  The paper briefly surveys theoretical models for the polar Kerr effect (PKE)
and time-reversal symmetry breaking in the pseudogap phase of cuprate
superconductors. By elimination, the most promising candidate is the tilted
loop-current model, obtained from the Simon-Varma model by tilting one
triangular loop up and another one down toward the apical oxygens. The model is
consistent with the PKE, spin-polarized neutron scattering, and optical
anisotropy measurements. Spontaneous currents in this model flow between the
in-plane and apical oxygens in such a manner that each oxygen belongs to one
current loop. This loop-current pattern is similar to the spin order in the
magnetoelectric antiferromagnet Cr2O3, where the PKE is observed
experimentally. By analogy, it should be possible to train the PKE sign in the
cuprates magnetoelectrically. Several experiments are proposed to confirm the
loop-current order: the magnetic-field-induced polarity, the nonlinear
anomalous Hall effect, and the second-harmonic generation.
