  Is pulsar make up of strange matter? The magnetic field decay of a pulsar may
be able to give us an answer. Since Cooper pairing of quarks occurs inside a
sufficiently cold strange star, the strange stellar core is superconducting. In
order to compensate the effect of rotation, different superconducting species
inside a rotating strange star try to set up different values of London fields.
Thus, we have a frustrated system. Using Ginzburg-Landau formalism, I solved
the problem of rotating a superconducting strange star: Instead of setting up a
global London field, vortex bundles carrying localized magnetic fields are
formed. Moreover, the number density of vortex bundles is directly proportional
to the angular speed of the star. Since it is energetically favorable for the
vortex bundles to pin to magnetic flux tubes, the rotational dynamics and
magnetic evolution of a strange star are coupled together, leading to the
magnetic flux expulsion as the star slows down. I investigate this effect
numerically and find that the characteristic field decay time is much less than
20~Myr in all reasonable parameter region. On the other hand, the
characteristic magnetic field decay time for pulsars is $\geq 20$~Myr. Thus, my
finding cast doubt on the hypothesis that pulsars are strange stars.
