Continuous Raman sideband cooling beyond the Lamb-Dicke regime in a trapped ion chain

Abstract

We report continuous Raman sideband cooling of a long ion chain close to the motional ground state beyond the Lamb-Dicke (LD) regime. By driving multiple sideband transitions simultaneously, we show that nearly all axial modes of a 24-ion chain are cooled to the ground state, with an LD parameter as large as $\ensuremath{\eta}=1.3$, spanning a frequency bandwidth of 4 MHz. Compared to traditional ground-state cooling methods such as pulsed sideband cooling or electromagnetically-induced-transparency cooling, our method offers two key advantages: disposal of pulse optimization, and a wide bandwidth covering the entire spectrum. This technique contributes as a crucial step for large-scale quantum information processing with linear ion chains and higher dimensions alike, and can be readily generalized to other atomic and molecular systems.