  The photon spin is an important resource for quantum information processing
as is the electron spin in spintronics. However, for subwavelength confined
optical excitations, polarization as a global property of a mode cannot be
defined. Here, we show that any polarization state of a plane-wave photon can
reversibly be mapped to a pseudo-spin embodied by the two fundamental modes of
a subwavelength plasmonic two-wire transmission line. We design a device in
which this pseudo-spin evolves in a well-defined fashion throughout the device
reminiscent of the evolution of photon polarization in a birefringent medium
and the behaviour of electron spins in the channel of a spin field-effect
transistor. The significance of this pseudo-spin is enriched by the fact that
it is subject to spin-orbit locking. Combined with optically active materials
to exert external control over the pseudo-spin precession, our findings could
enable spin-optical transistors, i.e. the routing and processing of quantum
information with light on a subwavelength scale.
