When an object is placed in contact with a thermal bath, its temperature will eventually stabilize to that of the bath itself. There are, however, exceptions to this rule. For example, it is known that if a semiconductor sample is immersed in a thermal bath with a temperature lower than 1 K, an electron population within the semiconductor can be hotter than the thermal bath. But how do you measure this temperature? Martin Kroner and colleagues at ETH Zürich, and Sarah Beavan, Alexander Högele and colleagues at Ludwig-Maximilians-Universität München have now shown in two independent studies that a single quantum dot can be used to measure the temperature of an electron reservoir that is in a helium bath that is a few hundred millikelvin.
The energy of an electron or a hole inside a quantum dot can only assume discrete values, and using a laser beam it is possible to create electron–hole complexes that absorb and emit light at very precise wavelengths. The two research groups used the optical lines that correspond to the spin states of a negatively charged exciton — two electrons and one hole — in a quantum dot. In a magnetic field, the two spin states split due to the Zeeman effect, and their relative population depends on the temperature. By measuring the ratio of the optical emissions it is therefore possible to establish the temperature of the charged exciton. The researchers were able to place the quantum dots close enough to the electron reservoir that the relative population of the charged excitons' states reflected that of the reservoir. In both cases, the teams confirmed that the temperature of the electron reservoir was higher than that of the bath.
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Pulizzi, F. A quantum dot thermometer. Nature Nanotech (2014). https://doi.org/10.1038/nnano.2014.205