Abstract
We propose a class of preparation schemes for orbital angular momentum and polarization qubits carried by single photons or classical states of light based on coherent feedback control by an ancillary degree of freedom of light. The preparation methods use linear optics and include the transcription of an arbitrary polarization state onto a two-level OAM system (swap) for arbitrary OAM values ${\pm}\ell$ within a light beam, i.e., without a spatial interferometer. The preparations can be carried out with unit efficiency independent from the potentially unknown initial state of the system. The swap scheme also allows us to implement arbitrary unitary gates on OAM qubits (${\pm}\ell$) by reducing them to polarization gates. In addition, we show how to translate measurement-based qubit control channels into coherent feedback schemes for optical implementation.
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