Fast thermometry and energy relaxation measurements on metallic calorimeters

The development of single photon detectors (SPD's) in the microwave regime is a fundamental challenge for obtaining a better understanding of the phenomena and components employed in quantum information technologies. Particularly, the field of quantum thermodynamics has until now been mainly theory oriented due to the lack of suitable detectors. In this thesis I present the first steps in developing a high fidelity SPD suitable for detecting itinerant microwave photons in a transmission line. The device is of mesoscopic size and directly integrable into superconducting quantum circuits. Out of the two main approaches in microwave SPD's, atom-like systems and thermal detectors, we pursue the latter. In a thermal detector, the energy of the photon is transferred into the absorber of the detector, causing an abrupt increase in its temperature. This leads to the destruction of the photon, which can be avoided with an atom-like detector. However, an advantage of the thermal approach is that a broadband microwave detector, operated in a continuous manner, can be realized. This is essential for several experiments for example in quantum thermodynamics. The absorber of our micro calorimeter is a normal metal nanowire. The electron gas in the wire is used for the photon absorption, whereas the phonons work as a heat sink. The absorber is connected to a superconducting electrode via a thin tunnel barrier and grounded through a direct normal metal-superconductor contact, which acts as a heat mirror. The temperature of the electron gas is probed via the differential conductance of a tunnel-junction. For realising a sensitive SPD, it is essential to minimize the heat capacity of the absorber. Thermal properties of small metal structures can deviate significantly from the properties of bulk materials due to the large surface-to-volume ratio. We have measured thermal relaxation in Cu and Ag thin-film nanowires at sub-kelvin temperatures and observed an anomalously long relaxation time in the Cu wires. In addition to a large specific heat, these results may also originate from the slow thermalisation in the granular structure observed in the evaporated Cu wires.