Experimental violation of a Bell-like inequality with optical vortex beams

Optical beams with topological singularities have a Schmidt decomposition. Hence, they display features typically associated with bipartite quantum systems; in particular, these classical beams can exhibit entanglement. This classical entanglement can be quantified by a Bell inequality formulated in terms of Wigner functions. We experimentally demonstrate the violation of this inequality for Laguerre-Gauss (LG) beams and confirm that the violation increases with increasing orbital angular momentum. Our measurements yield negativity of the Wigner function at the origin for \LG_{10} beams, whereas for \LG_{20} we always get a positive value.Comment: 6 pages, 4 eps-color figures. Comments welcome

Creation, storage, and on-demand release of optical quantum states with a negative Wigner function

Highly nonclassical quantum states of light, characterized by Wigner functions with negative values, have been created so far only in a heralded fashion. In this case, the desired output emerges rarely and randomly from a quantum-state generator. An important example is the heralded production of high-purity single-photon states, typically based on some nonlinear optical interaction. In contrast, on-demand single-photon sources were also reported, exploiting the quantized level structure of matter systems. These sources, however, lead to highly impure output states, composed mostly of vacuum. While such impure states may still exhibit certain single-photon-like features such as anti-bunching, they are not enough nonclassical for advanced quantum information processing. On the other hand, the intrinsic randomness of pure, heralded states can be circumvented by first storing and then releasing them on demand. Here we propose such a controlled release, and we experimentally demonstrate it for heralded single photons. We employ two optical cavities, where the photons are both created and stored inside one cavity, and finally released through a dynamical tuning of the other cavity. We demonstrate storage times of up to 300 ns, while keeping the single-photon purity around 50% after storage. This is the first demonstration of a negative Wigner function at the output of an on-demand photon source or a quantum memory. In principle, our storage system is compatible with all kinds of nonclassical states, including those known to be essential for many advanced quantum information protocols.Comment: 14 pages, 5 figure