Pumpette

Pumpette, a goblin gate-sentry of The SI Guard, cranks a tiny portcullis that lets one electron through per turn.

The Physics

A single-electron pump is a nanoscale island (a quantum dot or metal island) cut off by tunnel barriers. Its charging energy is large enough that electrons can only cross one at a time (Coulomb blockade). Driving the gates at frequency f moves one electron per cycle, so the current is set by counting.

I = ef

• I: current (A)
• e: elementary charge, 1.602 176 634 × 10⁻¹⁹ C exactly
• f: pumping frequency (Hz)

Since 20 May 2019 the ampere is defined by fixing e exactly, replacing the old definition based on the force between two parallel wires. At f = 1 GHz the pump gives about 160 pA.

Honest caveat: Real pumps occasionally miss or add an electron, and their currents are small (picoamps to nanoamps), so error counting matters. A pump is one way to realise the ampere directly; in practice it is also realised with Josephson voltage and quantum Hall resistance standards via Ohm's law.

On the sticker

Pumpette works the gate crank with one bright electron spark at the bars. Printed text: PUMPETTE with the line I = ef. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.