Sympathetic cooling and detection of a hot trapped ion by a cold one

TL;DR

This paper demonstrates a method to sympathetically cool and detect a hot trapped ion using a cold ion, providing insights into cooling dynamics and a new detection technique for high-energy ions.

Contribution

It introduces a novel technique for detecting hot ions via motional state monitoring of a sympathetically cooled ion, and models the cooling dynamics in weak Coulomb interaction regimes.

Findings

High neutral atom velocities observed with laser ablation.

Long sympathetic cooling times before ion crystallization.

Successful detection of hot ions through motional state monitoring.

Abstract

We investigate the dynamics of an ion sympathetically cooled by another laser-cooled ion or small ion crystal. To this end, we develop simple models of the cooling dynamics in the limit of weak Coulomb interactions. Experimentally, we create a two-ion crystal of Ca$^+$ and Al$^+$ by photo-ionization of neutral atoms produced by laser ablation. We characterize the velocity distribution of the laser-ablated atoms crossing the trap by time-resolved fluorescence spectroscopy. We observe neutral atom velocities much higher than the ones of thermally heated samples and find as a consequence long sympathethic cooling times before crystallization occurs. Our key result is a new technique for detecting the loading of an initially hot ion with energy in the eV range by monitoring the motional state of a Doppler-cooled ion already present in the trap. This technique not only detects the ion but also provides information about dynamics of the sympathetic cooling process.