Feedback Cooling and Thermometry of a Single Trapped Ion Using a Knife Edge

TL;DR

This paper demonstrates the first feedback cooling of a single trapped ion below the Doppler limit by real-time fluorescence monitoring and electronic feedback, achieving significant temperature reduction.

Contribution

It introduces a novel feedback cooling method for ions using fluorescence detection via a knife edge, surpassing the Doppler cooling limit.

Findings

Achieved cooling of a single ion up to 9 times below the Doppler limit.

Implemented real-time motion detection using fluorescence intensity modulation.

Enhanced fluorescence collection with a parabolic mirror to improve feedback efficiency.

Abstract

We report on the first feedback cooling of a single trapped ion below the Doppler limit of $\hbar\Gamma/2 k_\mathrm{B}$. The motion of a single ion is monitored in real-time and cooled up to 9-times below the Doppler cooling temperature by applying electronic feedback. Real-time motion detection is implemented by imaging the fluorescence photons emitted by the ion onto a knife edge and detecting the transmitted light, a method used so far to cool trapped nanoparticles. The intensity modulation of the fluorescence resulting from the ion motion is used to generate and apply the feedback signal and also to determine the ion temperature. The method benefits from a high rate of detected scattered photons, which can be a challenge, and which we address by using a parabolic mirror for collecting the fluorescence.