title: Discrete tunable color entanglement.
abstract: 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 nondegenerate photons onto the color DOF. We create, for the first time, high-quality, discretely color-entangled states (with energy band gap up to 8.4 THz) without any spectrally selective filtering, 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.
authors.affiliation: Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Boltzmanngasse 3, A-1090 Vienna, Austria.
authors.forename: S
authors.initials: S
authors.lastname: Ramelow
authors.forename: L
authors.initials: L
authors.lastname: Ratschbacher
authors.forename: A
authors.initials: A
authors.lastname: Fedrizzi
authors.forename: N K
authors.initials: NK
authors.lastname: Langford
authors.forename: A
authors.initials: A
authors.lastname: Zeilinger