Thermometry based on a superconducting qubit

In this thesis, we demonstrate the operation of a thermometer based on a superconducting transmon qubit. We experimentally show that by measuring the qubit’s population distribution and its respective effective temperature, it is possible to measure temperature of an object with which the qubit is thermalized. However, due to the qubit’s quantum nature, it is very sensitive to any other source of excitation present in the system, which can be treated as a separate noise source or a heat bath. Consideration of a qubit coupled to several uncorrelated noise sources constitutes an important part of the thesis. We investigate the applicability range of the qubit thermometer in terms of dynamic range, operation speed and precision. While the upper bound on the dynamic temperature range is mainly defined by the material properties of the superconductor used in the qubit fabrication, namely, by the superconducting gap, the lower temperature is set by coupling to parasitic excitation sources. The population measurements of a transmon qubit are based on an algorithm which uses π-pulses for swapping the populations of the three lowest energy levels of the transmon. The largest impact on the precision of the measurement is defined by the signal-to-noise ratio and the quality of the qubit control pulses. The precision limit is ultimately defined by the quantum Cramér-Rao bound, highlighting the statistical origin of this thermometry method. Finally, we utilize the transmon qubit for thermometry of a mesoscopic heat bath located on the same chip. The heat bath is a normal metal resistor, whose temperature was controlled with normal metal/insulator/superconductor (NIS) junctions. By coupling the transmon to the resistor capacitively and performing the population measurements, we could observe linear dependence between the qubit effective temperature and the temperature of the resistor. Moreover, while the designed mechanism of the qubit-resistor coupling was photonic, we managed to observe another channel of interaction. At large bias voltages applied to the NIS-junction, which sets the temperature of the resistor, the junction starts to emit nonequilibrium phonons that can break Cooper pairs in the superconducting qubit. This is a nonlocal effect, in which nonequilibrium quasiparticles lead both to change of the qubit population distribution and significant suppression of the relaxation time.