State Tomography: Hidden Differences
  We address the problem of completely characterizing multi-particle states including loss of information to unobserved degrees of freedom. In systems where non-classical interference plays a role, such as linear-optics quantum gates, such information can degrade interference in two ways, by decoherence and by distinguishing the particles. Distinguishing information, often the limiting factor for quantum optical devices, is not correctly described by previous state-reconstruction techniques, which account only for decoherence. We extend these techniques and find that a single modified density matrix can completely describe partially-coherent, partially-distinguishable states. We use this observation to experimentally characterize two-photon polarization states in single-mode optical fiber. 
R. B. A. Adamson (1), L. K. Shalm (1), M. W. Mitchell (2) and A. M.   Steinberg (1) ((1) Centre for Quantum Information & Qu...