  Two-dimensional honeycomb antiferromagnet becomes an important class of
materials as it can provide a route to Kitaev quantum spin liquid,
characterized by massive quantum entanglement and fractional excitations. The
signatures of its proximity to Kitaev quantum spin liquid in the honeycomb
antiferromagnet includes anisotropic bond-dependent magnetic responses and
persistent fluctuation by frustration in paramagnetic regime. Here, we propose
Cu3Co2SbO6 heterostructures as an intriguing honeycomb antiferromagnet for
quantum spin liquid, wherein bond-dependent and frustrated spins interact with
optical excitons. This system exhibits antiferromagnetism at 16 K with
different spin-flip magnetic fields between a bond-parallel and
bond-perpendicular directions, aligning more closely with the generalized
Heisenberg-Kitaev than the XXZ model. Optical spectroscopy reveals a strong
excitonic transition coupled to the antiferromagnetism, enabling optical
detection of its spin states. Particularly, such spin-exciton coupling presents
anisotropic responses between bond-parallel and bond-perpendicular magnetic
field as well as a finite spin-spin correlation function around 40 K, higher
than twice its N\'eel temperature. The characteristic temperature that remains
barely changed even under strong magnetic fields highlights the robustness of
the spin-fluctuation region. Our results demonstrate Cu3Co2SbO6 as a unique
candidate for the quantum spin liquid phase, where the spin Hamiltonian and
quasiparticle excitations can be probed and potentially controlled by light.
