Error characterization in Quantum Information Processing: a protocol for   analyzing spatial correlations and its experimental implementation
  We present a protocol for error characterization and its experimental implementation with 4 qubits in liquid state NMR. The method is designed to retrieve information about spatial correlations and scales as $O(n^w)$, where $w$ is the maximum number of qubits that have non-negligible interaction. We discuss the practical aspects regarding accuracy and implementation. 
C. C. L\'opez, B. L\'evi, D. G. Cory