Highly efficient generation of single-mode photon pairs using a   crystalline whispering gallery mode resonator
  We report a highly efficient source of narrow-band photon pairs based on parametric down-conversion in a crystalline whispering gallery mode resonator. Remarkably, each photon of a pair is strictly emitted into a single spatial and temporal mode, as witnessed by Glaubers autocorrelation function. We explore the phase-matching conditions in spherical geometries, and determine the requirements of the single-mode operation. Understanding these conditions has allowed us to experimentally demonstrate a single-mode pair-detection rate of $0.97 \cdot 10^6$ pairs/s per mW pump power per 20 MHz bandwidth without the need of additional filter cavities. 
Michael F\"ortsch, Gerhard Schunk, Josef U. F\"urst, Dmitry Strekalov,   Thomas Gerrits, Martin J. Stevens, Florian Sedlmeir...
quant-ph physics.optics