A witness of multipartite entanglement strata

We describe an entanglement witness for N -qubit mixed states based on the properties of N -point correlation functions. As the degree of violation increases beyond different threshold values, or strata, this witness can guarantee that no more than M qubits are separable from the rest of the state for any M 6 N , or that there is some genuine M-party or greater multipartite entanglement present. We illustrate the use our criterion by investigating the existence of entanglement in thermal stabilizer states, where we demonstrate that the witness is capable of witnessing bound-entangled states. Intriguingly, this entanglement can be shown to persist in the thermodynamic limit at arbitrary temperature. 4 Author to whom any correspondence should be addressed. New Journal of Physics 10 (2008) 053026 1367-2630/08/053026+13$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft