Coauthors: Fabio Sciarrino (Sapienza University of Rome) Eleonora Nagali (Sapienza University of Rome) Francesco De Martini (Sapienza University of Rome) Radim Filip (Palacky University Olomouc)

Quantum entanglement a key resource in quantum information tasks willingly interacts with surrounding systems. Due to this interaction the entangled system under interest couples to surrounding systems and the amount of entanglement is reduced or even lost and cannot serve to its application purpose. If the entanglement is completely destroyed by the coupling, distillation protocol does not work and other correcting protocols were suggested such as unlocking of hidden entanglement or entanglement localization [1]. Entanglement localization can concentrate back redistributed entanglement at least partially from the surroundings just by measurement on the surrounding system and proper feed-forward quantum correction. We deal with the situation when the input state is maximally entangled state of two qubits and another qubit serves as a surrounding system. Because the surrounding qubit is inaccessible before the coupling it is in an unknown state. The qubits in our case are represented by polarization states of single photons. In this presentation we extensively study the influence of coherence between the surrounding photon and one photon from the entangled pair on the localization pro- tocol which is parametrized by the probability p that the surrounding photon is indistinguishable. After the coupling between photons, represented by transmissivity T of the beamsplitter, the entanglement of the input state is reduced and for some T entanglement is completely redirected to the surrounding photon. We theoretically prove that for any linear coupling it is possible to localize non-zero entanglement back to the pair just by proper polarization sensitive detection of photon in surrounding photon (after coupling the surrounding photon is accessible). After measurement on the surrounding photon we may use additional single-copy filtration on both photons from the pair to further raise up the concurrence. Single-copy filtration probabilistically attenuates one polarization relatively to an orthogonal one. Qualitatively this localization is independent on the level of coherence between coupling photons. The theoretical results were experimentally tested using polarization entangled photons created in SPDC process [2]. An extension of the localization procedure was calculated for multiple consecutive couplings to the independent surrounding photons. [1] F. Verstraete, M. Popp and J.I. Cirac, Phys. Rev. Lett. 92, 027901 (2004) [2] Fabio Sciarrino, Eleonora Nagali, Francesco De Martini, Miroslav Gavenda, and Radim Filip, Phys. Rev. A 79, 060304 (2009)