  We propose the scheme of a novel spin-optronic device, optical analog of
Datta and Das spin transistor for the electrons. The role of the
ferromagnetic-nonmagnetic contact is played by a spatially confined cavity
polariton BEC. The condensate is responsible for the appearance of effective
magnetic field which rotates the spin state of a propagating pulse of
polaritons allowing to tune the transmittivity of the device.
