Public defence, Engineering Physics, MSc Aashish Sah

Title of the thesis: Programmable coupling to electromagnetic environments in superconducting qubits

Thesis defender: Aashish Sah 
Opponent: Associate Professor Eli levenson-Falk, University of Southern California, US
Custos: Academy Professor Mikko Möttönen, Aalto University School of Science

Protecting and controlling superconducting quantum bits 

Superconducting circuits provide a versatile platform for quantum computing and other quantum technologies. Their performance depends on preserving fragile quantum information while allowing fast and accurate control. These requirements create a fundamental challenge: the external coupling used to operate a quantum bit, or qubit, also introduces energy loss and environmental noise. 

In this thesis, we investigate how the electromagnetic environment of superconducting qubits can be engineered to balance isolation and control. We develop and experimentally demonstrate circuit designs that protect qubits while preserving the interactions needed to operate them. The work addresses an important challenge in the development of larger and more reliable quantum processors. 

First, we introduce filters integrated directly into the chip that suppress energy loss through the qubit control lines. These filters improve the measured qubit energy lifetime by up to a factor of 200. Using microwave signals at a fraction of the qubit's natural oscillation frequency, we demonstrate fast control while maintaining this protection. This approach can also reduce the heat generated by control signals in the cooling system. 

Second, we introduce the quantum dial, a tunable filter that adjusts how strongly a qubit interacts with its environment. The same circuit supports three operational regimes: an idle state with suppressed radiative decay, a control state enabling high-fidelity single-qubit gates, and a strongly coupled state for unconditional, state-independent qubit reset. We also demonstrate sensitive temperature measurements by using the qubit as a thermometer of its surroundings. These results illustrate how adjustable coupling can support both quantum computation and sensing. 

Alongside these developments, the thesis contributes compact microwave components and techniques for qubit reset and the characterization of propagating microwave fields. These provide further tools for studying and operating superconducting quantum circuits. 

The findings demonstrate how engineered qubit-environment coupling can support both qubit protection and useful interactions. The developed methods offer ways to reduce unwanted energy loss and adapt these interactions to the requirements of different operations in future superconducting quantum processors.

Keywords: Josephson junction, qubit, quantum computing 

Thesis available for public display 7 days prior to the defence at Aalto University's public display page.