I will present our recent progress in the coherent coupling of spectrally separated harmonic modes of motion of a trapped ion crystal and its applications. We realize high-fidelity coherent exchange of single motional quanta between modes – where the timing, strength, and phase of the coupling are controlled through an oscillating electric potential with suitable spatial variation. The coupling rate can be made much larger than the decoherence rates, enabling demonstrations of high-fidelity quantum state transfer, entanglement of motional modes, and Hong-Ou-Mandel-type interference. We demonstrate quantum non-demolition measurement of a quantum state stored in a ‘protected’ motional mode by using high-fidelity swaps. In addition to the QND demonstration, I will discuss a scheme that can potentially realize various bosonic quantum error correction codes. Mode-mode coupling can be used for speeding up operations in a mixed-species ion crystal. We demonstrate indirect ground-state cooling of weakly laser-coupled motional modes. I will also present how to boost logic operations between ions of different species via two coupled modes.