  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.
