Coherent Population Trapping in Diamond N-V Centers at Zero Magnetic   Field
  Coherent population trapping at zero magnetic field was observed for nitrogen-vacancy centers in diamond under optical excitation. This was measured as a reduction in photoluminescence when the detuning between two excitation lasers matched the 2.88 GHz crystal-field splitting of the color center ground states. This behavior is highly sensitive to strain, which modifies the excited states, and was unexpected following recent experiments demonstrating optical readout of single nitrogen-vacancy electron spins based on cycling transitions. These results demonstrate for the first time that three-level Lambda configurations suitable for proposed quantum information applications can be realized simultaneously for all four orientations of nitrogen-vacancy centers at zero magnetic field. 
Charles Santori, David Fattal, Sean M. Spillane, Marco Fiorentino,   Raymond G. Beausoleil, Andrew D. Greentree, Paolo Olive...