We present a simple protocol to purify bipartite entanglement in spin-1/2 particles by utilising only natural spin-spin interactions, i.e., those that can commonly be realised in realistic physical systems, and S_(z)-measurements on single spins. Even the standard isotropic Heisenberg interaction is shown to be sufficient to purify mixed state entanglement if there are at least three pairs of spins. This approach could be useful for quantum information processing in solid-state-based systems.
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