Robust Polarization Gradient Cooling of Trapped Ions

TL;DR

This paper demonstrates three-dimensional polarization gradient cooling of trapped calcium ions using near-resonant laser beams, achieving low phonon numbers and robustness against high initial motional states, with laser spectral purity being crucial.

Contribution

The study introduces a robust polarization gradient cooling method for trapped ions that remains effective even with high initial motional energy and addresses laser spectral impurity issues.

Findings

Achieved mean phonon numbers of 5.4 and 3.3 in axial and radial directions.

Cooling remains effective with high initial phonon occupation and micromotion.

Spectral impurity of the laser affects cooling efficiency, mitigated by cavity filtering.

Abstract

We implement three-dimensional polarization gradient cooling of trapped ions. Counter-propagating laser beams near $393\,$nm impinge in lin$\,\perp\,$lin configuration, at a frequency below the S$_{1/2}$ to P$_{3/2}$ resonance in $^{40}$Ca$^+$. We demonstrate mean phonon numbers of $5.4(4)$ at a trap frequency of $2\pi \times 285\,$kHz and $3.3(4)$ at $2\pi\times480\,$kHz, in the axial and radial directions, respectively. Our measurements demonstrate that cooling with laser beams detuned to lower frequencies from the resonance is robust against an elevated phonon occupation number, and thus works well for an initial ion motion far out of the Lamb-Dicke regime, for up to four ions, and for a micromotion modulation index $\beta\leq 0.1$. Still, we find that the spectral impurity of the laser field influences both, cooling rates and cooling limits. Thus, a Fabry-P\'{e}rot cavity filter is employed for efficiently suppressing amplified spontaneous emission of the diode laser.