Abstract
We present measurements of the helical mode of single photons and do so by sending heralded photons through a Mach–Zehnder interferometer that prepares the light in a helical mode with topological charge one, and interferes it with itself in the fundamental non-helical mode. Masks placed after the interferometer were used to diagnose the amplitude and phase of the mode of the light. Auxiliary measurements verified that the light was in a non-classical state. The results are in good agreement with theory. The experiments demonstrate in a direct way that single photons carry the entire spatial helical-mode information.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Light beams in helical spatial modes are interesting light forms that can be visualized easily with a wave description. Light in a helical mode, more strictly a Laguerre–Gauss beam with a radial index zero and an azimuthal index one, has the shape of a ring but carries a phase that increases with the transverse angle. As a consequence, the points of equal phase (the wavefront) form a helix. In addition, the mode carries orbital angular momentum due to a locally tilted linear momentum along its corkscrew wavefront. Quantum mechanics, however, maintains that light is made up of packets of energy, photons, in the mode of the field. Photons must therefore also carry helical modes. Since 2001, numerous experiments have demonstrated that a photon can exist in helical modes, including carrying orbital angular momentum. However, a direct measurement has not been performed until now.
Main results. This paper presents measurements where single photons sent through an interferometer interfere with themselves in two spatial modes; a helical mode and a fundamental, laser-beam-like mode, with planar wavefronts. The interference produces a helical mode that is spatially asymmetric but that rotates about its center when the phase between the interfering modes is changed.
Wider implications. The results confirm the predictions of quantum mechanics: single photons carry the entire spatial mode, or image, information. This method can be used to study other non-classical claims from quantum mechanics, including the contention that photon pairs entangled in spatial modes can carry an image together that is different from the image that each photon carries individually.
Figure. Images of the spatial amplitude distributions of single photons in a helical mode (top left) interfering with themselves in a fundamental, planar, spatial mode (top right). The resulting mode is helical but asymmetric, rotating about the beam axis when the phase between the interfering modes is varied (four bottom images). The images were taken one photon at a time, pixel per pixel, by scanning a single-photon detector in a transverse plane.