  It is well-known that the ground state of homogeneous superconducting systems
with spin-orbit coupling (SOC) in the presence of the Zeeman field is the
so-called helical state, which is characterized by the phase modulation of the
order parameter, but zero supercurrent density. In this work we investigate the
realization of the helical state in a hybrid system with spatially separated
superconductivity and exchange field by considering S/F bilayer on top of a 3D
topological insulator. This system is characterized by strong spin-momentum
locking and, consequently, provides the most favorable conditions for the
helical state generation. The analysis is based on the microscopic theory in
terms of the quasiclassical Green's functions. We demonstrate that in the
bilayer the helical state survives if the exchange field has non-zero component
perpendicular to the S/F interface even in spite of the fact that the
superconducting order parameter and the exchange field are spatially separated.
At the same time, in this spatially inhomogeneous situation the helical state
is accompanied by the spontaneous currents distributed over the bilayer in such
a way as to have zero average. Further, we show that this hybrid helical state
gives rise to nonreciprocity in the system. We demonstrate the realization of
the superconducting diode effect and show that the degree of nonreciprocity is
very large in the considered system due to the strongest possible value of the
spin-momentum locking.
