19.11.2009 17:15

Rotating photons: The angular momentum of light from the spin to the orbital form

L. Marrucci

A beam of light, or even a single photon, can carry two different forms of angular momentum: the “spin” form, associated with a circular polarization, and the “orbital” form, associated with a helical wavefront and with an optical vortex at the beam core. Both forms of angular momentum can be coupled with matter and thus induce the rotation of a small particle. A few years ago, we have found that an optical medium that is both birefringent and inhomogeneous can couple these two forms of angular momentum to each other and thus give rise to a conversion from one form to the other. A novel optical element having this conversion capability, named “q-plate”, has been thus developed by us using patterned liquid crystals technology. A q-plate allows a much more flexible control of the orbital angular momentum (OAM) of light, both at a classical and quantum level, than other existing methods. For example, the possibility of transferring a qubit of quantum information encoded in a single photon from the polarization (spin) degree of freedom to the OAM and vice versa has been demonstrated. This quantum transfer works well even with pairs of photons exhibiting quantum correlations. Exploiting this good control of the OAM of photons, we have then demonstrated for the first time the Hong-Ou-Mandel quantum coalescence effect of two OAM-carrying photons impinging on a beam-splitter and the optimal quantum cloning of the OAM quantum state of a single photon.

Kategorie: Kolloquium