surface NV centers using suitable chemical surface termination. However, approaches that also enable robust control over adsorbed molecule density, orientation, and binding configuration are needed. We demonstrate a diamond surface preparation for mixed nitrogen- and oxygen-termination that simultaneously improves NV center coherence times for emitters <10-nm-deep and enables direct and recyclable chemical functionalization via amine-reactive crosslinking. Using this approach, we probe single NV centers embedded in nanopillar waveguides to perform $^{19}\mathrm{F}$ NMR sensing of covalently bound trifluoromethyl tags in the ca. 50-100 molecule regime. This work signifies an important step toward nuclear spin localization and structure interrogation at the single-molecule level., Comment: 21 pages and 16 pages supporting information

We present a simple, continuous, cavity-enhanced optical absorption measurement technique based on high-bandwidth Pound-Drever-Hall (PDH) sideband locking. The technique provides a resonant amplitude quadrature readout that can be mapped onto the cavity's internal loss rate, and is naturally compatible with weak probe beams. With a proof-of-concept 5-cm-long Fabry-Perot cavity, we measure an absorption sensitivity of $6.6 \times 10^{-11} ~\text{cm}^{-1}/\sqrt{\text{Hz}}$ at 100 kHz (roughly the cavity bandwidth) and $\sim 10^{-10}~\text{cm}^{-1}/\sqrt{\text{Hz}}$ from 30 kHz to 1 MHz, with $38~\mu$W collected from the cavity's circulating power., Comment: 13 pages, 6 figures