Temporal coherence and correlation of counterpropagating twin photons
à 11h en salle C. Brot
Backward parametric down-conversion, theoretically predicted almost fifty years ago, was only recently experimentally realized , when the challenges involved in the fabrication of the required submicrometer poling period could be solved. A peculiarity of this geometry is the presence of distributed feedback, responsible for the appearance of a threshold pump intensity, beyond which the system behaves as a “Mirrorless Optical Parametric Oscillator” (MOPO) .
This seminar focuses on the quantum properties of the counter-propagating twin beams/twin photons generated below the MOPO threshold. The first part describes the temporal coherence and correlation of the twin beams in the threshold region, where stimulated pair production is the dominant mechanism. In this regime the feedback plays a fundamental role, and we predict  a progressive shrinking of the emission spectra and a critical slowing down of the temporal coherence and correlation as threshold is approached.
The second part concentrates on the regime on spontaneous pair production, and describes the effects of the spectral properties of the pump on the degree of entanglement of the quantum state of twin photons. We show  that under proper conditions the state becomes almost separable, with the possibility of generating high purity single-photon heralded states.
 C. Canalias, V. Pasiskevicius, Mirrorless optical parametric oscillator, Nature Photonics 1, 459 (2007)
 T.Corti, A. Gatti and E. Brambilla, Phys. Rev. A, in press (2016).E-print : arXiv:1511.04910
 Alessandra Gatti, Tommaso Corti, Enrico Brambilla, Phys. Rev. A 92, 053809 (2015)
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