Observing a Quantum Phase Transition by Measuring a Single Spin
  We show that the ground-state quantum correlations of an Ising model can be detected by monitoring the time evolution of a single spin alone, and that the critical point of a quantum phase transition is detected through a maximum of a suitably defined observable. A proposed implementation with trapped ions realizes an experimental probe of quantum phase transitions which is based on quantum correlations and scalable for large system sizes. 
Manuel Gessner, Michael Ramm, Hartmut Haeffner, Andreas Buchleitner,   and Heinz-Peter Breuer
quant-ph