We report the realization of such a delayed-choice experiment in a scheme close to the ideal original proposal.
The delayed-choice scheme is implemented as follows. Linearly polarized single photons are sent by a polarization beamsplitter BS input through an interferometer (length 48 m) with two spatially separated paths associated with orthogonal S and P polarizations (Fig. 2). The movable output beamsplitter BS output consists of the combination of a half-wave plate, a polarization beamsplitter BS′, an electro-optical modulaor (EOM) with its optical axis oriented at 22.5° from input polarizations, and a Wollaston prism. The two beams of the interferometer, which are spatially separated and orthogonally polarized, are first overlapped by BS′ but can still be unambiguously identified by their polarization. Then, the choice between the two interferometer configurations, closed or open, is realized with the EOM, which can be switched between two different configurations within 40 ns by means of a homebuilt fast driver: Either no voltage is applied to the EOM, or its half-wave voltage V p is applied to it. In the first case, the situation corresponds to the removal of BS output and the two paths remain uncombined (open configuration). Because the original S and P polarizations of the two paths are oriented along prism polarization eigenstates, each “click” of one detector D1 or D2 placed on the output ports is associated with a specific path (path 1 or path 2, respectively). When the V p voltage is applied, the EOM is equivalent to a half-wave plate that rotates the input polarizations by an angle of 45°. The prism then recombines the two rotated polarizations that have traveled along different optical paths, and interference appears on the two output ports. We then have the closed interferometer configuration.