Quantum coherence and control in one- and two-photon optical systems
  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. 
Andrew J. Berglund