A. Ludwig

A stack of thin plates made of different materials lies on a table. If you heat it from below, heat will rise, but will it spread sideways just as quickly? In layered semiconductors, the answer is no: heat runs faster along the layers than across them. This is like water in a stack of paper: it spreads easily along the sheets but seeps through the stack with difficulty.

An assembly line spits out parts one by one per cycle. If the cycle is short and defects are few, the line runs fast. In quantum optics, a single-photon source plays the role of such a line: it must emit exactly one quantum of light per cycle, and the more cycles per second, the more photons go into the fiber.