  The key characteristic of an optical mode in a microcavity is its quality
factor describing the optical losses. The numerical computation of this
quantity can be very demanding for present-day devices. Here we show for a
certain class of whispering-gallery cavities that the quality factor is related
to dynamical tunneling, a phenomenon studied in the field of quantum chaos. We
extend a recently developed approach for determining dynamical tunneling rates
to open cavities. This allows us to derive an analytical formula for the
quality factor which is in very good agreement with full solutions of Maxwell's
equations.
