Universal, Unambiguous Preparation of Bell Pairs

The ability of preparing perfect Bell pairs with a practical scheme is of great relevance for quantum communication, distributed quantum computing, as well as multicore quantum computing. We propose a scheme which probabilistically, but universally and unambiguously produces the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\vert \Phi_{+}\rangle$</tex> Bell pair from four copies of qubit pairs initially in the same unknown pure quantum state. The same scheme, extended to eight qubit pairs initially in the same, moderately mixed quantum state, unambiguously produces the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\vert \Phi_{+}\rangle$</tex> Bell pair with quadratically suppressed noise. The core step of the proposed scheme consists of a pair of local two-qubit operations applied at each of the two distant locations, followed by a partial projective measurement and postselection at each party, with results communicated classically. While the scheme resembles standard entanglement distillation protocols, it achieves success within just three iterations, making it attractive for real-world applications.