Experimental observation of the X-shaped near field spatio-temporal correlation of ultra-broadband twin beams

In this work we present the experimental observation of the non factorable near field spatio-temporal correlation of ultra-broadband twin beams generated by parametric down conversion (PDC), in an interferometric-type experiment using sum frequency generation, where both the temporal and spatial degrees of freedom of PDC light are controlled with very high resolution. The revealed X-structure of the correlation is in accordance with the predictions of the theory.Comment: 5 pages, 3 figure

Detection of the ultranarrow temporal correlation of twin beams via sum-frequency generation

We demonstrate the ultranarrow temporal correlation (6 fs full width half maximum) of twin beams generated by parametric down-conversion, by using the inverse process of sum-frequency generation. The result relies on an achromatic imaging of a huge bandwith of twin beams and on a careful control of their spatial degrees of freedom. The detrimental effects of spatial filtering and of imperfect imaging are shown toghether with the theoretical model used to describe the results

Quantum spatial correlations in high-gain parametric down-conversion measured by means of a CCD camera

We consider travelling-wave parametric down-conversion in the high-gain regime and present the experimental demonstration of the quantum character of the spatial fluctuations in the system. In addition to showing the presence of sub-shot noise fluctuations in the intensity difference, we demonstrate that the peak value of the normalized spatial correlations between signal and idler lies well above the line marking the boundary between the classical and the quantum domain. This effect is equivalent to the apparent violation of the Cauchy-Schwartz inequality, predicted by some of us years ago, which represents a spatial analogue of photon antibunching in time. Finally, we analyse numerically the transition from the quantum to the classical regime when the gain is increased and we emphasize the role of the inaccuracy in the determination of the symmetry center of the signal/idler pattern in the far-field plane.Comment: 21 pages, 11 figures, submitted to J. Mod. Opt. special issue on Quantum Imagin

Coherence properties of high-gain twin beams generated in pump-depletion regime

Twin-beam coherence properties are analyzed both in the spatial and spectral domains at high-gain regime including pump depletion. The increase of the size of intensity auto- and cross-correlation areas at increasing pump power is replaced by a decrease in the pump depletion regime. This effect is interpreted as a progressive loss in the mode selection occurring at high-gain amplification. The experimental determination of the number of spatio-spectral modes from $g^{(2)}$-function measurements confirms this explanation.Comment: 7 pages, 7 figure

X-shaped space-time coherence in optical parametric generation

We study the spatiotemporal coherence properties of superfluorescence radiation generated in optical parametric amplification of quantum noise. We show that the angular dispersion properties of the spatiotemporal spectra, measured in different phase-matching conditions, lead to a clear X-shaped structure of the mutual correlation function of the radiation. Within a statistical picture, we interpret the generated superfluorescence as a stochastic \u201cgas\u201d of quasistationary modes characterized by a skewed correlation in the spatiotemporal domain, with characteristics similar to linear and nonlinear X waves not describable within a separable approach in space and time

Emergence of X-shaped spatiotemporal coherence in optical waves

Considering the problem of parametric nonlinear interaction, we report the experimental observation of electromagnetic waves characterized by an X-shaped spatiotemporal coherence; i.e., coherence is neither spatial nor temporal, but skewed along specific spatiotemporal trajectories. The application of the usual, purely spatial or temporal, measures of coherence would erroneously lead to the conclusion that the field is fully incoherent. Such hidden coherence has been identified owing to an innovative diagnostic technique based on simultaneous analysis of both the spatial and temporal spectra

Disclosing the spatio-temporal structure of PDC entanglement through frequency up-conversion

In this work we propose and analyse a scheme where the full spatio-temporal correlation of twin photons/beams generated by parametric down-conversion is detected by using its inverse process, i.e. sum frequency generation. Our main result is that, by imposing independently a temporal delay \Delta t and a transverse spatial shift \Delta x between two twin components of PDC light, the up-converted light intensity provides information on the correlation of the PDC light in the full spatio-temporal domain, and should enable the reconstruction of the peculiar X-shaped structure of the correlation predicted in [gatti2009,caspani2010,brambilla2010]. Through both a semi-analytical and a numerical modeling of the proposed optical system, we analyse the feasibility of the experiment and identify the best conditions to implement it. In particular, the tolerance of the phase-sensitive measurement against the presence of dispersive elements, imperfect imaging conditions and possible misalignments of the two crystals is evaluated.Comment: 20 pages, 19 figure