We demonstrate radiofrequency thermometry on a micrometer-sized metallic island below 100 mK. Our device is based on a normal metal-insulator-superconductor tunnel junction coupled to a resonator with transmission readout. In the first generation of the device, we achieve 90 {\mu}K/Hz^1/2 noise-equivalent temperature with 10 MHz bandwidth. We measure the thermal relaxation time of the electron gas in the island, which we find to be of the order of 100 {\mu}s. Such a calorimetric detector, upon optimization, can be seamlessly integrated into superconducting circuits, with immediate applications in quantum-thermodynamics experiments down to single quanta of energy.
@article{arxiv.1405.7568,
title = {Fast electron thermometry towards ultra-sensitive calorimetric detection},
author = {S. Gasparinetti and K. L. Viisanen and O. -P. Saira and T. Faivre and M. Arzeo and M. Meschke and J. P. Pekola},
journal= {arXiv preprint arXiv:1405.7568},
year = {2015}
}