  We report continuous Raman sideband cooling (CRSC) of a long ion chain 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
$\eta = 1.3$, spanning a frequency bandwidth of 4 MHz. Compared to traditional
ground-state cooling methods such as pulsed sideband cooling or
electromagnetic-induced-transparency (EIT) cooling, our method offers two key
advantages: robustness to timing errors; and an ultra-wide bandwidth unlimited
by the number of ions. 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.
