  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.
