Letter | Published:

Experimental entanglement purification of arbitrary unknown states

Abstract

Distribution of entangled states between distant locations is essential for quantum communication1,2,3 over large distances. But owing to unavoidable decoherence in the quantum communication channel, the quality of entangled states generally decreases exponentially with the channel length. Entanglement purification4,5—a way to extract a subset of states of high entanglement and high purity from a large set of less entangled states—is thus needed to overcome decoherence. Besides its important application in quantum communication, entanglement purification also plays a crucial role in error correction for quantum computation, because it can significantly increase the quality of logic operations between different qubits6. Here we demonstrate entanglement purification for general mixed states of polarization-entangled photons using only linear optics7. Typically, one photon pair of fidelity 92% could be obtained from two pairs, each of fidelity 75%. In our experiments, decoherence is overcome to the extent that the technique would achieve tolerable error rates for quantum repeaters in long-distance quantum communication8. Our results also imply that the requirement of high-accuracy logic operations in fault-tolerant quantum computation can be considerably relaxed6.

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Acknowledgements

We thank H. Briegel, T. Jennewein and C. Simon for discussions. This work was supported by the Austrian Science Foundation (FWF), and by the TMR and the QuComm programs of the European Commission.

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Correspondence to Jian-Wei Pan or Anton Zeilinger.

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The authors declare that they have no competing financial interests.

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Figure 1: Schematic drawing showing the principle of entanglement purification using linear optics.
Figure 2: Experimental set-up for entanglement purification. A pulse of ultraviolet light passes through a BBO crystal twice to produce two polarization-entangled photon pairs, that is, pair 1 in a1–b1 and pair 2 in a2–b2.
Figure 3: Experimental results showing the procedures to achieve perfect temporal overlap and to adjust the phase φ4 = 0.
Figure 4: Experimental results.

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