Leakage dynamics and reduction in disordered transmon arrays and tunable couplers
Thesis event information
Date and time of the thesis defence
Place of the thesis defence
TA105, Linnanmaa Campus
Topic of the dissertation
Leakage dynamics and reduction in disordered transmon arrays and tunable couplers
Doctoral candidate
Master of Science Taneli Tolppanen
Faculty and unit
University of Oulu Graduate School, Faculty of Science, Nano and Molecular Systems Research Unit
Subject of study
Physics
Opponent
Associate Professor Alexandru Petrescu, Mines Paris
Custos
Associate Professor Matti Silveri, University of Oulu
Leakage removal for transmon qubits
A quantum bit (qubit) can be realized with a quantum system that has at least two distinct states. A popular realization of a qubit is the transmon device, which is made from superconducting material and in which quantum information can be controlled with electrical signals. The two lowest energy states of a transmon are used to represent the |0> and |1> states of a qubit, but the transmon also has higher energy states. Due to imperfect control, some of these higher energy states can be populated. This is called leakage, and the excitations beyond the qubit states are called leakage excitations. This thesis uses numerical methods to study superconducting devices that can remove leakage excitations.
A good leakage removal device, or more formally a leakage reduction unit (LRU), should remove leakage excitations without disturbing the quantum state of a qubit. We studied an LRU based on a chain of transmons, configured so that only leakage excitations can move along the chain. The last transmon of the chain is connected to the environment, which allows leakage excitations to be removed from the chain. We found that this kind of a device can remove leakage excitations from transmon qubits, while its effect on the quantum state of the qubit can be reliably suppressed. In addition to leakage, we studied how repeated measurement affects entanglement in transmon chains, which we found to induce a phase transition in the chains entanglement properties. Furthermore, we studied how leakage behaves in systems with tunable couplers and tunable coupler based LRUs. We found that while tunable couplers can turn off the interaction between qubits, leakage excitations can still move between them. Understanding leakage dynamics is helpful for designing systems more resistant to leakage errors. It also enables LRUs based on tunable couplers, which we found to remove leakage faster than the previously mentioned transmon chain based device while also disturbing the qubit less.
Leakage challenges quantum computation by causing errors that are difficult for standard error correction methods to handle, and in general these methods do not remove leakage. This thesis seeks to address this problem by studying new LRUs for transmon qubits. The LRUs studied here are passive, meaning that leakage can be removed while the qubit is in use. So far, passive LRUs have not received much attention.
A good leakage removal device, or more formally a leakage reduction unit (LRU), should remove leakage excitations without disturbing the quantum state of a qubit. We studied an LRU based on a chain of transmons, configured so that only leakage excitations can move along the chain. The last transmon of the chain is connected to the environment, which allows leakage excitations to be removed from the chain. We found that this kind of a device can remove leakage excitations from transmon qubits, while its effect on the quantum state of the qubit can be reliably suppressed. In addition to leakage, we studied how repeated measurement affects entanglement in transmon chains, which we found to induce a phase transition in the chains entanglement properties. Furthermore, we studied how leakage behaves in systems with tunable couplers and tunable coupler based LRUs. We found that while tunable couplers can turn off the interaction between qubits, leakage excitations can still move between them. Understanding leakage dynamics is helpful for designing systems more resistant to leakage errors. It also enables LRUs based on tunable couplers, which we found to remove leakage faster than the previously mentioned transmon chain based device while also disturbing the qubit less.
Leakage challenges quantum computation by causing errors that are difficult for standard error correction methods to handle, and in general these methods do not remove leakage. This thesis seeks to address this problem by studying new LRUs for transmon qubits. The LRUs studied here are passive, meaning that leakage can be removed while the qubit is in use. So far, passive LRUs have not received much attention.
Created 20.10.2026 | Updated 21.10.2026