Laser Cooling Planar Motion of a 2D Ion Crystal with a Strong Rotating Wall

2D planar ion crystals stored in Penning traps offer an attractive platform for quantum sensing and simulation protocols on hundreds of qubits¹. However, to realize these applications, the crystal must be cooled to milli-Kelvin temperatures. The motion of the crystal consists of three mode branches: out of plane axial modes (along B-field) and in plane cyclotron and magnetron like modes (perpendicular to B-field)². Conventional doppler laser cooling techniques on axial and cyclotron modes reliably reach milli-Kelvin temperatures, however, a nonuniform beam intensity is required to begin cooling the potential energy dominated magnetron modes³. Recent work indicates that magnetron modes are not cooled below 10mK in experiment. Uncooled magnetron modes broaden the axial mode spectra, limiting the performance of quantum protocols⁴. Our full-dynamics simulations⁵ suggest that magnetron modes can be cooled to milli-Kelvin temperatures by raising the strength of an oscillating rf quadrupolar field (rotating wall) and utilizing a narrower beam width. In tandem, a stronger rotating wall overcomes torques from the nonuniform planar beam, and the narrower beam width increases the intensity gradient that cools magnetron modes. Furthermore, we present simulation and theoretical work characterizing the disparate planar cooling rates of the rapidly cooled (millisecond) cyclotron mode branch and slowly cooled (hundred millisecond) magnetron mode branch.

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