Public defence in Engineering Physics, M.Sc. Diego Subero Rengel

Title of the thesis: Quantum transport and phase transitions in superconducting systems

Doctoral student: Diego Subero Rengel
Opponent: Professor Manuel Houzet, CEA Grenoble, France
Custos: Professor Jukka Pekola, Aalto University, School of Science, Department of Applied Physics

The dissertation focuses on charge and thermal transport in mesoscopic systems, with emphasis on the Coulomb blockade effect in the transport properties at cryogenic temperatures. We study tunneling in systems with the Coulomb blockade effect, as well as the role of the electromagnetic environment and the impact of these effects on the bolometry of microwave photons through superconducting circuits.

We propose and test a variant of the Maxwell demon experiment, the gambling demon. Within this context, we derive and verify second law-like inequalities accounting for the average work done when gambling is involved. For experimental verification, we use two single electron boxes connected to each other capacitively, where the dynamics of electron tunneling into the metallic island is governed by an electrostatic potential.

We study photonic heat transport through a superconducting circuit by exploiting the Johnson-Nyquist archetype, where two thermal reservoirs are connected via a frequency-dependent transmission line controlled either by an electric or magnetic field. We achieve precise control of photon thermal conductance near its quantum limit. We also studied the environmental back-action effect on photon-mediated heat transport, finding that while strong fluctuations produced by the environment affect charge transport through the SQUID as expected, they do not significantly impact heat transport. This indicates that, unlike in the DC charge transport experiment, the Josephson effect survives in our thermal transport measurement regardless of the strength of the dissipation, which is a complementary test of the anticipated dissipative phase transition proposed by Schmid and Bulgadaev.

In this scenario, we have also performed a conventional DC charge transport experiment through a Josephson junction JJ connected to a voltage source via an ohmic resistor with resistance either greater or smaller than resistance quantum RQ to revisit the debated dissipative phase transition in a JJ. Our results support the existence of this transition, evidenced by a distinct dip in electrical conductance at zero voltage bias for the JJ connected to an environmental resistance exceeding RQ. Conversely, for the JJ coupled to an environmental resistance less than RQ, a conductance peak appears at zero voltage bias, indicative of the Josephson effect.

Key words: gambling demons, thermometry, quantum heat transport, superconducting circuits, quantum dissipative phase transition

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/ 

Contact information:

Doctoral theses at the School of Science: https://aaltodoc.aalto.fi/handle/123456789/52