We uncover the fine structure of a silicon vacancy in isotopically purified silicon carbide (4H-${}^{28}\mathrm{SiC}$) and reveal not yet considered terms in the spin Hamiltonian, originated from the trigonal pyramidal symmetry of this spin-$3/2$ color center. These terms give rise to additional spin transitions, which would be otherwise forbidden, and lead to a level anticrossing in an external magnetic field. We observe a sharp variation of the photoluminescence intensity in the vicinity of this level anticrossing, which can be used for a purely all-optical sensing of the magnetic field. We achieve dc magnetic field sensitivity better than $100\mathrm{nT}/\sqrt{\mathrm{Hz}}$ within a volume of $3\times {10}^{-7}\mathrm{m}{\mathrm{m}}^3$ at room temperature and demonstrate that this contactless method is robust at high temperatures up to at least 500 K. As our approach does not require application of radio-frequency fields, it is scalable to much larger volumes. For an optimized light-trapping waveguide of $3{\mathrm{mm}}^3$, the projection noise limit is below $100\mathrm{fT}/\sqrt{\mathrm{Hz}}$.

We report a giant thermal shift of 2.1 MHz/K related to the excited-state zero-field splitting in the silicon vacancy centers in 4H silicon carbide. It is obtained from the indirect observation of the optically detected magnetic resonance in the excited state using the ground state as an ancilla. Alternatively, relative variations of the zero-field splitting for small temperature differences can be detected without application of radiofrequency fields, by simply monitoring the photoluminescence intensity in the vicinity of the level anticrossing. This effect results in an all-optical thermometry technique with temperature sensitivity of 100 mK/Hz^1/2 for a detection volume of approximately 10⁻⁶ mm³. In contrast, the zero-field splitting in the ground state does not reveal detectable temperature shift. Using these properties, an integrated magnetic field and temperature sensor can be implemented on the same center.

One of the challenges in the field of quantum sensing and information processing is to selectively address and coherently manipulate highly homogeneous qubits subject to external perturbations. Here, we present room-temperature coherent control of high-dimensional quantum bits, the so-called qudits, associated with vacancy-related spins in silicon carbide enriched with nuclear spin-free isotopes. In addition to the excitation of a spectrally narrow qudit mode at the pump frequency, several other modes are excited in the electron spin resonance spectra whose relative positions depend on the external magnetic field. We develop a theory of multipole spin dynamics and demonstrate selective quantum control of homogeneous spin packets with sub-MHz spectral resolution. Furthermore, we perform two-frequency Ramsey interferometry to demonstrate absolute dc magnetometry, which is immune to thermal noise and strain inhomogeneity.

Silicon carbide (SiC) hosts many interesting defects that can potentially serve as qubits for a range of advanced quantum technologies. Some of them have very interesting properties, making them potentially useful, e.g., as interfaces between stationary and flying qubits. Here we present a detailed overview of the relevant properties of the spins in silicon vacancies of the $6H$-SiC polytype. This includes the temperature-dependent photoluminescence, optically detected magnetic resonance, and the relaxation times of the longitudinal and transverse components of the spins during free precession as well as under the influence of different refocusing schemes.

We report the influence of static mechanical deformation on the zero-field spin splitting of silicon vacancies in silicon carbide at room temperature. We use AlN/6H-SiC heterostructures deformed by growth conditions and monitor the stress distribution as a function of distance from the heterointerface with spatially resolved confocal Raman spectroscopy. The zero-field spin splitting of the V1/V3 and V2 centers in 6H-SiC, measured by optically detected magnetic resonance, reveals significant changes at the heterointerface compared to the bulk value. This approach allows unambiguous determination of the spin-deformation interaction constant, which is 0.75 GHz/strain for the V1/V3 centers and 0.5 GHz/strain for the V2 centers. Provided piezoelectricity of AlN, our results offer a strategy to realize fine tuning of spin transition energies in SiC by deformation.

We grow AlN/4H-SiC and AlN/6H-SiC heterostructures by physical vapor deposition and characterize the heterointerface with micro-scale resolution. We investigate the spatial stress and strain distribution in these heterostructures using confocal Raman spectroscopy. We measure the spectral shifts of various vibrational Raman modes across the heterointerface and along the entire depth of the 4H- and 6H-SiC layers. Using the earlier experimental prediction for the phonon-deformation potential constants, we determine the stress tensor components in SiC as a function of the distance from the AlN/SiC heterointerface. Despite the fact that the lattice parameter of SiC is smaller than that of AlN, the SiC layers are compressively strained at the heterointerface. This counterintuitive behavior is explained by different coefficients of thermal expansion of SiC and AlN when the heterostructures are cooled from growth to room temperature. The compressive stress values are maximum at the heterointerface, approaching 1 GPa, and relax to the equilibrium value on the scale of several tens of micrometers from the heterointerface.