Dissipation at the Nanoscale: Cooper-pair Pumping and Electron Thermometry

Abstract

Dissipation in electrical circuits is a theme of great relevance for present and future information technology. On the one hand, dissipation makes it increasingly demanding to scale down the tiny integrated circuits that run our computers. On the other hand, it poses a serious challenge to the realization of a "quantum" computer. In this thesis we study dissipation in three types of nanometer-sized electrical circuits cooled down to subkelvin temperatures.

We first consider Cooper-pair pumps, whose output current is related to quantum geometric phases as well as to the amount of dissipation experienced by the device. We present the first observation of single Cooper-pair pumping without quasiparticle poisoning and a quantitative characterization of adiabaticity breakdown in pumping. We also propose to use a Cooper-pair pump to realize Landau-Zener-Stückelberg interferometry with geometric phases and envisage a novel scheme for Cooper-pair pumping.

We then turn to thermometry in two-dimensional electron gases (2DEGs). Thermalizing 2DEGs is problematic at low temperatures, so that dissipation becomes a critical issue. We aim at measuring the 2DEG temperature while minimizing the self-heating of the thermometer. We first demonstrate a variation on the well-known quantum dot thermometry scheme. We then propose and demonstrate a contactless scheme that combines quantum dot thermometry with charge sensing using a quantum point contact.

The third type of circuit we consider is a fast electronic thermometer built out of a normal metal-insulator-superconductor (NIS) tunnel junction. This thermometer may be used to realize a fast microcalorimeter for single-photon detection in the microwave range. We demonstrate its use by measuring thermal relaxation times of a small copper island at the lowest temperature to date, in a regime where the relevant heat-relaxation mechanisms are strongly suppressed.

Finally, this thesis contains three theoretical studies on the dynamics of periodically driven quantum systems in the presence of dissipation. We focus on the effects of the environment on the steady-state dynamics and on the distribution of energy exchanges between the driven system and the environment. These studies may serve as a starting point for further experiments beyond those reported in this thesis.