Universal, unambiguous concentration and distillation of Bell pairs

Abstract

The ability of preparing perfect Bell pairs with a practical scheme is of great relevance for quantum communication as well as distributed quantum computing. We propose a scheme which probabilistically, but unambiguously produces the $\left|Φ_{+}\right>$ Bell pair from four copies of qubit pairs initially in the same arbitrary pure quantum state. The same scheme, extended to eight qubit pairs initially in the same, moderately mixed quantum state, unambiguously produces the $\left|Φ_{+}\right>$ 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.

Figures (1)

• Figure 1: The schematic representation of the core step of the protocol. $A$ and $B$ denote two distant parties, Alice and Bob, who apply a local two-qubit unitary $U$, and a subsequent measurement on qubits $2$ and $4$, respectively, after which they only keep qubits 1 and 3 if the measurements resulted 0, which they can communicate classically.