arXiv subjects

Alexander Ungar

Publications and source records attributed to Alexander Ungar.

3 recordsLinked to original sources

arXiv2025

Robust AC vector sensing at zero magnetic field with pentacene

Quantum sensors based on electronic spins have emerged as powerful probes of microwave-frequency fields. Among other solid-state platforms, spins in molecular crystals offer a range of advantages, from high spin density to functionalization via chemical tunability. Here, we demonstrate microwave vector magnetometry using the photoexcited spin triplet of deuterated pentacene molecules, operating at zero external magnetic field and room temperature. We achieve full three-dimensional microwave field reconstruction by detecting the Rabi frequencies of anisotropic spin-triplet transitions associated with two crystallographic orientations of pentacene in naphthalene crystals. We further introduce a phase alternated protocol that extends the rotating-frame coherence time by an order of magnitude and enables sensitivities of $1~\mu\mathrm{T}/\sqrt{\mathrm{Hz}}$ with sub-micrometer spatial resolution. These results establish pentacene-based molecular spins as a practical and high-performance platform for microwave quantum sensing, and the control techniques are broadly applicable to other molecular and solid-state spin systems.

cond-mat.mes-hall↗

arXiv2025

Zero-field identification and control of hydrogen-related electron-nuclear spin registers in diamond

Spin defects in diamond serve as powerful building blocks for quantum technologies, especially for applications in quantum sensing and quantum networking. Electron-nuclear defects formed in the environment of optically active spins, such as the nitrogen-vacancy (NV) center, provide a resource for multi-qubit quantum registers. However, many of these defects have yet to be characterized, limiting their control and integration in quantum devices. Here, we apply two hybrid electron-nuclear spin control schemes to self-consistently characterize unknown spin defects at the single-spin level. We perform double electron-electron resonance at zero field (ZF-DEER) to extract hyperfine components and introduce a nuclear-electron-electron triple resonance (NEETR) protocol to control and identify the nuclear spin through the stronger electronic spin interaction. These results provide a guide to resolving the defect structures using ab initio calculations, leading to the identification of a new hydrogen-related defect structure as well as an accurate match to a previously identified nitrogen-related defect. We further apply our NEETR protocol to demonstrate initialization, unitary control, and long-lived coherence of the hydrogen nuclear spin qubit with $T_2 = 1.0(3)\,\mathrm{ms}$. Together, these characterization and control tools establish a framework to harness previously unknown electron-nuclear defects for quantum register applications.

quant-ph↗

arXiv2023

Control of an environmental spin defect beyond the coherence limit of a central spin

Electronic spin defects in the environment of an optically-active spin can be used to increase the size and hence the performance of solid-state quantum registers, especially for applications in quantum metrology and quantum communication. Previous works on multi-qubit electronic-spin registers in the environment of a Nitrogen-Vacancy (NV) center in diamond have only included spins directly coupled to the NV. As this direct coupling is limited by the central spin coherence time, it significantly restricts the register's maximum attainable size. To address this problem, we present a scalable approach to increase the size of electronic-spin registers. Our approach exploits a weakly-coupled probe spin together with double-resonance control sequences to mediate the transfer of spin polarization between the central NV spin and an environmental spin that is not directly coupled to it. We experimentally realize this approach to demonstrate the detection and coherent control of an unknown electronic spin outside the coherence limit of a central NV. Our work paves the way for engineering larger quantum spin registers with the potential to advance nanoscale sensing, enable correlated noise spectroscopy for error correction, and facilitate the realization of spin-chain quantum wires for quantum communication.

quant-ph↗