COUNTING ELECTRONS
As of 2011, the SET chip developed
at the PTB may well be the key to a
novel definition of the ampere which
links it to a fundamental physical
constant, the charge of an electron.
The chip transports and counts elec-
trons. This quantum effect – called
‘single electron tunneling’ – only
occurs at low temperatures, there-
fore the chip needs to be cooled
down to 20 mK with a cryostat – at
right – and liquid helium.