Quantum sensors exploit the quantum states of atoms, ions, photons, spins, or solid-state defects to detect extremely small changes in magnetic and electric fields, temperature, pressure, rotation, forces, time, or chemical environments. Their exceptional performance arises because quantum states are highly sensitive to external perturbations and can be controlled and read out with remarkable precision. In this talk, I will present recent progress in solid-state defect–based quantum sensors achieved in my research group.

The most established platform is the negatively charged nitrogen-vacancy (NV⁻) center in diamond, which exhibits excellent coherence and optical properties in bulk at room temperature. Its electron spin can be initialized and read out optically using optically detected magnetic resonance (ODMR) under green illumination [1,2]. However, these favorable properties deteriorate when NV centers are engineered close to the diamond surface for sensing applications. Despite decades of effort within the NV community, the simultaneous optimization of surface chemistry and near-surface defect creation remains a major unsolved challenge.

We have shown through first-principles calculations and quantum-mechanical modeling that the introduction of NV centers by ion implantation and annealing inevitably leaves residual vacancies and vacancy clusters that act as hole sources under the green laser used for NV excitation. These holes are rapidly captured by negatively charged NV centers, converting them into their neutral charge state and suppressing ODMR contrast [3,4]. Our simulations indicate that this effect can be mitigated by raising the Fermi level near the surface using appropriate surface terminations, such as fluorination [5]. However, the experimental development of such methods is ongoing [6]. A further limitation of the diamond NV platform is that green illumination causes autofluorescence and heating in biological environments, motivating the search for alternative quantum sensors for bio-related applications.

For more than a decade, our first-principles studies have suggested that neutral divacancy defects in silicon carbide (SiC) could provide such an alternative [7,8]. These defects can be excited in the infrared and emit in the second biological window, making them intrinsically attractive for biological sensing. Indeed, specific divacancy and divacancy-related color centers—such as the PL6 center—have been demonstrated to possess ODMR and spin properties comparable to those of the NV center in diamond [9].

The next step is to engineer the SiC surface and introduce shallow divacancy species suitable for nanoscale sensing. Under ambient conditions, however, SiC readily oxidizes and forms a high density of optical and paramagnetic defects at the interface, necessitating new materials-processing strategies [10]. We have shown with first-principles calculations that the ODMR contrast of divacancy centers can be enhanced through strain engineering [11], a prediction verified experimentally by our collaborators. Additionally, we proposed replacing the native oxide with carbon-chain terminations to prevent oxidation. Our simulations revealed that such surfaces serve as ideal hosts for shallow divacancy quantum sensors capable of detecting external paramagnetic species [12]. This theoretical proposal has since been experimentally demonstrated by our collaborators, paving the way toward non-invasive, room-temperature quantum sensor devices.

[1] The nitrogen-vacancy colour centre in diamond, Marcus W Doherty, Neil B Manson, Paul Delaney, Fedor Jelezko, Jörg Wrachtrup, Lloyd CL Hollenberg, Physics Reports, 528, 1 (2013).

[2] Ab initio theory of the nitrogen-vacancy center in diamond, Adam Gali, Nanophotonics, 8, 1907 (2019).

[3] The Role of Electrolytes in the Relaxation of Near-Surface Spin Defects in Diamond, Fabian A. Freire-Moschovitis, Roberto Rizzato, Anton Pershin, Moritz R. Schepp, Robin D. Allert, Lina M. Todenhagen, Martin S. Brandt, Adam Gali, and Dominik B. Bucher, ACS Nano, 17, 10474 (2023).

[4] Controlled Surface Modification to Revive Shallow NV- Centers, Jeffrey Neethi Neethirajan, Toni Hache, Domenico Paone, Dinesh Pinto, Andrej Denisenko, Rainer Stöhr, Péter Udvarhelyi, Anton Pershin, Adam Gali, Joerg Wrachtrup, Klaus Kern, and Aparajita Singha, Nano Letters, 23, 2563 (2023).

[5] The proper surface termination for luminescent near-surface NV-centres in diamond, Moloud Kaviani, Peter Deák, Bálint Aradi, Thomas Frauenheim, Jyh-Pin Chou , and Adam Gali, Nano Letters, 14, 4772 (2014).

[6] Diamond surface functionalization via visible light-driven C-H activation for nanoscale quantum sensing, Lila V. H. Rodgers, Suong T. Nguyen, James H. Cox, Kalliope Zervas, Zhiyang Yuan, Sorawis Sangtawesin, Alastair Stacey, Cherno Jaye, Conan Weiland, Anton Pershin, Adam Gali, Lars Thomsen, Simon A. Meynell, Lillian B. Hughes, Ania C. Bleszynski Jayich, Xin Gui, Robert J. Cava, Robert R. Knowles, and Nathalie P. de Leon, Proceedings of the National Academy of Sciences, 121, e2316032121 (2024).

[7] Feature Article: Time-dependent density functional study on the excitation spectrum of point defects in semiconductors, Adam Gali, physica status solidi b, 248, 1337 (2011).

[8] Near-infrared luminescent cubic silicon carbide nanocrystals for in vivo biomarker applications: an ab initio study, B. Somogyi, V. Zólyomi, and Adam Gali, Nanoscale, 4, 7720 (2012).

[9] Room temperature coherent manipulation of single-spin qubits in silicon carbide with a high readout contrast, Qiang Li, Jun-Feng Wang, Fei-Fei Yan, Ji-Yang Zhou, Han-Feng Wang, He Liu, Li-Ping Guo, Xiong Zhou, Adam Gali, Zheng-Hao Liu, Zu-Qing Wang, Kai Sun, Guo-Ping Guo, Jian-Shun Tang, Hao Li, Li-Xing You, Jin-Shi Xu, Chuan-Feng Li, Guang-Can Guo, National Science Review 9, nwab122 (2022).

[10] Interview with Adam Gali in Nature Materials, 24, 996 (2025) by Anna Pertsova and Amos Martinez

[11] Editor's suggestion: Strain-Enhanced Spin Readout Contrast in Silicon Carbide Membranes, Haibo Hu, Guodong Bian, Ailun Yi, Chunhui Jiang, Junhua Tan, Qi Luo, Bo Liang, Zhengtong Liu, Xinfang Nie, Dawei Lu, Shumin Xiao, Xin Ou, Ádám Gali, Yu Zhou, and Qinghai Song, Physical Review Letters 135, 110601 (2025). Commentary paper: https://phys.org/news/2025-10-strain-readout-quantum-technologies.html

[12] Non-invasive bioinert room-temperature quantum sensor from silicon carbide qubits, Pei Li, Ji-Yang Zhou, Song Li, Péter Udvarhelyi, Jin-Shi Xu, Chuan-Feng Li, Bing Huang, Guang-Can Guo, and Adam Gali, Nature Materials, 24, 1913 (2025). Commentary paper: https://phys.org/news/2025-11-quantum-sensor-based-silicon-carbide.html