Although frequency multiplexing of information has revolutionized the field of classical communications, the color degree of freedom (DOF) has been used relatively little for quantum applications. We experimentally demonstrate a new hybrid quantum gate that transfers polarization entanglement of non-degenerate photons onto the color DOF. We create, for the first time, high quality, discretely color-entangled states (with energy bandgap up to 8.4 THz) without any filtering or postselection, and unambiguously verify and quantify the amount of entanglement (tangle, 0.611±0.009) by reconstructing a restricted density matrix; we generate a range of maximally entangled states, including a set of mutually unbiased bases for an encoded qubit space. The technique can be generalized to transfer polarization entanglement onto other photonic DOFs, like orbital angular momentum.
@article{arxiv.0904.4916,
title = {Discrete, Tunable Color Entanglement},
author = {S. Ramelow and L. Ratschbacher and A. Fedrizzi and N. K. Langford and A. Zeilinger},
journal= {arXiv preprint arXiv:0904.4916},
year = {2010}
}