  We propose an Aharonov-Bohm interferometer consisted of two topological
superconducting chains (TSCs) to generate coherence of Majorana qubits, each
qubit is made of two Majorana zero modes (MZMs) with the definite fermion
parity. We obtain the generalized exact master equation as well as its solution
and study the real-time dynamics of the MZM qubit states under various
operations. We demonstrate that by tuning the magnetic flux, the decoherence
rates can be modified significantly, and dissipationless MZMs can be generated.
By applying the bias voltage to the leads, one can manipulate MZM qubit
coherence and generate a nearly pure superposition state of Majorana qubit.
Moreover, parity flipping between MZM qubits with different fermion parities
can be realized by controlling the coupling between the leads and the TSCs
through gate voltages.
