Physicists in the US have developed a new type of self-calibrating thermometer that exploits the shot noise in a tunnel junction to measure very low temperatures. Robert Schoelkopf of Yale University and colleagues at Yale and the JILA laboratory in Colorado say that their device is faster, more compact and simpler to use than conventional cryogenic thermometers (L Spietz et al. 2003 Science 300 1929).
Existing low-temperature thermometers are unsatisfactory because they are either too slow, too expensive or not accurate enough. They can also be complicated to use and often only work over a limited temperature range. Moreover, it is difficult to calibrate these thermometers at temperatures in the range near 1 Kelvin.
Schoelkopf and colleagues have developed a ‘shot noise thermometer’ that consists of a sensor made of two layers of aluminium that are separated by a thin insulating barrier of aluminium oxide. When a voltage is applied to the device, electrons are forced to tunnel through the barrier and this generates an electrical current that contains shot noise.
The researchers found that the temperature could be related to the voltage by a noise measurement that depends only on two fundamental constants – the electron charge and Boltzmann’s constant – and the assumption that the electrons obey Fermi-Dirac statistics. Therefore, by measuring the noise and the voltage, they were able to calculate exact temperatures.
The Yale team found that the shot noise thermometer could measure temperatures with a precision of 0.02% in the range near 1 Kelvin. Schoelkopf and colleagues plan to improve this performance further and hope that the device could eventually be used as a low-temperature standard in measurement science and metrology.
