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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Tomography of spatially entangled biphotons

D. Korn, Dirk Puhlmann, Carsten Henkel, Robert Elsner, Martin R. Ostermeyer

Published Jun 1, 2009• 0 citations
Publisher

Abstract

Correlations in light can be used for a variety of applications in metrology and imaging. Several schemes in quantum imaging rely on multi-photon absorption. Therefore a detailed knowledge of the temporal and spatial correlations of the photons used is desired. This paper investigates the correlations of space-momentum entangled biphotons generated from type II parametric down conversion. The type II generation allows for individual manipulation in each path to control the spatial modes involved in the subsequent biphoton beam creation utilizing a Hong-Ou-Mandel interference.

DOI
10.1109/cleoe-eqec.2009.5192008

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Correlations in light can be used for a variety of applications in metrology and imaging. Several schemes in quantum imaging rely on multi-photon absorption. Therefore a detailed knowledge of the temporal and spatial correlations of the photons used is desired. This paper investigates the correlations of space-momentum entangled biphotons generated from type II parametric down conversion. The type II generation allows for individual manipulation in each path to control the spatial modes involved in the subsequent biphoton beam creation utilizing a Hong-Ou-Mandel interference.
The entangled photon pairs (biphotons) produced from parametric down-conversion (PDC) in a χ(2)crystal are the key element for several quantum information and communication schemes. Recent theoretical investigations [1–2] have revealed the peculiar structure of the biphotonic correlation characterizing the entanglement of twin beams in the space-time domain. The biphotonic correlation in type I PDC is characterized in the near field by a X-shaped geometry, which turns out to be non-factorable in space and time, offering thus the relevant possibility of manipulating the temporal bandwidth of biphoton entanglement by acting on their spatial degrees of freedom. The name “X-entanglement” was coined to describe this geometry [1]. An other key feature which emerged is the extreme spatial and temporal localization of the biphotonic correlation, on the micrometer and femtosecond range, respectively.
The Hong-Ou-Mandel interference effect lies at the heart of many emerging quantum technologies whose performance can be significantly enhanced with increasing numbers of entangled modes one could measure and thus utilize. Photon pairs generated through the process of spontaneous parametric down conversion are known to be entangled in a vast number of modes in the various degrees of freedom (DOF) the photons possess such as time, energy, and momentum, etc. Due to limitations in detection technology and techniques, often only one such DOFs can be effectively measured at a time, resulting in much lost potential. Here, we experimentally demonstrate, with the aid of a time tagging camera, high speed measurement and characterization of two-photon interference. With a data acquisition time of only a few seconds, we observe a bi-photon interference and coalescence visibility of ∼64% with potentially up to ∼2 × 103 spatial modes. These results open up a route for practical applications of using the high dimensionality of spatiotemporal DOF in two-photon interference, and in particular, for quantum sensing and communication.
Here we report our efforts in the characterization and engineering of spatio-temporally [1] entangled photon states. We use photon pairs obtained from spontaneous parametric down-conversion (SPDC) and present a novel way to reconstruct the complex spectral wavefunction from information obtained in a two-dimensional Hong-Ou-Mandel experiment [2]. This represents a novel and straightforward way to access otherwise hidden information in the wavefunction. Furthermore, the choice of the detection mode gives us some control over the spatio-temporal structure of the wavefunction.
We study the spatial coherence of entangled photon pairs that are generated via type-I spontaneous parametric down-conversion (SPDC). By manipulating the spatial overlap between the two down-converted beams in a Hong-Ou-Mandel interferometer we observe the spatial interference of multiple transverse modes for an even and an odd number of mirrors in the interferometer. We demonstrate that the two-photon spatial coherence, which is quantified in terms of a transverse coherence length, differs completely for the two mirror geometries and support this result by a theoretical and experimental explanation in terms of photon labeling.