Experimental entanglement restoration on entanglement-breaking channels (original) (raw)

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. The resource of quantum entanglement lies at the basis of many protocols of quantum communication and information . In order to face the problems introduced by the real world, it is primary of importance to consider the unavoidable interaction between noise and signal. The presence of noise alters or even invalidates the transmission of quantum information through communication channel, by spoiling entanglement. Nowadays many investigations have been developed in order to restore the entanglement lost, performing purification and distillation protocols . However, all these techniques can operate only on systems characterized by non-vanishing entanglement. When a channel is entanglement-breaking , that is, no entanglement is left after the interaction with noise, it is insufficient to work only at the input and output of the channel, and one has to go into the channel process and try to get more information to reveal entanglement. Thus a physical structure of the channel starts to be relevant. An interesting approach is based on an extension of quantum erasing idea [4], [5], [6], [7]: a state of the system can be restored if a proper measurement is performed on the environment [8]. This procedure, recently introduced as environmental channel correction [9], [10], [11], offers a theoretical possibility to deterministically restore complete entanglement. This is an interesting perspective, but the environment is EPR M I X I N G Alice Bob Measurement Filter B Filter A Environment FIG. 1: Schematic representation of the entanglement restoration procedure. Generation by the EPR source of the entangled photons shared between Alice and Bob. On Bob mode is showed the interaction between the entangled photon and the environment and the measure on the environmental photon that drives the filtration process.