arXiv subjects

Peter Knittel

Publications and source records attributed to Peter Knittel.

8 recordsLinked to original sources

arXiv2026

Investigation of Widefield NV-Center Magnetic Imaging for Non-Destructive Materials Testing

Due to progressing miniaturization, material fatigue is expected to gain importance on microscopic scales. Nevertheless, most non-destructive testing techniques are optimized for macroscopic scales. Therefore, new approaches applicable to miniaturized samples need to be explored. High spatial resolution imaging of a material's magnetic stray field enables sensitive detection of microstructural changes due to the local interplay of magnetic properties, strain, and defects. Up to now, this approach has rarely been used for materials testing since high-resolution magnetic sensing is traditionally challenging. Novel quantum sensing techniques offer the potential to close this gap. One of the most prominent quantum sensors, the nitrogen vacancy center in diamond, is investigated for non-destructive testing applications within the scope of this work. An experimental system based on a widefield sensing approach was developed to image magnetic stray field distributions within seconds to minutes over areas up to 1 x 1 $mm^2$. Magnetic sensitivities below 10 $μ$T/$\sqrt{\text{Hz}}$ and a spatial resolution of 1-2 $μ$m were achieved. The technique offers high mechanical stability, which is crucial for non-destructive testing applications. Comparison with magneto-optical Kerr effect measurements showed that the magnetic field maps are strongly correlated with the material's surface domains while containing additional information from deeper inside the sample. Characteristic changes in the magnetic stray field distribution were detected in an electrical steel sample after cyclic loading. A potential marker for early fatigue damage was deduced from the splitting gradient distribution, while 2D Fourier transform analysis provided additional insight.

physics.ins-det↗

arXiv2026

Revealing and reducing growth-induced interfacial disorder in preferentially aligned nitrogen-vacancy centers in diamond

Nitrogen-vacancy (NV) centers in chemical-vapor-deposition (CVD) diamond can form preferentially oriented ensembles with high sensing performance and low densities of lattice defects. Thin films of this material are a cornerstone of various imaging modalities. However, nitrogen injection needed to produce such films can transiently drive growth out of equilibrium, generating interfacial strain and spin defects that degrade NV coherence. Here, we investigate this disorder in $^{12}\text{C}$-enriched, preferentially oriented NV layers grown on (111) diamond using two nitrogen-injection procedures, combined with nanometer-scale selective plasma etching and NV spin-coherence measurements. Pulsed nitrogen injection produces a pronounced nitrogen overshoot within a 60--80 nm interfacial region, generating excessive amounts of defects. By contrast, smooth nitrogen delivery through mass flow controllers substantially suppresses interfacial disorder, yielding coherence properties close to the theoretical limit imposed by spin-bath noise. A 50-nm NV layer is used to demonstrate proton nuclear magnetic resonance detection. This work reveals the role of interfacial disorder associated with the nitrogen-doping procedure and provides a route to growing high-quality, thin NV-doped layers for quantum-sensing applications.

quant-ph↗

arXiv2026

High-cooperativity coupling and spin-resolved extinction of tin-vacancy centers in a diamond-like microcavity

The tin-vacancy (SnV) center in diamond is a promising spin-photon interface for quantum networks, combining favorable optical properties with spin coherence above 1K. Unfolding the full potential requires cavity enhancement to increase photon-emitter coupling efficiency. Here, we demonstrate cavity-enhanced light-matter coupling of SnV centers in a fully tunable Fabry-Pérot microcavity operating at temperatures down to 1K with in-situ magnetic field control. We access the diamond-like regime of hybrid cavity modes through integration of low-roughness diamond membranes, where the field is concentrated inside the diamond and Purcell enhancement is maximized. Diamond-like modes deliver a more than two-fold increase in the effective Purcell factor over air-like modes, reaching $C_0 = 4.1(1)$ compared to $C_0 = 1.85(5)$ in the air-like case, while simultaneously relaxing mechanical stability requirements. Resonant probing reveals coherent cavity-emitter coupling with 96% extinction contrast and a coherent cooperativity of $C = 4.0(14)$. By applying a magnetic field, we further achieve spin-resolved cavity extinction, observing spin-selective optical transitions with a contrast of ${\cal C}_{\rm spin} = 0.91$. These results establish SnV centers in diamond coupled to open Fabry-Pérot microcavities as a promising platform for efficient spin-photon interfaces.

quant-ph↗

arXiv2026

Delta-Doped Diamond via in-situ Plasma-Distance Control

We present an approach for the CVD growth of diamond, where the sample is placed in a defined distance from the reactor baseplate, to which the plasma couples. We observe two previously unknown growth regimes. In the first case, the sample is positioned within three to five millimeters of the plasma, leading to a decreased growth rate, compared to a position inside the plasma and, additionally, to an increased nitrogen incorporation, allowing the fabrication of delta-doped layers with a thickness below 30 nm. In another regime, where the sample is more than 10 mm away from the plasma, no growth is observed. Instead, we assume a deposition of nitrogen-rich species on the diamond surface, which is incorporated during the growth of the following layer. All fabricated layers show NV emission, where the intensity correlates with the nitrogen incorporation. The growth techniques could allow the fabrication of highly doped thin films for quantum sensing applications, as well as layers with low NV concentration, for quantum computing. The new approaches are applicable not only for nitrogen incorporation but also for other defects, for example, phosphorus, which could open up new avenues for diamond-based electronics.

cond-mat.mtrl-sci↗

arXiv2025

Readout of a solid state spin ensemble at the projection noise limit

Spin ensembles are central to quantum science, from frequency standards and fundamental physics searches to magnetic resonance spectroscopy and quantum sensing. Their performance is ultimately constrained by spin projection noise, yet solid-state implementations have so far been limited by much larger photon shot noise. Here we demonstrate a direct, quantum non-demolition readout of a mesoscopic ensemble of nitrogen-vacancy (NV) centers in diamond that surpasses the photon shot-noise limit and approaches the intrinsic spin projection noise. By stabilizing the $^{14}$N nuclear spin bath at high magnetic fields and employing repetitive nuclear-assisted spin readout, we achieve a noise reduction of 3.8 dB below the thermal projection noise level. This enables direct access to the intrinsic fluctuations of the spin ensemble, allowing us to directly observe the signatures of correlated spin states. Our results establish projection noise-limited readout as a practical tool for solid-state quantum sensors, opening pathways to quantum-enhanced metrology, direct detection of many-body correlations, and the implementation of spin squeezing in mesoscopic solid-state ensembles.

quant-ph↗

arXiv2025

The properties of the nitrogen-vacancy center in milled chemical vapor deposition nanodiamonds

Fluorescent nanodiamonds (FNDs) containing negatively charged nitrogen-vacancy (NV-) centers are vital for many emerging quantum sensing applications from magnetometry to intracellular sensing in biology. However, developing a scalable fabrication method for FNDs hosting color centers with consistent bulk-like photoluminescence (PL) and spin coherence properties remains a highly desired but unrealized goal. Here, we investigate optimized ball milling of single-crystal diamonds produced via chemical vapor deposition (CVD) and containing 2 ppm of substitutional nitrogen and 0.3 ppm of NV- to achieve this goal. The NV charge state, PL lifetime, and spin properties of bulk CVD diamond samples are directly compared to milled CVD FNDs and commercial high-pressure high-temperature (HPHT) FNDs. We find that on average, the relative contribution of the NV- charge state to the total NV PL is lower and the NV PL lifetime is longer in CVD FNDs compared to HPHT FNDs, both likely due to the lower Ns0 concentration in CVD FNDs. The CVD bulk and CVD FNDs on average show similar average T1 spin relaxation times of 3.2 $\pm$ 0.7 ms and 4.7 $\pm$ 1.6 ms, respectively, compared to 0.17 $\pm$ 0.01 ms for commercial HPHT FNDs. Our results demonstrate that ball milling of CVD diamonds enables the large-scale fabrication of NV ensembles in FNDs with bulk-like T1 spin relaxation properties.

cond-mat.mes-hall↗

arXiv2025

Spin coherence in strongly coupled spin baths in quasi-two-dimensional layers

We investigate the spin-coherence decay of NV$^-$-spins interacting with the strongly-coupled bath of nitrogen defects in diamond layers. For thin diamond layers, we demonstrate that the spin-coherence times exceed those of bulk diamond, thus allowing to surpass the limit imposed by high defect concentrations in bulk. We show that the stretched-exponential parameter for the short-time spin-coherence decay is governed by the hyperfine interaction in the bath, thereby constraining random-noise models. We introduce a novel method based on the cluster-correlation expansion applied to strongly-interacting bath partitions. Our results facilitate material development for quantum-technology devices.

quant-ph↗

arXiv2022

Microfluidic quantum sensing platform for lab-on-a-chip applications

Lab-on-a-chip (LOC) applications have emerged as invaluable physical and life sciences tools. The advantages stem from advanced system miniaturization, thus, requiring far less sample volume while allowing for complex functionality, increased reproducibility, and high throughput. However, LOC applications necessitate extensive sensor miniaturization to leverage these inherent advantages fully. Atom-sized quantum sensors are highly promising to bridge this gap and have enabled measurements of temperature, electric and magnetic fields on the nano- to microscale. Nevertheless, the technical complexity of both disciplines has so far impeded an uncompromising combination of LOC systems and quantum sensors. Here, we present a fully integrated microfluidic platform for solid-state spin quantum sensors, such as the nitrogen-vacancy (NV) center in diamond. Our platform fulfills all technical requirements, such as fast spin manipulation, enabling full quantum sensing capabilities, biocompatibility, and easy adaptability to arbitrary channel and chip geometries. To illustrate the vast potential of quantum sensors in LOC systems, we demonstrate various NV center-based sensing modalities for chemical analysis in our microfluidic platform, ranging from paramagnetic ion detection to high-resolution microscale NV-NMR. Consequently, our work opens the door for novel chemical analysis capabilities within LOC devices with applications in electrochemistry, high throughput reaction screening, bioanalytics, organ-on-a-chip, or single-cell studies.

quant-ph↗