  Some giant pulses and fast radio bursts exhibit notable circular
polarization, which remains inexplicable and carries significant implications
for their emission mechanisms. In this study, we unveil the presence of
multiple nanoshot pairs uniformly spaced by approximately 21 $\mu$s within a
giant pulse emitted by the Crab pulsar. Among these pairs, a subset displays
left-hand and right-hand circular polarization in two distinct nanoshots. We
propose that the occurrence of such nanoshot pairs with dual circular
polarizations arises from the fragmentation of a linearly-polarized nanoshot
under the extreme Faraday effect, leveraging highly-asymmetrical pair plasma
and the ultra-intense field of nanoshots. The asymmetry in pair plasmas is
likely linked to discharge activities in pulsars. Moreover, the intense field
of nanoshots induces cyclotron resonance within the magnetosphere, effectively
slowing down the propagation velocity of the circularly polarized mode. Our
findings suggest that giant pulses originate within the inner magnetosphere and
lend support to a magnetospheric origin for fast radio bursts.
