High-energy-resolution measurement of ultracold atom-ion collisional cross section

TL;DR

This paper introduces a high-resolution method to measure ultracold atom-ion collisional cross sections, revealing quantum effects and matching classical predictions, with improved energy resolution over previous experiments.

Contribution

The authors developed a novel technique for directly measuring atom-ion collision cross sections at ultracold energies with unprecedented resolution, enabling detailed quantum collision studies.

Findings

Measured inelastic collision cross sections agree with classical Langevin predictions.

Achieved energy resolution below 200 μK·k_B, surpassing previous experiments.

Method can be applied to various inelastic processes and quantum resonance searches.

Abstract

The cross section of a given process fundamentally quantifies the probability for that given process to occur. In the quantum regime of low energies, the cross section can vary strongly with collision energy due to quantum effects. Here, we report on a method to directly measure the atom-ion collisional cross section in the energy range of 0.2-12 mK$\cdot$ k$_B$, by shuttling ultracold atoms trapped in an optical-lattice across a radio-frequency trapped ion. In this method, the average number of atom-ion collisions per experiment is below one such that the energy resolution is not limited by the broad (power-law) steady-state atom-ion energy distribution. Here, we estimate that the energy resolution is below 200 $\mu$K$\cdot$k$_B$, limited by drifts in the ion's excess micromotion compensation and can be reduced to the 10's $\mu$K$\cdot$k$_B$ regime. This resolution is one order-of-magnitude better than previous experiments measuring cold atom-ion collisional cross section energy dependence. We used our method to measure the energy dependence of the inelastic collision cross sections of a non-adiabatic Electronic-Excitation-Exchange (EEE) and Spin-Orbit Change (SOC) processes. We found that in the measured energy range, the EEE and SOC cross sections statistically agree with the classical Langevin cross section. This method allows for measuring the cross sections of various inelastic processes and opens up possibilities to search for atom-ion quantum signatures such as shape-resonances.