Vacancy-related centres in silicon carbide are attracting growing attention because of their appealing optical and spin properties. These atomic-scale defects can be created using electron or neutron irradiation; however, their precise engineering has not been demonstrated yet. Here, silicon vacancies are generated in a nuclear reactor and their density is controlled over eight orders of magnitude within an accuracy down to a single vacancy level. An isolated silicon vacancy serves as a near-infrared photostable single-photon emitter, operating even at room temperature. The vacancy spins can be manipulated using an optically detected magnetic resonance technique, and we determine the transition rates and absorption crosssection, describing the intensity-dependent photophysics of these emitters. The on-demand engineering of optically active spins in technologically friendly materials is a crucial step toward implementation of both maser amplifiers, requiring high-density spin ensembles, and qubits based on single spins.

Quantum systems can provide outstanding performance in various sensing applications, ranging from bioscience to nanotechnology. Atomic-scale defects in silicon carbide are very attractive in this respect because of the technological advantages of this material and favorable optical and radio frequency spectral ranges to control these defects. We identified several, separately addressable spin-3/2 centers in the same silicon carbide crystal, which are immune to nonaxial strain fluctuations. Some of them are characterized by nearly temperature independent axial crystal fields, making these centers very attractive for vector magnetometry. Contrarily, the zero-field splitting of another center exhibits a giant thermal shift of −1.1 MHz/K at room temperature, which can be used for thermometry applications. We also discuss a synchronized composite clock exploiting spin centers with different thermal response.

We uncover the fine structure of a silicon vacancy in isotopically purified silicon carbide (4H-28 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 nT/ √ Hz within a volume of 3 × 10 −7 mm 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 radiofrequency fields, it is scalable to much larger volumes. For an optimized light-trapping waveguide of 3 mm 3 the projection noise limit is below 100 fT/ √ Hz.

We demonstrate that silicon carbide (SiC) with natural isotope abundance can preserve a coherent spin superposition in silicon vacancies over unexpectedly long time approaching 0.1 seconds. The spin-locked subspace with drastically reduced decoherence rate is attained through the suppression of heteronuclear spin cross-talking by applying a moderate magnetic field in combination with dynamic decoupling from the nuclear spin baths. We identify several phonon-assisted mechanisms of spin-lattice relaxation, ultimately limiting quantum coherence, and find that it can be extremely long at cryogenic temperature, equal or even longer than 8 seconds. Our approach may be extended to other polyatomic compounds and open a path towards improved qubit memory for wafer-scale quantum techmologies.PACS numbers: 76.30. Mi, 42.50.Dv, 76.70.Hb Introduction -Long electron quantum coherence in solid-state systems is the ultimate prerequisite for new technologies based on quantum phenomena [1,2]. Particularly, the sensitivity of quantum sensors scales with the electron spin coherence time T 2 [3,4]. One of the common sources of decoherence is the interaction with fluctuating nuclear spins, and the usual way to prolong spin coherence is to perform isotope purification of the crystal. Indeed, the longest electron T 2 times of about 1 s and 0.6 s have been reported for spin-free silicon 28 Si and diamond 12 C crystals, respectively [5,6]. However, isotope purification is a technologically demanding procedure, which is not always possible. Therefore, one of the key challenges in quantum information science is to achieve long-lived spin coherence in natural materials.To address this goal, we combine two approaches. First, we exploit the suppression of mutual spin flip-flop processes between different types of nuclei in binary compounds, which occur in strong enough magnetic fields according to the theoretical simulations of Ref. [7]. Second, we use a periodic train of radiofrequency (RF) pulses to refocus spin coherence and decouple electron spins from inhomogeneous environment, similar to that applied for color centers [6] and quantum dots (QDs) [8].In recent years, SiC is attracting continuously growing interest as a technologically perspective platform for quantum spintronics [9][10][11][12][13][14][15][16][17] with the ability for single spin engineering and control [18][19][20][21]. The longest T 2 in SiC reported to date is 1 ms at cryogenic temperature [19]. We observe that in a finite magnetic field, a coherent spin superposition can be locked over longer time, which continuously increases up to about 75 ms with the number of decoupling pulses. The absence of saturation indicates that the longest possible spin locking time T

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/Hz1/2 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.

We show that uniaxial color centers in silicon carbide with hexagonal lattice structure can be used to measure not only the strength but also the polar angle of the external magnetic field with respect to the defect axis with high precision. The method is based on the optical detection of multiple spin resonances in the silicon vacancy defect with quadruplet ground state. We achieve a perfect agreement between the experimental and calculated spin resonance spectra without any fitting parameters, providing angle resolution of a few degrees in the magnetic field range up to several millitesla. Our approach is suitable for ensembles as well as for single spin-3/2 color centers, allowing for vector magnetometry on the nanoscale at ambient conditions. PACS numbers: 76.30. Mi, 71.70.Ej, 76.70.Hb, 61.72.jd Optically addressable atomic-scale spin centers constitute the basis for nanomagnetometry with high sensitivity and high spatial resolution [1,2]. The most prominent example is the nitrogen-vacancy (NV) defect in diamond and several benchmark experiments have been performed using this system [3][4][5], including proton nuclear magnetic resonance on the nanometer scale [6,7]. The principle of magnetometry with spin-carrying color centers is based on optical detection of magnetic resonance (ODMR), subject to external magnetic field. In case of individual NV defects with spin S = 1, the projection of the magnetic field on the defect axis is measured. The NV defect in the diamond cubic lattice is oriented along one out of four 111 crystallographic axes and, therefore, using ensemble experiments the magnetic field vector B can be reconstructed [8,9]. Ensembles of the NV defects are also suggested for the implementation of high precision magnetic field sensors with femtotesla sensitivity [10,11] and solid-state frequency standards [12]. These implementations require high homogeneity of the NV centers. The NV defects can be fabricated with preferential alignment [13,14], and using nonlinear shift of the ODMR lines in relatively high magnetic fields of several tens of millitesla the transverse field component can be reconstructed [15]. However, in many demanding applications much lower magnetic fields should be detected, and the information on the magnetic field orientation is difficult to extract in this approach.Here, we demonstrate an alternative approach to implement vector magnetometry for magnetic fields below several millitesla, which is suitable for ensemble as well as for individual uniaxial spin centers with S = 3/2 [16]. As a model system, we consider a silicon vacancy (V Si ) in silicon carbide (SiC) [17][18][19][20]. Due to the polymorphism of SiC, there is a large variety of vacancy-related defects with appealing quantum properties [16,[21][22][23][24][25][26][27][28][29][30][31][32][33]. All experiments presented here have been performed at room temperature on a 4H-SiC bulk crystal, possessing hexagonal lattice structure. The crystal has been grown by the standard sublimation technique, such that the [0001]...

Constructing quantum devices comprises various challenging tasks, especially when concerning their nanoscale geometry. For quantum color centers, the traditional approach is to fabricate the device structure after the nondeterministic placement of the centers. Reversing this approach, we present the controlled generation of quantum centers in silicon carbide (SiC) by focused proton beam in a noncomplex manner without need for pre- or postirradiation treatment. The generation depth and resolution can be predicted by matching the proton energy to the material's stopping power, and the amount of quantum centers at one specific sample volume is tunable from ensembles of millions to discernible single photon emitters. We identify the generated centers as silicon vacancies through their characteristic magnetic resonance signatures and demonstrate that they possess a long spin-echo coherence time of 42 ± 20 μs at room temperature. Our approach hence enables the fabrication of quantum hybrid nanodevices based on SiC platform, where spin centers are integrated into p-i-n diodes, photonic cavities, and mechanical resonators.

We discuss the fine structure and spin dynamics of spin-3/2 centers associated with silicon vacancies in silicon carbide. The centers have optically addressable spin states which makes them highly promising for quantum technologies. The fine structure of the spin centers turns out to be highly sensitive to mechanical pressure, external magnetic and electric fields, temperature variation, etc., which can be utilized for efficient room-temperature sensing, particularly by purely optical means or through the optically detected magnetic resonance. We discuss the experimental achievements in magnetometry and thermometry based on the spin state mixing at level anticrossings in an external magnetic field and the underlying microscopic mechanisms. We also discuss spin fluctuations in an ensemble of vacancies caused by interaction with environment.