Securing QKD Links in the Full Hilbert Space
  Many quantum key distribution analyses examine the link security in a subset of the full Hilbert space that is available to describe the system. In reality, information about the photon state can be embedded in correlations between the polarization space and other dimensions of the Hilbert space in such a way that Eve can determine the polarization of a photon without affecting it. This paper uses the concept of suitability to quantify the available information for Eve, and then describe a systematic way to calculate and measure these possibilities. 
Deborah J. Jackson, George M. Hockney (Jet Propulsion Laboratory,   California Institute of Technology)