Phase protection of Fano-Feshbach resonances

Abstract

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. Their energy positions were shown to follow quantum chaotic statistics. In contrast, their lifetimes have so far escaped a similarly comprehensive understanding. Here, we show that bound states, despite being resonantly coupled to a scattering state, become 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.

Quantum Effects in Cold and Controlled Molecular Dynamics

“…All three examples share a rigorous theoretical description based on first principles. More importantly for the present contribution, the quantum effects discussed here have been observed in experiments with molecular beams [3,4,7]. They thus testify to the topicality and continuing significance of the molecular beam technique developed by Otto Stern and colleagues.…”

“…This chapter reviews three examples of such quantum effects beyond entanglement from recent work of my group-tunneling resonances that emerge in cold collisions and that can be used to probe interparticle interactions [3], Fano-Feshbach resonances that can be protected against decay by a suitable phase condition [4], and quantum pathway interference in the circular dichroism of photoelectrons that is observed after the photoionization of chiral molecules [5,6]. All three examples share a rigorous theoretical description based on first principles.…”

Direct observation of a Feshbach resonance by coincidence detection of ions and electrons in Penning ionization collisions

“…presents the observed exponential decay of t b for Ar + with a radial velocity between 209 and 295 m/s, with an observed lifetime of 1.01 μs. Our lifetime measurement is not sensitive enough to resolve lifetimes arising from different rovibrational states; however, the measured lifetime falls into the range predicted by theory31 . Further support of our observation is…”

“…For the HeAr + system, this enabled the observation of a Feshbach resonance identified as the predissociation of the HeAr + A 2 molecular state. Although the kinetic energy distributions of He * + Ar PI products were obtained in previous experiments 31,33,34 , the lack of correlation between ionic and electronic data left the details of the postionization dynamics completely hidden. Coincidence detection of ions and electrons provides the mapping between initial and final states and enables the direct observation of the scattering resonance.…”

“…where t b is the absolute TOF of the quasi-bound HeAr + in the A 2 state before dissociation, ΔTOF is the addition to the ions TOF relative to the center TOF peak of Ar + , and m Ar m HeAr is the mass ratio. given by the lack of Feshbach ions in previous studies of Ne * + Ar PI reaction due to the long lifetime of the quasi-bound NeAr + A 2 state 31,32 .…”