A photon-pair source with controllable delay based on shaped   inhomogeneous broadening of rare-earth doped solids
  Spontaneous Raman emission in atomic gases provides an attractive source of photon pairs with a controllable delay. We show how this technique can be implemented in solid state systems by appropriately shaping the inhomogeneous broadening. Our proposal is eminently feasible with current technology and provides a realistic solution to entangle remote rare-earth doped solids in a heralded way. 
Pavel Sekatski, Nicolas Sangouard, Nicolas Gisin, Hugues de   Riedmatten, Mikael Afzelius
quant-ph