!the presence repels the successor
In a macroscopic circuit, electrons flow as a continuous fluid. Trillions of them crowd together, their individual electrical repulsion washed out by the sheer scale of the conductor.
But when the conductor is reduced to a microscopic island—a quantum dot or a single-electron transistor—the fluid nature breaks down, and the individual repulsion becomes absolute law.
Because the island is so small, its capacitance is tiny. Adding just one single electron increases the electrostatic energy of the entire island significantly. This sudden spike in negative charge creates a Coulomb blockade: a literal barrier of electrostatic repulsion that prevents any other electron from tunneling onto the island.
The first electron claims the space, and its very presence repels its successor.
Only by carefully tuning an external gate voltage can the barrier be lowered just enough to allow the first electron to leave, making room for exactly one more to take its place. The continuous river is reduced to a turnstile, passing charge one indivisible particle at a time.
