We present experiments where a single subwavelength scatterer is used to
examine and control the back-scattering induced coupling between
counterpropagating high-Q modes of a microsphere resonator. Our measurements
reveal the standing wave character of the resulting symmetric and antisymmetric
eigenmodes, their unbalanced intensity distributions, and the coherent nature
of their coupling. We discuss our findings and the underlying classical physics
in the framework common to quantum optics and provide a particularly intuitive
explanation of the central processes.Comment: accepted for publication in Pysical Review Letter