  We investigate coherence in one- and two-photon optical systems, both
theoretically and experimentally. In the first case, we develop the density
operator representing a single photon state subjected to a non-dissipative
coupling between observed (polarization) and unobserved (frequency) degrees of
freedom. We show that an implementation of ``bang-bang'' quantum control
protects photon polarization information from certain types of decoherence. In
the second case, we investigate the existence of a "decoherence-free" subspace
of the Hilbert space of two-photon polarization states under the action of a
similar coupling. The density operator representation is developed analytically
and solutions are obtained numerically.
  NOTE: This manuscript is taken from the author's undergraduate thesis (A.B.
Dartmouth College, June 2000, advised by Dr. Walter E. Lawrence), under the
supervision of Dr. Paul G. Kwiat.
