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arXiv:0907.2780 · Physical Review A

Entanglement localization after coupling to an incoherent noisy system

Fabio Sciarrino, Eleonora Nagali, F. De Martini, Miroslav Gavenda, Radim Filip

Published Jun 10, 2009• 12 citations
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Abstract

We report the experimental realization of entanglement localization which restores the polarization entanglement completely redirected after a linear coupling with incoherent and noisy surrounding photon. The method based only on measurements of the surrounding photon after the coupling and on postselection can localize the entanglement back to original systems for any linear coupling.

DOI
10.1103/physreva.79.060304

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We report the experimental realization of entanglement localization which restores the polarization entanglement completely redirected after a linear coupling with incoherent and noisy surrounding photon. The method based only on measurements of the surrounding photon after the coupling and on postselection can localize the entanglement back to original systems for any linear coupling.
We discuss both theoretically and experimentally elementary two-photon polarization entanglement localization after break of entanglement caused by linear coupling of environmental photon with one of the system photons. The localization of entanglement is based on simple polarization measurement of the surrounding photon after the coupling. We demonstrate that nonzero entanglement can be localized back irrespectively to the distinguishability of coupled photons. Further, it can be increased by local single-copy polarization filters up to an amount violating Bell inequalities. The present technique allows restoration of entanglement in those cases, when the entanglement distillation produces no entanglement from the coupling.
In the last years, different procedures have been developed in order to recover entanglement after propagation over a noisy channel. However, above a certain amount of noise, entanglement is completely lost. In this case the channel is called "entanglement-breaking" and any multi-copy distillation methods cannot help to reveal even a bit of entanglement. We report the experimental realization of a new protocol which restores entanglement from a single photon entanglement-breaking channel by measuring the information leaking out into the environment. Such restoration entanglement procedure provides new elements to overcome decoherence effects, a subject of renowed interest in quantum communication in the last decade.
Quantum entanglement, a fundamental property ensuring security of key distribution and efficiency of quantum computing, is extremely sensitive to decoherence. Different procedures have been developed in order to recover entanglement after propagation over a noisy channel. However, besides a certain amount of noise, entanglement is completely lost. In this case the channel is called entanglement breaking and any multi-copy distillation methods cannot help to restore even a bit of entanglement. We report the experimental realization of a new method which restores entanglement from a single photon entanglement breaking channel. The method based on measurement of environmental light and quantum feed-forward correction can reveal entanglement even if this one completely disappeared. This protocol provides new elements to overcome decoherence effects.
Different procedures have been developed in order to recover entanglement after propagation over a noisy channel. Besides a certain amount of noise, entanglement is completely lost and the channel is called entanglement breaking. Here we investigate both theoretically and experimentally an entanglement concentration protocol for a mixed three-qubit state outgoing from a strong linear coupling of two-qubit maximally entangled polarization state with another qubit in a completely mixed state. Thanks to such concentration procedure, the initial entanglement can be probabilistically recovered. Furthermore, we analyse the case of sequential linear couplings with many depolarized photons showing that thanks to the concentration a full recovering of entanglement is still possible.
It is commonly believed that photon polarization entanglement can only be obtained via pair creation within the same source or via postselective measurements on photons that overlapped within their coherence time inside a linear optics setup. In contrast to this, we show here that polarization entanglement can also be produced by distant single photon sources in free space and without the photons ever having to meet, if the detection of a photon does not reveal its origin—the which way information. In the case of two sources, the entanglement arises under the condition of two emissions in certain spatial directions and leaves the dipoles in a maximally entangled state.
Multipartite entangled states are key elements for quantum information processing. Here we experimentally investigate the particular properties of a polarization-entangled four-photon state, which can be generated directly by second order parametric down-conversion. The perfect correlations and the invariance under local transformations enable one to encode one qubit of quantum information in a decoherence-free subspace and thus to communicate it safely over noisy quantum channels. Furthermore, we present an experimental method to detect genuine fourpartite entanglement using entanglement witness operators. The implementation of such operators requires only of few local polarization measurements but uniquely proves the genuine multipartite entanglement.