  The insensitivity of photons towards external magnetic fields forms one of
the hardest barriers against efficient magneto-optical control, aiming at
modulating the polarization state of light. However, there is even scarcer
evidence of magneto-optical effects that can spatially modulate light. Here, we
demonstrate the latter by exploiting strongly coupled states of semimagnetic
matter and light in planar semiconductor microcavities. We nonresonantly excite
two spatially adjacent exciton-polariton condensates which, through inherent
ballistic near field coupling mechanism, spontaneously synchronise into a
dissipative quantum fluidic supermode of definite parity. Applying a magnetic
field along the optical axis, we continuously adjust the light-matter
composition of the condensate exciton-polaritons, inducing a supermode switch
into a higher order mode of opposite parity. Our findings set the ground
towards magnetic spatial modulation of nonlinear light.
