In our recent work, we have proposed a directly measurable parameter quantifying eff ective 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.

qubit transfer scheme

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Operational approach to quantum indistinguishability