Young's double-slit interference experiments with undulator vortex radiation were conducted, focusing on photon-counting regime. To isolate the second harmonic radiation in the ultraviolet range emitted from the helical undulator and achieve successful counting measurements, an ultranarrow bandpass filter was utilized under an extremely low-current mode of the electron storage ring. It was observed that the photon spots on the detector, after passing through the double slits, appeared to be randomly distributed. However, upon integrating these photon spots, it was confirmed that interference fringes with characteristic features of optical vortices, such as dark and broken/distorted stripes in the center, were formed. The reproducibility of these interference fringes was confirmed by calculating the optical path difference for the optical vortex reaching the double slits, as well as the optical path difference resulting from normal double-slit interference. Consequently, these findings indicate that even in the state of a single photon, the radiation emitted spontaneously by a high-energy electron in spiral motion possesses the nature of an optical vortex, characterized by a spiral wavefront.
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