Servizio Comunitario di Informazione in materia di Ricerca e Sviluppo - CORDIS

Spatial correlation of the photons generated by spontaneous parametric down conversion

In the first year of the project, a work concerning the surface-plasmon assisted transmission of entangled photons attracted widespread attention. In that experiment the effect on the polarization entanglement of photon pairs passing through arrays of subwavelength holes was investigated. The optical transmission of these hole arrays is anomalously large. It is believed to be associated with the excitation and coupling of surface plasmons at both sides of the whole arrays. It was experimentally found that the polarization entanglement of the transmitted photons is essentially identical to that of the incident photons.

However, in presence of tight focusing of the beams on a hole array the polarization entanglement rapidly decreases. This effect is related to the non-local nature of the electronic excitation (plasmon) at the metal surfaces. This non-local character imprints "Welcher Weg" information on the photons, resulting in less entanglement. A second aspect is connected to the orbital-angular momentum (OAM) degree of freedom of a photon, and its entanglement in the case of a photon pair. The OAM degree of freedom is attractive since it provides a high-dimensional alphabet for quantum-information processing. In order to maximally exploit the high-dimensionality of OAM Hilbert space, photons carrying half-integer OAM in units of ?, were investigated. A major advantage of these half-integer states is that they are not cylindrically symmetric, in contrast to the integer states, and can be oriented, similar, in a way, as the polarization states in polarization entanglement. Partner 2 proposed a novel setup to investigate the entanglement of OAM states living in a high-dimensional Hilbert space.

Non-integerspiral phase plates in spatial analysers were incorporated, enabling in this way the use of only two detectors. The two-photon states produced in the present setup are not confined to a 2x2 dimensional Hilbert space and this allows the probing of correlations in a high-dimensional space. In this setup Partner 2 could measure the quantum correlations while rotating the spiral phase plates contained in both the signal and idler arms. Parabolic coincidence fringes were observed in accord with theory. The coincidence fringes are shown to only depend on the relative orientation of the spiral phase plates in signal and idler arms.

These states were used to perform a Bell-type measurement on entanglement of spatial degrees of freedom, employing two analyser/detector combinations only. It is well known that, in the proper limit, integer orbital angular momentum is conserved in parametric down conversion. Moreover, this conservation rule also applies to the non-integer case. The theoretical value of the CHSH version of the Bell parameter equal to S=3.2, while this parameter cannot exceed the value 2v2 for qubits (Cirelson’s bound). In parallel the measured value of the S-parameter beyond Cirelson’s bound, namely S=3.07+/-0.06, corresponds to four standard deviations above the value 2v2.

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9500, 2300 RA LEIDEN
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