The effect of focusing on polarization qubits
  When a photon with well-defined polarization and momentum passes through a focusing device, these properties are no longer well defined. Their loss is captured by describing polarization by a 3x3 effective density matrix. Here we show that the effective density matrix corresponds to the actual photodetection model and we provide a simple formula to calculate it in terms of classical fields. Moreover, we explore several possible experimental consequences of the "longitudinal" term: limits on single-photon detection efficiency, polarization-dependent atomic transitions rates and the implications on quantum information processing. 
Netanel H. Lindner and Daniel R. Terno