  We utilize single-crystal neutron diffraction to study the $C2/m$ structure
of potassium hydrogen carbonate (KHCO$_3$) and macroscopic quantum entanglement
above the phase transition at $T_c = 318$ K. Whereas split atom sites could be
due to disorder, the diffraction pattern at 340 K evidences macroscopic proton
states identical to those previously observed below $T_c$ by F. Fillaux et al.,
(2006 \textit{J. Phys.: Condens. Matter} \textbf{18} 3229). We propose a
theoretical framework for decoherence-free proton states and the calculated
differential cross-section accords with observations. The structural transition
occurs from one ordered $P2_1/a$ structure ($T < T_c$) to another ordered
$C2/m$ structure. There is no breakdown of the quantum regime. It is suggested
that the crystal is a macroscopic quantum object which can be represented by a
state vector. Raman spectroscopy and quasi-elastic neutron scattering suggest
that the $|C2/m>$ state vector is a superposition of the state vectors for two
$P2_1/a$-like structures symmetric with respect to $(a,c)$ planes.
