  Semiconductors offer a promising platform for physical implementation of
qubits, but their broad adoption is presently hindered by limited scalability
and/or very low operating temperatures. Learning from the nitrogen-vacancy
centers in diamond, 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. Transition metal (TM)
impurities in silicon are common paramagnetic deep defects, but a comprehensive
theoretical study of the whole 3$d$ series that considers generalized Koopmans'
condition is missing. We apply the HSE06(+U) method to examine their potential
as optically active spin qubits and identify seven TM impurities that have
optically allowed triplet-triplet transitions within the silicon band gap.
These results provide the first step toward silicon-based qubits with higher
operating temperatures for quantum sensing. Additionally, these point defects
could lead to spin-photon interfaces in silicon-based qubits and devices for
mid-infrared free-space communications.
