Direct thermometry and enhanced cooling of planar modes in a 2D ion crystal in a Penning trap

Quantum sensing and simulation experiments in two-dimensional ion crystals in Penning traps depend on the frequency stability of the ions’ collective axial motional modes. Precise control of the axial motion is achieved by Doppler laser cooling, which effectively cools out-of-plane axial modes and in-plane high frequency cyclotron modes but poorly cools in-plane low frequency magnetron modes. Recent research has demonstrated that elevated magnetron mode temperatures reduce axial mode stability resulting in a significant detrimental effect on the efficacy of quantum operations. To overcome this issue, we present a new method for enhancing the cooling of magnetron modes by resonantly coupling them to the well-cooled cyclotron modes. The coupling enables energy exchange between the two mode branches resulting in a reduction of the magnetron mode temperature. To characterize the effectiveness of the improved magnetron mode cooling, we also present a new technique to directly measure in-plane mode temperatures using a spatial light modulator (SLM). The SLM is used to imprint a radial gradient AC Stark shift (ACSS) pattern in the crystal plane. Ion in-plane motion causes a modulation of the ACSS as seen by each ion resulting in spin dephasing that is proportional to the size of the ion’s motion. Measurement of the ion spin dephasing therefore provides a quantitative measurement of the in-plane mode temperature. Preliminary work indicates a reduction of the magnetron mode temperature from 10 mK to ~1 mK with the application of mode coupling.