The identification of a microscopic configuration of point defects acting as quantum bits is a key step in the advance of quantum information processing and sensing. Among the numerous candidates, silicon-vacancy related centers in silicon carbide (SiC) have shown remarkable properties owing to their particular spin-3/2 ground and excited states. Although, these centers were observed decades ago, two competing models, the isolated negatively charged silicon vacancy and the complex of negatively charged silicon vacancy and neutral carbon vacancy [Phys. Rev. Lett. 115, 247602 (2015)], are still argued as an origin. By means of high-precision first-principles calculations and high-resolution electron spin resonance measurements, we here unambiguously identify the Si-vacancy related qubits in hexagonal SiC as isolated negatively charged silicon vacancies. Moreover, we identify the Si-vacancy qubit configurations that provide room-temperature optical readout. DOI: 10.1103/PhysRevB.96.161114 Point defects in solids acting as quantum bits (qubits) are a highly promising platform for quantum information processing (QIP) and nanoscale sensor applications where typically their electron spin provides the functional quantum states. There are qubits that have long electron spin coherence times [1][2][3][4][5], and some of them have been demonstrated to persist up to room temperature [1,4]. These electron spins can be optically initialized and read out [2,6-9], making them very attractive candidates for QIP and related applications [10][11][12]. Among these qubits, silicon-vacancy related defects in hexagonal polytypes of SiC, such as 4H -and 6H -SiC, have shown favorable spin properties [13,14], demonstrated even at a single defect level at room temperature [4]. Two and three different silicon-vacancy related centers were observed in 4H -and 6H -SiC, where the corresponding photoluminescence (PL) lines are denoted as V1 and V2, and V1, V2, and V3, [15,16], respectively. The V2 line in 4H -SiC [4] and the V2 and V3 lines in 6H -SiC [13] are sufficiently strong to observe their corresponding electron spin via optically detected magnetic resonance (ODMR) measurements at room temperature. In particular, it has been demonstrated that the V2 color center in 4H -SiC can be used for magnetometer [17][18][19][20] and nanoscale thermometer [21] applications and as a room-temperature maser [14].Today, it is widely accepted that V1-V3 PL lines and T V1 -T V3 electron paramagnetic resonance (EPR) signals in 4H -and 6H -SiC are related to spin-3/2 negatively charged silicon vacancies [22][23][24][25]. On the other hand, the actual microscopic configuration of these vacancy related centers is still debated. The unanswered question is whether these centers are isolated silicon vacancies [V Si difference between the two models is how the observed finite zero-field splitting (ZFS) of the ground-state spin sublevels is explained. In model I, it is assumed that the C 3v symmetric crystal field, allowing nonzero ZFS [26], is strong enough t...