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Physical Review A

Observation of near-field correlations in spontaneous parametric down-conversion

H. Di Lorenzo Pires, Martin P. van Exter

Published Apr 3, 2009• 15 citations
Publisher

Abstract

We experimentally demonstrate how the two-photon field generated by spontaneous parametric down-conversion contains an intriguing fine structure associated with the positional spread within the photon pair. The obtained results provide a three-dimensional picture of the near-field correlations, which are determined by the phase-matching conditions. These correlations are compared with previous results on second-harmonic generation, spatial antibunching, and transverse entanglement in parametric down-conversion.

DOI
10.1103/physreva.79.041801

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We experimentally demonstrate how the two-photon field generated by spontaneous parametric down-conversion contains an intriguing fine structure associated with the positional spread within the photon pair. The obtained results provide a three-dimensional picture of the near-field correlations, which are determined by the phase-matching conditions. These correlations are compared with previous results on second-harmonic generation, spatial antibunching, and transverse entanglement in parametric down-conversion.
In experiments with spontaneous parametric down conversion the near-field plane is usually defined as the output facet of the generating crystal. Experimentally, however, the spatial correlations are accessed via an imaging system and observed in a region around the image of the output facet. We show that the imaged two-photon field has a very rich transverse and longitudinal structure, which is determined by the phase-matching conditions. We observe many intriguing three-dimensional structures that demonstrate the presence of spatial antibunching, an extreme localization of twin photons, and spatial correlations that resemble Bessel beams in propagation. We link these observations with previous results in second harmonic generation and predict the presence of fourth-order phase singularities. Both experiments and theory are presented, yielding further insight into the nature of the two-photon field.
We directly observe far-field correlations of x-ray photon pairs generated by spontaneous parametric down-conversion (SPDC). Using an energy-resolved, two-dimensional photon counting detector we record the full ring-shaped emission of both photons across a broad bandwidth and extract pair correlations directly from raw events without imposing angular constraints. The ring radii scale with photon energy, in quantitative agreement with transverse phase matching, providing a stringent momentum-space validation of x-ray SPDC. These observations open a route to leveraging quantum correlations in x-ray imaging and metrology, including correlation-enhanced magnification and reduced blurring.
Structured optical fields have led to several ground-breaking techniques in classical imaging and microscopy. At the same time, in the quantum domain, position-momentum entangled photon fields have been shown to have several unique features that can lead to beyond-classical imaging and microscopy capabilities. Therefore, it is natural to expect that position-momentum entangled two-photon fields that are structured can push the boundaries of quantum imaging and microscopy even further beyond. Nonetheless, the existing experimental schemes are able to produce either structured two-photon fields without position-momentum entanglement, or position-momentum entangled two-photon fields without structured correlations. In this article, by manipulating the phase-matching condition of the spontaneous parametric down-conversion process, we report experimental generation of two-photon fields with various structures in their spatial correlations. We experimentally measure the minimum bound on the entanglement of formation and thereby verify the position-momentum entanglement of the structured two-photon field. We expect this work to have important implications for quantum technologies related to imaging and sensing.
In this Letter, we present a theoretical and experimental study about the spatial correlations of paired photons generated by Type II spontaneous parametric down-conversion. In particular, we show how these correlations can be positive or negative, depending on the direction in which the far-field plane is scanned and the polarization postselected. Our results provide a straightforward way to observe different kind of correlations that complement other well-known methods to tune the spatial correlations of paired photons.