We present the novel embodiment of a photonic qubit that makes use of one continuous spatial degree of freedom of a single photon and relies on the the parity of the photon's transverse spatial distribution. Using optical spontaneous parametric downconversion to produce photon pairs, we demonstrate the controlled generation of entangled-photon states in this new space. Specifically, two Bell states, and a continuum of their superpositions, are generated by simple manipulation of a classical parameter, the optical-pump spatial parity, and not by manipulation of the entangled photons themselves. An interferometric device, isomorphic in action to a polarizing beam splitter, projects the spatial-parity states onto an even--odd basis. This new physical realization of photonic qubits could be used as a foundation for future experiments in quantum information processing.
@article{arxiv.0708.3064,
title = {Synthesis and Analysis of Entangled Photonic Qubits in Spatial-Parity Space},
author = {Timothy Yarnall and Ayman F. Abouraddy and Bahaa E. A. Saleh and Malvin C. Teich},
journal= {arXiv preprint arXiv:0708.3064},
year = {2008}
}