Michal Kern

This work presents an industrially scalable, power-efficient quantum magnetometer chip based on 4H-silicon carbide (SiC) technology, leveraging wafer-scale fabrication techniques to optimize V2 silicon-vacancy color centers for reproducible fabrication. The integration of these color center ensembles into a planar silicon carbide waveguide enables efficient excitation of a large ensemble and simplifies fluorescence extraction compared with standard confocal methods. We report continuous wave (cw) optically detected magnetic resonance measurements, complemented by Rabi, Ramsey, and Hahn-echo sequences, which demonstrate the coherent capabilities of the large embedded ensemble of V2 centers. Based on the data, our device exhibits sensor shot-noise-limited sensitivities of around 30 nT / Hz , 2–3 orders of magnitude lower compared with more complex confocal techniques. This concept simplifies the quantum sensor architecture, enhances sensitivity, and streamlines optical excitation and collection, thereby paving the way for the development of next-generation SiC-quantum sensing technologies.

Dead time following radio frequency excitation limits pulsed magnetic resonance (MR) experiments by obscuring early-time spin dynamics and short-lived signals. We demonstrate continuous concurrent excitation and detection using two oscillator-based MR detectors: a high-frequency (477 megahertz) injection-locked voltage-controlled oscillator (VCO) array enabling a scalable segmented-coil architecture and a low-frequency (17 megahertz) single-coil VCO. Both detectors are embedded in custom-designed phase-locked loops, enabling phase-coherent multipulse operation. Using a planar segmented coil driven by a chip-integrated VCO array operating at 477-megahertz proton frequency, we perform conventional Fourier transform nuclear MR, continuously monitor driven Rabi oscillations throughout the excitation pulse, and observe the immediate onset of free-induction decay, all without dedicated transmit-receive switches or receiver gating. We further demonstrate phase-coherent, phase-cycled multipulse operation, including a Hahn echo sequence realized on a second system operating at 17 megahertz. Together, these results demonstrate the general applicability and versatility of the oscillator-based detection principle across pulse sequences and operating frequencies.

This work presents a comprehensive study on the development of planar integrated photonics for 4H silicon carbide (SiC) quantum sensors, with a focus on achieving high-performance and scalable devices. By leveraging high-volume SiC technology, we achieve a substantial reduction in optical losses, reaching 3.5 dB/cm at 785 nm, thereby surpassing the performance of previously reported planar SiC waveguides. A comprehensive analysis of the signal mode profile yields valuable insights into the modal characteristics of the waveguides, enabling the optimization of N-doping profiles for the optimal incorporation of V2 silicon vacancy color centers, which possess unique optical and spin properties essential for quantum sensing applications. Notably, our platform demonstrates broadband capability, supporting the development of both multi-mode and single-mode waveguides. Furthermore, we minimize surface roughness of the coupling facets to an root mean square value of 0.2 nm, thereby reducing scattering losses and enhancing device performance while maintaining compatibility with high-volume manufacturing processes. V2 color centers are created within the waveguide through proton implantation, as evidenced by the detection of fluorescence light above 850 nm within the guided waveguide mode, highlighting the potential of our platform for highly sensitive and compact SiC-based quantum sensors.

Quantum sensing has rapidly become one of the most promising frontiers in modern measurement science, leveraging quantum mechanical principles to detect physical quantities with sensitivities that can far exceed classical sensor performance. Among the various quantum platforms under investigation, nitrogen vacancy (NV) centers in diamond have emerged as particularly compelling due to their stability, room temperature operation, and compatibility with microelectronics and photonic integration. This article explores the current landscape of scalable quantum sensing, with particular attention to the technological and device-level innovations required to transition laboratory-based systems into commercially viable platforms. We focus in particular on the interface electronics and optical and electronic integration strategies that are enabling scalable chip-level implementations of quantum sensors.

Electrical detection of magnetic resonance (EDMR), a variant of electron paramagnetic resonance (EPR) in which the magnetic resonance effect is detected via changes in the electrical properties of materials or devices, is of high interest for studying electrically active defects in SiC metal–oxide–semiconductor field-effect transistors (MOSFETs), but requires dedicated instrumentation that is not widely available. Integration of microwave sources and detection circuits has led to the design of very compact EPR instruments, the so-called EPR-on-a-chip (EPRoC), which also offers opportunities for EDMR-on-a-chip (EDMRoC). Here, we report the demonstration of EDMRoC on lateral SiC MOSFETs under charge pumping (CP) conditions. The detected CP current gives direct access to microscopic information about the recombination centers within the transistor gate inversion region under the gate dielectric. Efficient and selective microwave excitation of the region of interest of the device can be obtained by only modest modifications to both the MOSFET and the EPRoC electronic board. A comparative study between EDMRoC and a traditional resonant cavity configuration reveals comparable signal-to-noise ratios for CP-detected EDMR spectra. In addition to space- and cost-efficiency, EDMRoC offers alternative detection modes with scanning and modulation of the microwave frequency, as well as potentially easier sample mounting and exchange. We end with a discussion of the advantages, limitations, and perspectives of the EDMRoC setup compared to EDMR in a conventional EPR spectrometer, offering promise for widespread integration of EDMR in semiconductor laboratories.

Low-field nuclear magnetic resonance (NMR) instruments are an indispensable tool in industrial research and quality control. However, the intrinsically low spin polarization at low magnetic fields severely limits their detection sensitivity and measurement throughput, preventing their widespread use in biomedical analysis. Overhauser dynamic nuclear polarization (ODNP) effectively addresses this problem by transferring the spin polarization from free electrons to protons, significantly enhancing sensitivity. In this paper, we explore the potential of using ODNP for signal enhancement in a custom-designed portable chipbased DNP-enhanced NMR platform, which is centered around a miniaturized microwave (MW) transmitter, a custom-designed NMR-on-a-chip transceiver, and two application-specific ODNP probes. The MW transmitter provides frequency synthesis, signal modulation, and power amplification, providing sufficient output power for efficient polarization transfer. The NMR-on-a-chip transceiver combines a radio frequency (RF) transmitter with a fully differential quadrature receiver, providing pulsed excitation and NMR signal down-conversion and amplification. Two customdesigned ODNP probes are used for proof-of-concept DNPenhanced NMR relaxometry and spectroscopy measurements. The presented chip-based ODNP platform achieves a maximum MW output power of 34dBm, resulting in a signal enhancement of –162 using the relaxometry ODNP probe with 1.4 μL of 10mM non-degassed TEMPOL solution, and an enhancement of –63 with the spectroscopy ODNP probe using 50 nL of the same solution. The proton polarization was increased from 0.5×10 ^–6 to 81×10 ^–6 at a low field of 0.16T. Proof-of-concept measurements on radical-doped tattoo inks and acetic acid verify the potential of our chip-based ODNP platform for the analysis of biologically and medically relevant parameters such as relaxation times, chemical shifts, and hyperfine interactions.

This monograph presents the emerging field of second-generation quantum sensing, which leverages phenomena such as superposition and entanglement, thereby offering measurement sensitivities far surpassing classical methods. In our discussion, we emphasize its potential to revolutionize various scientific and technological domains. Starting with a foundational overview of quantum sensors, distinguishing them from quantum computing and communication technologies, we then highlight the relative maturity of quantum sensing, especially in room-temperature operations, which positions it closest to market adoption. The main part of the monograph is dedicated to solid-state defects, particularly nitrogen-vacancy (NV) centers in diamond, which have emerged as promising candidates for scalable quantum sensors. The unique optical and spin properties of NV centers are explored in detail, emphasizing their possible applications in fields such as biomedical imaging, materials science, and semiconductor inspection. We delve into the technical aspects of integrating NV centers with conventional electronic and photonic systems, discussing the challenges and innovations in electronic interface circuits, photonic integration, and system-level integration technologies. Additionally, we examine gas-based quantum sensors, particularly those utilizing Rydberg atoms, which offer high precision due to their long coherence times. The challenges of integrating gas-based sensors, compared to their solid-state counterparts, are also briefly discussed. Overall, the monograph underscores the potential of second-generation quantum sensors, particularly those based on NV centers, to be the first scalable, high-volume quantum devices on the market, with applications spanning various fields due to their high sensitivity and room-temperature operation.

In this article we provide an overview of chip-integrated voltage-controlled oscillator (VCO)-based EPR detection as a new paradigm in EPR sensing. After a brief motivation for this alternative detection method, we provide a self-contained overview of the detection principle, both for continuous-wave and pulsed detection. Based on this introduction, we will highlight the advantages and disadvantages of VCO-based detection compared to conventional resonator-based detection. This is followed by an overview of the current state of the art in VCO-based EPR and interesting emerging applications of the technology. The paper concludes with a brief summary and outlook on future research directions.