  The origin of quantum fluctuations responsible for the spin liquid state in
Tb$_2$Ti$_2$O$_7$ has remained a long standing problem. By synchrotron-based
terahertz measurements, we show evidence of strong coupling between the
magnetic and lattice degrees of freedom that provides a path to the quantum
melting process. As revealed by hybrid crystal electric field-phonon
excitations that appear at 0.67 THz below 200 K, and around 0.42 THz below 50
K, the double vibronic process is unique for Tb$^{3+}$ in the titanate family
due to adequate energy matching and strong quadrupolar moments. The results
suggest that lattice motion can indeed be the driving force behind spin flips
within the hybridized ground and first excited states, promoting quantum terms
in the effective Hamiltonian that describes Tb$_2$Ti$_2$O$_7$.
