Abstract

In 1987 Vaidman, Aharonov, and Albert (VAA) showed “How to ascertain the values of σ_x, σ_y, and σ_z of a spin $-\frac{1}{2}$ particle” [1]. Their proposal later on became known as “spin measurement retrodiction”, i.e. determining the result of a spin measurement without knowing its basis (x, y, or z). As can easily be seen, this is not possible for a single particle (see fig. 1a). Yet, utilizing entanglement, there is a solution to this problem.

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