The interaction between light and magnetism is considered a promising route to the development of energy-efficient data storage technologies. To date, however, ultrafast optical magnetization control has been limited to a binary process, whereby light in either of two polarization states generates (writes) or adopts (reads) a magnetic bit carrying either a positive or negative magnetization. Here, we report how the fundamental limitation of just two states can be overcome, allowing an arbitrary optical polarization state to be written magnetically. The effect is demonstrated using a three-sublattice antiferromagnet-hexagonal YMnO 3. Its three magnetic oscillation eigenmodes are selectively excited by the three polarization eigenstates of the light. The magnetic oscillation state is then transferred back into the polarization state of an optical probe pulse, thus completing an arbitrary optomagnonic write-read cycle.
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