Emitters of $N$-photon bundles. C. Sánchez Muñoz, E. del Valle, A. González Tudela, K. Müller, S. Lichtmannecker, M. Kaniber, C. Tejedor, J.J. Finley and F.P. Laussy in Nature Photon. 8:550 (2014).   What the paper says?

The bundler—or emitter of $N$-photon bundles—is an hypothetic device (as of today, still only a theoretical design) that releases all its light in groups (or bundles) of exactly $N$ photons, for a tuneable integer $N=1, 2, 3, etc.$ The particular case $N=1$ is well-known as the single-photon source and thus would not qualify as a bundler. The case $N=2$ is also very particular and emission of this type has been approached through a variety of mechanisms. What makes the bundler special, is $N=3$ and above.