Optimal quantum cloning of orbital angular momentum photon qubits through Hong-Ou-Mandel coalescence

The orbital angular momentum (OAM) of light, associated with a helical structure of the wavefunction, has great potential in quantum photonics, as it allows a higher dimensional quantum space to be attached to each photon(1,2). Hitherto, however, the use of OAM has been hindered by difficulties in its manipulation. Here, by making use of the recently demonstrated spin-OAM information transfer tools(3,4), we report the first observation of the Hong-Ou-Mandel coalescence(5) of two incoming photons having non-zero OAM into the same outgoing mode of a beamsplitter. The coalescence can be switched on and off by varying the input OAM state of the photons. Such an effect has then been used to carry out the 1 -> 2 universal optimal quantum cloning of OAM-encoded qubits(6-8), using the symmetrization technique already developed for polarization(9,10). These results are shown to be scalable to quantum spaces of arbitrary dimensions, even combining different degrees of freedom of the photons.