Multi-mode Ground-state Cooling of a Trapped Ion Chain beyond the Lamb-Dicke Regime

Laser cooling of a trapped ion chain to the motional ground state is the starting point of quantum information processing with trapped ions. Traditionally, ground-state preparation is carried out by resolved sideband cooling, which requires reaching the Lamb-Dicke regime to work efficiently. However, this requirement is challenging for axial modes of Be+ in a Paul trap, due to the light mass and low motional frequency, leading to Lamb-Dicke factors approaching 1. Here, we report a novel method to cool multiple motional modes in an ion chain to the ground state in parallel. We simultaneously drive all the red sideband transitions, and add a weak optical pumping to constantly reset the spin-state. We demonstrate cooling a single ion to the ground-state for η up to 0.78 within 200us. We also show effective parallel cooling for the axial modes of 30 ions to the near ground state. Our method has a speeding effect compared to traditional resolved sideband cooling, works robustly far outside the Lamb-Dicke regime, and can apply to any atomic system with arbitrary motional spectrums.