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Multipartite indistinguishability


The generation of indistinguishable multipartite states of quantum systems is an important topic -- such states are manifest in all experiments exhibiting quantum interference, and are the basis for many implementations of quantum information processors.  So far, experiments have been limited to few bosons -- here we investigate the question of how to test and/or characterise the indistinguishability of many.  We adopt a particle picture for describing multipartite states of N identical bosons.  A distinction must be drawn between 'practical' indistinguishability imposed by real detectors that are only sensitive to a subset of the ostensibly complete set of degrees of freedom attributable to each particle, and 'complete' indistinguishability where those inaccessible degrees of freedom are all in the same state.  We show that these pictures are compatible, in that they give rise to the same number of experimentally accessible measurement outcomes.  We will discuss several implications, such as that informationally complete tomography in a practically indistinguishable situation does not require exponentially many measurements, as would be true in the completely distinguishable case.  The hope is to generate a stimulating informal discussion about the topic of multipartite indistinguishability and how it can be understood.