Density Matrix Tomography of Entangled Electron and Nuclear Spin States   in 15N:C60
  We discuss details of the preparation and detection of entangled electron-nuclear spin states in 15N:C60 together with a quantitative evaluation of the complete density matrix. All four Bell states of a two qubit subsystem were analyzed. In addition we find a quantum critical temperature of Tq = 7.73 K for this system at an electron spin resonance frequency of 95 GHz. 
Werner Scherer and Michael Mehring