Experimental Realization of Multi-ion Sympathetic Cooling on a Trapped Ion Crystal

Sympathetic cooling in large ion crystals is a necessary technology for ion-trap-based quantum memory, large-scale quantum computing and simulation that requires long runtime. In order to resist the decoherence induced by the motional heating or background collisions, it is of crucial importance to cool large ion crystals at runtime without affecting the internal states of the computational qubits. In this work, we propose that it is feasible to achieve cooling effects near global Doppler cooling limit by optimizing the choice of several adjacent cooling ions according to the collective normal modes. We also experimentally demonstrate the cooling dynamics of multi-ion sympathetic cooling with only two cooling ions on an   ion chain. With increasing the crystal scale, a tiny fraction of ions suffices to efficiently cool the whole ion crystal. Despite employing a single ion species in this work, our scheme is also appropriate for dual-type qubits or ions with small mass difference. Our proposal can directly be generalized to larger 2D and 3D ion crystals. It thus provides an important enabling step for the future large-scale trapped ion quantum computing and long-time quantum memory.