Title: Controlling Majorana hybridization in magnetic chain-superconductor systems

Abstract: We propose controlling the hybridization between Majorana zero modes at the ends of magnetic adatom chains on superconductors by an additional magnetic adatom deposited close by. By tuning the additional adatom's magnetization, position, and coupling to the superconductor, we can couple and decouple the Majorana modes as well as control the ground state parity. The scheme is independent of microscopic details in ferromagnetic and helical magnetic chains on superconductors with and without spin-orbit coupling, which we show by studying their full microscopic models and their common low-energy description. Our results show that scanning tunneling microscopy and electron spin resonance techniques are promising tools for controlling the Majorana hybridization in magnetic adatoms-superconductor setups, providing a basis for Majorana parity measurements, fusion, and braiding techniques.