Semiconductors offer a promising platform for the physical implementation of qubits, demonstrated by the successes in quantum sensing, computing, and communication. The broad adoption of semiconductor qubits is presently hindered by limited scalability and/or very low operating temperatures. Learning from the NV$^{-}$ centers in diamond, whose optical properties enable high operating temperature, our goal is to find equivalent optically active point defect centers in crystalline silicon, which could be advantageous for their scalability and integration with classical devices. Motivated by the fact that transition metal impurities in silicon typically produce deep carrier trapping centers, we apply first-principles methods to investigate electronic and optical properties of these deep-level defects and subsequently examine their potential for Si-based qubits. We identify nine transition metal impurities that have optically allowed triplet-triplet transitions within the Si band gap, which could be considered candidates for a Si-based qubit. These results provide the first step toward Si-based qubits with higher operating temperatures and spin-photon interfaces for quantum communication.
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