  Single photon emitters in hexagonal boron nitride offer a gateway to the
future of quantum technologies, yet their identification remains challenging
and subject to ongoing debate. We demonstrate through ab initio calculations
that the optical properties of a carbon chain tetramer are in excellent
agreement with the characteristics of a blue quantum emitter in hexagonal boron
nitride emitting at 435 nm. Its calculated zero-phonon line energy (2.77 eV)
and radiative lifetime (1.6 ns) perfectly align with experimental observations.
The relatively weak electron--phonon coupling (Huang-Rhys factor of 1.5)
indicates intense emission at the zero-phonon line. Despite the absence of an
inversion center in the carbon tetramer, we demonstrate that it exhibits a
negligible linear Stark effect, consistent with experimental findings.
Additionally, our hypothesis explains the experimental observation that the
formation of blue emitters is only possible in samples containing numerous
ultraviolet emitters, which are likely identical to carbon dimers.
