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    Ascertaining the Values of σx\sigma_x, σy\sigma_y, and σz\sigma_z of a Polarization Qubit

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    In the 1987 spin retrodiction puzzle of Vaidman, Aharonov, and Albert one is challenged to ascertain the values of σx\sigma_x, σy\sigma_y, and σz\sigma_z of a spin-1/2 particle by utilizing entanglement. We report the experimental realization of a quantum-optical version in which the outcome of an intermediate polarization projection is inferred by exploiting single-photon two-qubit quantum gates. The experimental success probability is consistently above the 90.2% threshold of the optimal one-qubit strategy, with an average success probability of 95.6%.Comment: 4 pages, 2 figures; final version with new title and new abstract; to appear in Physical Review Letter

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