A fast, primary Coulomb blockade thermometer

We have measured the third derivative of the current-voltage characteristics, d^3I/dV^3, in a two-dimensional array of small tunnel junctions using a lock-in amplifier. We show that this derivative is zero at a voltage which scales linearly with the temperature and depends only on the temperature and natural constants, thus providing a primary thermometer. We demonstrate a measurement method which extracts the zero crossing voltage directly using a feedback circuit. This method requires only one voltage measurement, which makes it substantially faster than the original Coulomb blockade thermometry method.