There is an urgent quest for room-temperature qubits in nanometer-sized, ultrasmall nanocrystals for quantum biosensing, hyperpolarization of biomolecules, and quantum information processing. Thus far, the preparation of such qubits at the nanoscale has remained futile. Here, we present a synthesis method that avoids any interaction of the solid with high-energy particles and uses self-propagated high-temperature synthesis with a subsequent electrochemical method, the no-photon exciton generation chemistry to produce room-temperature qubits in ultrasmall nanocrystals of sizes down to 3 nm with high yield. We first create the host silicon carbide (SiC) crystallites by high-temperature synthesis and then apply wet chemical etching, which results in ultrasmall SiC nanocrystals and facilitates the creation of thermally stable defect qubits in the material. We demonstrate room-temperature optically detected magnetic resonance signal of divacancy qubits with 3.5% contrast from these nanoparticles with emission wavelengths falling in the second biological window (1000-1380 nm). These results constitute the formation of nonperturbative bioagents for quantum sensing and efficient hyperpolarization.

Department of Atomic Physics Budapest University of Technology and Economics Budafoki út 8 Budapest H 1111 Hungary

Department of Physics Budapest University of Technology and Economics and MTA BME Lendület Spintronics Research Group Budafoki út 8 Budapest H 1111 Hungary

Faculty of Mathematics and Physics Department of Chemical Physics and Optics Charles University Ke Karlovu 3 Prague 12116 Czechia

Institute for Solid State Physics and Optics Wigner Research Centre for Physics PO Box 49 Budapest H 1525 Hungary

Institute for Technical Physics and Materials Science Centre for Energy Research Konkoly Thege M út 29 33 Budapest H 1121 Hungary

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