Coherence of Symmetry-Protected Rotational Qubits in Cold Polyatomic Molecules

Polar polyatomic molecules provide an ideal but largely unexplored platform to encode qubits in rotational states. Here, we trap cold (100-600 mK) formaldehyde (H$_2$CO) inside an electric box and perform a Ramsey-type experiment to observe long-lived (~100 $\mu$s) coherences between symmetry-protected molecular states with opposite rotation but identical orientation, representing a quasi-hidden molecular degree of freedom. As a result, the observed qubit is insensitive to the magnitude of an external electric field, and depends only weakly on magnetic fields. Our findings provide a basis for future quantum and precision experiments with trapped cold molecules.