Recent measurements have highlighted that even in vacuum spatially shaped photons travel slower than $c$, the speed of monochromatic plane waves. Here we investigate the intrinsic delay introduced by “twisting” a photon, i.e., by introducing orbital angular momentum (OAM), and measure the photon time of flight with a Hong–Ou–Mandel interferometer. When all other parameters are held constant, the addition of OAM reduces the delay (accelerates) with respect to the same beam with no OAM. We support our results using a theoretical method to calculate the group velocity and gain an intuitive understanding of the measured OAM acceleration by considering a geometrical ray-tracing approach.

中文翻译：

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扭曲的光子有多快？

最近的测量结果突出表明，即使在真空中，空间形状的光子的传播速度也比 $C$，单色平面波的速度。在这里，我们研究了通过“扭曲”光子（即通过引入轨道角动量（OAM））引入的固有延迟，并使用Hong-Ou-Mandel干涉仪测量光子的飞行时间。当所有其他参数保持恒定时，添加OAM会相对于没有OAM的同一波束减少延迟（加速）。我们使用理论方法来支持我们的结果，以计算组速度，并通过考虑几何光线跟踪方法来获得对所测OAM加速度的直观理解。