  When time-reversal and inversion symmetry are broken, superconducting
circuits may exhibit a so-called diode effect, where the critical currents for
opposite directions of the current flow differ. In recent years, this effect
has been observed in a multitude of systems and the different physical
ingredients that may yield such an effect are well understood. On a microscopic
level, the interplay between spin-orbit coupling and a Zeeman field may give
rise to a diode effect in a single Josephson junction. However, so far there is
no analytical description of the effect within a simple model. Here we study a
single channel nanowire with Rashba spin-orbit coupling and in the presence of
a Zeeman field. We show that the different Fermi velocities and spin
projections of the two pseudo-spin bands lead to a diode effect. Simple
analytical expressions for the diode efficiency can be obtained in limiting
cases.
