Fig. 5

Hong–Ou–Mandel (HOM) interference. HOM dips for photons in the OAM \(\ell = \pm 1\) subspace. The red points show data when the photons in paths B and C are both in the mode \(\left| 1 \right\rangle\); the blue points show data when the photons in paths B and C are both in the mode \(\frac{1}{{\sqrt 2 }}\left( {\left| 1 \right\rangle + \left| { - 1} \right\rangle } \right)\). We fit a Gaussian curve with a visibility of 0.86 ± 0.04 and 0.70 ± 0.05, respectively. The error bars correspond to the standard deviation of the count rate assuming Poisson statistics, and these data are background-subtracted