Alexander Blech

We suggest a protocol for the sympathetic cooling of a molecular asymmetric top rotor co-trapped with laser-cooled atomic ions, based on resonant coupling between the molecular ion's electric dipole moment and a common normal mode of the trapped particles. By combining sympathetic sideband laser cooling with coherent microwave excitation, we demonstrate the efficient depopulation of arbitrary rotational subspaces and the ability to cool an incoherent distribution of rotational states into a single, well-defined quantum state. This capability opens the door to exploiting the rotational Hilbert space for applications in quantum information processing and high-precision spectroscopy.

Decay of bound states due to coupling with free particle states is a general phenomenon occurring at energy scales from MeV in nuclear physics to peV in ultracold atomic gases. Such a coupling gives rise to Fano-Feshbach resonances (FFR) that have become key to understanding and controlling interactions - in ultracold atomic gases, but also between quasiparticles such as microcavity polaritons. The energy positions of FFR were shown to follow quantum chaotic statistics. In contrast, lifetimes which are the fundamental property of a decaying state, have so far escaped a similarly comprehensive understanding. Here we show that a bound state, despite being resonantly coupled to a scattering state, becomes protected from decay whenever the relative phase is a multiple of $π$. We observe this phenomenon by measuring lifetimes spanning four orders of magnitude for FFR of spin-orbit excited molecular ions with merged beam and electrostatic trap experiments. Our results provide a blueprint for identifying naturally long-lived states in a decaying quantum system.