  We show that magnetic reconnection in a magnetically-dominated fast-cooling
plasma can naturally produce bright flares accompanied by rotations in the
synchrotron polarization vector. With particle-in-cell simulations of
reconnection, we find that flares are powered by efficient particle
acceleration at the interface of merging magnetic flux ropes, or "plasmoids".
The accelerated particles stream through the post-merger plasmoid towards the
observer, thus progressively illuminating regions with varying plane-of-sky
field direction, and so leading to a rotation in the observed polarization
vector. Our results provide evidence for magnetic reconnection as the physical
cause of high-energy flares from the relativistic jets of blazars (which recent
observations have shown to be frequently associated with polarization
rotations), and provide a first-principle physical mechanism for such flares.
