  We propose a novel chiral order parameter to explain the unusual polar Kerr
effect in underdoped cuprates. It is based on the loop-current model by Varma,
which is characterized by the in-plane anapole moment N and exhibits the
magnetoelectric effect. We propose a helical structure where the vector N^(n)
in the layer n is twisted by the angle pi/2 relative to N^(n-1), thus breaking
inversion symmetry. We show that coupling between magnetoelectric terms in the
neighboring layers for this structure produces optical gyrotropy, which results
in circular dichroism and the polar Kerr effect.

Superconductivity