Operational approach to quantum indistinguishability

In our recent work, we have proposed a directly measurable parameter quantifying effective indistinguishability of particles used as information carriers in quantum information transfer and processing [M.Miková et al., Phys. Rev. A 87, 042327 (2013); arXiv:1209.0908]. We experimentally tested its relevance for photons produced in parametric down-conversion and demonstrated that distinguishability of their internal states directly determines the quality of a simple linear-optical quantum-state-transfer protocol. Further, we shown that even if these internal–noninformational–degrees of freedom of two particles are nonfactorable, the particles can still serve as good carriers of qubits.