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arXiv:0809.4575 · Physical Review Letters

Multipath Entanglement of Two Photons

Alessandro Rossi, Giuseppe Vallone, Andrea Chiuri, F. De Martini, Paolo Mataloni

Published Apr 13, 2009• 120 citations
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Abstract

We present a novel optical device based on an integrated system of microlenses and single-mode optical fibers. It allows us to collect and direct into many modes two photons generated by spontaneous parametric down-conversion. By this device multiqubit entangled states and/or multilevel qudit states of two photons, encoded in the longitudinal momentum degree of freedom, are created. The multipath photon entanglement realized by this device is expected to find important applications in modern quantum information technology.

DOI
10.1103/physrevlett.102.153902

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We present a novel optical device based on an integrated system of microlenses and single-mode optical fibers. It allows us to collect and direct into many modes two photons generated by spontaneous parametric down-conversion. By this device multiqubit entangled states and/or multilevel qudit states of two photons, encoded in the longitudinal momentum degree of freedom, are created. The multipath photon entanglement realized by this device is expected to find important applications in modern quantum information technology.
By using an optical device based on the integration of a graded-index (GRIN) rod lens and a single-mode optical fiber we efficiently collected photon pairs generated by spontaneous parametric down-conversion. We show that this system preserves the polarization entanglement of the two-photon states. Hence this device, characterized by a remarkable easiness of alignment and allowing for high coupling efficiency of single-mode radiation, can be used with photons entangled in various degrees of freedom, such as polarization and spatial momentum and to interconnect different sides of complex optical circuits.
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We review recent theoretical and experimental works where are proposed and demonstrated how to use photon pairs created by spontaneous parametric down-conversion to generate entangled states of D-dimensional quantum systems, or qudits. This is the first demonstration of high-dimensional entanglement based on the intrinsic transverse momentum entanglement of the type-II down-converted photons. The qudit space is defined by an aperture made up of an opaque screen with D slits (paths), placed in the arms of the twin photons. By manipulating the pump beam profile we can prepare different entangled states of these possible paths. We focus our attention on an important case for applications in quantum information: the maximally entangled states. Experimental results for qudits with D=4 and D=8 are shown and measuring a two-photon conditional interference, we also demonstrate the nonclassical character of the correlations.
We show theoretically how two-photon fields generated by spontaneous parametric down-conversion can be used for creating maximally entangled states of multidimensional quantum systems, qudits. Logical states are defined by using different transverse spatial modes of the twin photons. These spatial modes are created by sending the components of the two-photon field through an array of multislits. The present experimental facilities allow generating maximally entangled states up to an eight-dimensional system.
Summary form only given. Correlated and entangled single photon states have made possible many fundamental studies in quantum optics such as the Bell-type test of hidden variables and non-locality, explorations of quantum interference properties, and investigations of quantum information processing and quantum communications technologies such as quantum cryptography, quantum computing, and quantum teleportation. These correlated single photon states are traditionally generated via a spontaneous parametric down-conversion process using a /spl chi//sup (2)/ material. We propose and analyze the generation of entangled single photons using /spl chi//sup (2)/ parametric processes in optical fibers. Such processes provide a simple and inexpensive correlated photon source readily compatible with commercial communications technologies. It further affords high efficiency for coupling into single mode fibers for applications in quantum cryptography.