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arXiv:0912.2975 · Physical Review A

Demonstration of a programmable source of two-photon multiqubit entangled states

S. Cialdi, Davide Brivio, Matteo G. A. Paris

Published Apr 26, 2010• 19 citations
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Abstract

We suggest and demonstrate a novel source of two-photon multipartite entangled states which exploits the transverse spatial structure of spontaneous parametric down-conversion together with a programmable spatial light modulator (SLM). The one-dimensional SLM is used to perform polarization entanglement purification and to realize arbitrary phase gates between polarization and momentum degrees of freedom of photons. We experimentally demonstrate our scheme by generating two-photon three-qubit linear cluster states with high fidelity using a diode laser pump with a limited coherence time and power on the crystal as low as $~2.5$ mW.

DOI
10.1103/physreva.81.042322

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We suggest and demonstrate a novel source of two-photon multipartite entangled states which exploits the transverse spatial structure of spontaneous parametric down-conversion together with a programmable spatial light modulator (SLM). The one-dimensional SLM is used to perform polarization entanglement purification and to realize arbitrary phase gates between polarization and momentum degrees of freedom of photons. We experimentally demonstrate our scheme by generating two-photon three-qubit linear cluster states with high fidelity using a diode laser pump with a limited coherence time and power on the crystal as low as  2.5~2.5 2.5 mW.
Photonic entanglement sources are nowadays of central interest in the scientific landscape for their demonstrated applications in quantum information, computation and communication. Required features for a real implementation are obviously high brigthness and purity, but also a precise control of decoherence processes during propagation and the use of many degrees of freedom to enhance the amount of information carried. We developed a new photonic entangled source based on parametric down-conversion within two type-I crystals in a non collinear configuration. A first peculiarity of such source is the very broad angular and spectral distribution exploited. In this way we obtained high brightness even using low pump power. A second peculiarity is the coupling of the source with a Spatial Light Modulator (SLM) allowing the compensation of intrinsic phase term which naturally reduces the state purity till about 0.5. Coupling 10 mrad on both channel, we obtained a purity of about 0.97 with a spectrum of 10 nm, and a purity around 0.90 with more than 60 nm. Starting from such purified source we also realized multi-qubit cluster states exploiting the angular degree of freedom (d.o.f.) of the photons. Here the SLM acts as a C-phase gate entangling polarization and momentum qubits. Furthermore we exploited signal-idler angular correlation to demonstrate the ghost imaging of a pure phase object and we realized a new cryptographic protocol based on non-local phase objects superposition. Our source has also paved the way in simulating system-environment interaction since the SLM allows precise decoherence control. We observed different dynamics of the system entanglement (polarization d.o.f.) modulating the environment spectrum (angular distribution). Then studying the trace distance evolution we demonstrated initial system-environment correlation.
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Spontaneous parametric down conversion (SPDC)-based sources of photonic entangled states are usually generated using coherent pump fields. For temporal, spatial, and polarization degree of freedom, the down-converted light entanglement is limited by the coherence of the pump in the same degree of freedom. However, the effect of pump coherence on the entanglement in a different degree of freedom remains largely unexplored. Here we experimentally demonstrate the generation of polarization entanglement from SPDC pumped by a light emitting diode (LED), a spatiotemporally incoherent pump. The two-photon state is found to have a concurrence of 0.562 and purity of 0.647.
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