Doohee Cho

A drop of metal on a semiconductor is like an island in the sea. For an electron to reach that island, it must pay a "toll": overcome a barrier that arises because the island is tiny, and each extra charge significantly changes its energy. This phenomenon is called Coulomb blockade. It underpins single-electron transistors, where current flows one electron at a time.

Current flowing through a wire is never perfectly smooth. If you listen closely enough, you hear a rustle — shot noise. It arises because charge is not carried as a continuous stream but in discrete portions, like raindrops on a roof. From the character of this noise, you can tell how much charge each 'drop' carries.