Beyond one and zero: multiple-internal-state read-out for enhanced signal and suppression of systematic errors in experiments with molecules

A waggish but short-sighted physicist once scorned diatomic molecules as having "one atom too many". Yet the benefits that molecules can offer to the fields of quantum information (QI) and particle physics (HEP) have become increasingly compelling. Long-range interactions between molecules are key features of novel QI schemes, and the large internal fields, and closely spaced states of opposite parity, can greatly enhance the signature of novel particle physics in eV-scale spectra. The large number of internal molecular quantum states can be seen as a liability, for instance making laser cooling more difficult. Yet if the populations of multiple internal states can each be determined distinctly, especially in a single experimental shot, one can begin to understand the diversity of states as a distinct asset. The variety of methods known to physical chemists as "action spectroscopy" offers opportunities for enhancing signal-to-noise and suppressing systematic errors. We present examples in the context of JILA's project to measure the electron's electric dipole moment using trapped molecular ions.