We investigate the dynamics of a driven optical parametric oscillator under the injection of orbital angular momentum. The injected mode is adiabatically driven through arbitrary transformations on the Poincaré sphere of first-order paraxial beams. As a result, the down-converted beam conjugated to the seed is shown to follow a path imposed by a nontrivial symmetry on the Poincaré sphere. This symmetry allows controllable distinguishability between the spatial modes of the down-converted beams. In this Letter, we provide convincing experimental evidence of this effect.

We present an experimental proof-of-principle for the generation and detection of pure two-qubit states which have been encoded in degrees of freedom that are common to both classical-light beams and single photons. Our protocol requires performing polarization tomography on a single qubit from a qubit pair. The degree of entanglement in the qubit pair is measured by concurrence, which can be directly extracted from intensity measurements-or photon counting-entering single-qubit polarization tomography. Entangled qubit pairs are basic units in schemes devised to implement quantum information processes such as quantum communication, quantum cryptography, etc., as well as in schemes designed to address foundational issues of quantum mechanics. The exploitation of entanglement is one of the most challenging goals of quantum information technologies. There are good reasons to believe that entanglement plays a key role in the advantage that quantum circuits would have over classical circuits [1]. Entanglement is however difficult to characterize experimentally. So-called entanglement witnesses are state specific, in the sense that they are tailored to detecting some types of entanglement while they are blind to others. Alternatively, one can rely on entanglement measures, which are designed to be state independent. A prominent example is concurrence, which is defined for any pure, bipartite state Φ as C(Φ) = | Φ|(σ y ⊗ σ y)|Φ * |, where σ y is the Pauli matrix and |Φ * the complex-conjugate of |Φ in the computational basis of the tensor-product space to which Φ belongs. Now, confronted with this measure, the experimentalist sees no obvious way to implement it directly in the laboratory. To begin with, complex conjugation is an unphysical operation, because it does not conserve positive-definiteness. Thus, the only way to obtain C(Φ) from measurements seems to be by means of full tomographic determination of state Φ, which is experimentally demanding and prone to inaccuracies. The evaluation of C(Φ), which nonlinearly depends on the parameters fixing Φ, can then be too inaccurate. Back in 2005, Mintert et al. [2] found a way out of the

We introduce a scalable photonic platform that enables efficient generation of entangled photon pairs from a semiconductor quantum dot. Our system, which is based on a self-aligned quantum dot- micro-cavity structure, erases the need for complex steps of lithography and nanofabrication. We experimentally show collection efficiency of 0.17 combined with a Purcell enhancement of up to 1.7. We harness the potential of our device to generate photon pairs entangled in time bin, reaching a fidelity of 0.84(5) with the maximally entangled state. The achieved pair collection efficiency is 4 times larger than the state-of-the art for this application. The device, which theoretically supports pair extraction efficiencies of nearly 0.5 is a promising candidate for the implementation of bright sources of time-bin, polarization- and hyper entangled photon pairs in a straightforward manner.

We report experiments in which the Bell parameter S that enters the Clauser-Horne-Shimony-Holt inequality:. In our experiments, we used two spatially separated optical beams, the electric fields of which were correlated to one another. The amount of correlation was quantified by the spectral degree of coherence , h a b ( ). This quantity measures the correlation between fields that exist at two distant locations and whose respective polarizations are given in terms of angles α and β, which can be set independently from one another. Such a correlation qualifies for the construction of the Bell parameter S. By changing the amount of field correlation, we could cover a range that goes from S. Our experimental findings should provide useful material for the ongoing, theoretical discussions about the quantum-classical border.

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