Modeling temperature-dependent population dynamics in the excited state of the nitrogen-vacancy center in diamond

The nitrogen-vacancy (NV) center in diamond is well known in quantum metrology and quantum information for its favorable spin and optical properties, which span a wide temperature range from near zero to over 600 K. Despite its prominence, the NV center's photo-physics is incompletely understood, especially at intermediate temperatures between 10-100 K where phonons become activated. In this work, we present a rate model able to describe the cross-over from the low-temperature to the high-temperature regime. Key to the model is a phonon-driven hopping between the two orbital branches in the excited state (ES), which accelerates spin relaxation via an interplay with the ES spin precession. We extend our model to include magnetic and electric fields as well as crystal strain, allowing us to simulate the population dynamics over a wide range of experimental conditions. Our model recovers existing descriptions for the low- and high-temperature limits, and successfully explains various sets of literature data. Further, the model allows us to predict experimental observables, in particular the photoluminescence (PL) emission rate, spin contrast, and spin initialization fidelity relevant for quantum applications. Lastly, our model allows probing the electron-phonon interaction of the NV center and reveals a gap between the current understanding and recent experimental findings

Rapid flipping of parametric phase states

Since the invention of the solid-state transistor, the overwhelming majority of computers followed the von Neumann architecture that strictly separates logic operations and memory. Today, there is a revived interest in alternative computation models accompanied by the necessity to develop corresponding hardware architectures. The Ising machine, for example, is a variant of the celebrated Hopfield network based on the Ising model. It can be realized with artifcial spins such as the `parametron' that arises in driven nonlinear resonators. The parametron encodes binary information in the phase state of its oscillation. It enables, in principle, logic operations without energy transfer and the corresponding speed limitations. In this work, we experimentally demonstrate flipping of parametron phase states on a timescale of an oscillation period, much faster than the ringdown time \tau that is often (erroneously) deemed a fundamental limit for resonator operations. Our work establishes a new paradigm for resonator-based logic architectures.Comment: 6 pages, 3 figure

Temperature dependence of photoluminescence intensity and spin contrast in nitrogen-vacancy centers

We report on measurements of the photoluminescence (PL) properties of single nitrogen-vacancy (NV) centers in diamond at temperatures between 4-300 K. We observe a strong reduction of the PL intensity and spin contrast between ca. 10-100 K that recovers to high levels below and above. Further, we find a rich dependence on magnetic bias field and crystal strain. We develop a comprehensive model based on spin mixing and orbital hopping in the electronic excited state that quantitatively explains the observations. Beyond a more complete understanding of the excited-state dynamics, our work provides a novel approach for probing electron-phonon interactions and a predictive tool for optimizing experimental conditions for quantum applications.Comment: Companion paper: arXiv:2304.02521 | Model: https://github.com/sernstETH/nvratemode

Towards Quantum Sensing of Chiral-Induced Spin Selectivity: Probing Donor-Bridge-Acceptor Molecules with NV Centers in Diamond

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 pair

Near-resonant nuclear spin detection with high-frequency mechanical resonators

Mechanical resonators operating in the high-frequency regime have become a versatile platform for fundamental and applied quantum research. Their exceptional properties, such as low mass and high quality factor, make them also very appealing for force sensing experiments. In this Letter, we propose a method for detecting and ultimately controlling nuclear spins by directly coupling them to high-frequency resonators via a magnetic field gradient. Dynamical backaction between the sensor and an ensemble of nuclear spins produces a shift in the sensor's resonance frequency, which can be measured to probe the spin ensemble. Based on analytical as well as numerical results, we predict that the method will allow nanoscale magnetic resonance imaging with a range of realistic devices. At the same time, this interaction paves the way for new manipulation techniques, similar to those employed in cavity optomechanics, enriching both the sensor's and the spin ensemble's features.Comment: Includes Supplemental Materia

Spatially resolved surface dissipation over metal and dielectric substrates

We report spatially resolved measurements of static and fluctuating electric fields over conductive (Au) and non-conductive (SiO2) surfaces. Using an ultrasensitive `nanoladder' cantilever probe to scan over these surfaces at distances of a few tens of nanometers, we record changes in the probe resonance frequency and damping that we associate with static and fluctuating fields, respectively. We find that the two quantities are spatially correlated and of similar magnitude for the two materials. We quantitatively describe the observed effects on the basis of trapped surface charges and dielectric fluctuations in an adsorbate layer. Our results provide direct, spatial evidence for surface dissipation in adsorbates that affects nanomechanical sensors, trapped ions, superconducting resonators, and color centers in diamond

Single Nitrogen-Vacancy-NMR of Amine-Functionalized Diamond Surfaces

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 informatio

Multicone Diamond Waveguides for Nanoscale Quantum Sensing

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 figure

Diamond surface engineering for molecular sensing with nitrogen-vacancy centers

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 page

High-speed domain wall racetracks in a magnetic insulator

Recent reports of current-induced switching of ferrimagnetic oxides coupled to a heavy metal layer have opened realistic prospects for implementing magnetic insulators into electrically addressable spintronic devices. However, key aspects such as the configuration and dynamics of magnetic domain walls driven by electrical currents in insulating oxides remain unexplored. Here, we investigate the internal structure of the domain walls in Tm3Fe5O12 (TmIG) and TmIG/Pt bilayers and demonstrate their efficient manipulation by spin-orbit torques with velocities of up to 400 m s$^{-1}$ and minimal current threshold for domain wall flow of 5 x 10$^{6}$ A cm$^{-2}$. Domain wall racetracks embedded in TmIG are defined by the deposition of Pt current lines, which allow us to control the domain propagation and magnetization switching in selected regions of an extended magnetic layer. Scanning nitrogen-vacancy magnetometry reveals that the domain walls of thin TmIG films are N\'eel walls with left-handed chirality, with the domain wall magnetization rotating towards an intermediate N\'eel-Bloch configuration upon deposition of Pt. These results indicate the presence of a sizable interfacial Dzyaloshinskii-Moriya interaction in TmIG, which leads to novel possibilities to control the formation of chiral spin textures in magnetic insulators. Ultimately, domain wall racetracks provide an efficient scheme to pattern the magnetic landscape of TmIG in a fast and reversible wa