  Quantum bit or qubit is a two-level system, which builds the foundation for
quantum computation, simulation, communication and sensing. Quantum states of
higher dimension, i.e., qutrits (D = 3) and especially qudits (D = 4 or
higher), offer significant advantages. Particularly, they can provide
noise-resistant quantum cryptography, simplify quantum logic and improve
quantum metrology. Flying and solid-state qudits have been implemented on the
basis of photonic chips and superconducting circuits, respectively. However,
there is still a lack of room-temperature qudits with long coherence time and
high spectral resolution. The silicon vacancy centers in silicon carbide (SiC)
with spin S = 3/2 are quite promising in this respect, but until now they were
treated as a canonical qubit system. Here, we apply a two-frequency protocol to
excite and image multiple qudit modes in a SiC spin ensemble under ambient
conditions. Strikingly, their spectral width is about one order of magnitude
narrower than the inhomogeneous broadening of the corresponding spin resonance.
By applying Ramsey interferometry to these spin qudits, we achieve a spectral
selectivity of 600 kHz and a spectral resolution of 30 kHz. As a practical
consequence, we demonstrate absolute DC magnetometry insensitive to thermal
noise and strain fluctuations.
