This summer, we are happy to welcome Odréanne Breton, BSc student at Université de Montréal in Mathematics and Physics, for a four-month internship. We wish you a great summer in the group!

Fluxonium qubits are gaining attention as a promising alternative to transmon qubits, thanks to their long coherence times and their ability to support fast, high-fidelity quantum gates. However, one aspect that has remained less clear is how well these qubits can be read out.

In transmon systems, recent studies have identified measurement-induced state transitions as a leading source of readout error. For fluxonium qubits, though, the role of these effects has not been fully understood. In this work, we take a systematic approach to this question, exploring a wide range of fluxonium designs that cover essentially all experimentally relevant regimes.

We find that “lighter” fluxonium variants are generally less susceptible to these readout-induced transitions, and we identify the key physical mechanisms responsible for this behavior. Together, these results provide a clearer picture of the limitations of fluxonium readout and offer practical guidance for the design of future fluxonium-based superconducting quantum processors.

High-fidelity measurement of superconducting qubits remains a central challenge in circuit QED, even though the physical mechanisms that limit measurement performance are now well understood. Existing approaches to mitigate these limitations often rely on alternative readout circuits that are complex and demanding to engineer.

In collaboration with ÉPFL, we experimentally demonstrate our proposed Josephson-junction based readout. It consists of a simple superconducting circuit that is intrinsically robust against deleterious effects induced by strong measurement drives. Despite being a first-generation implementation, our circuit already achieves a measurement fidelity of 99.4% within 68 ns, as well as leaving the state unchanged with a 98.4% probability. Importantly, these results do not require the use of an external Purcell filter or a near-quantum-limited amplifier.

On December 15, 2025, before his peers and a jury, Lautaro defended his doctoral thesis in physics at the auditorium of the Institut Quantique. His research focused on the modeling of superconducting circuits incorporating non-reciprocal devices, as well as sensing with nonclassical states of light. To learn more about his journey and the achievements leading to his PhD, we invite you to read his student portrait. Congratulations Lautaro, and much success in your future endeavors!

Our work on understanding the effect of charge noise on the ionization of transmon qubits has been published in Physical Review Letters.

One of the advantages of transmon qubits is their resilience against unavoidable electric noise in their environment. However, it was recently suggested that the measurement of these qubits is not protected against such noise. In collaboration with the Karlsruhe Institute of Technology, we studied the effect of electric noise on transmon measurement. Our theory explains observations and suggests strategies to improve measurement in the near future.

On September 23, Simon Richer, a graduating student from the group, presented his master’s seminar at the Institut quantique auditorium, titled Optimal Quantum Control Applied to the Readout of the Fluxonium Qubit. While completing the writing of his thesis, he is beginning a four-month internship at AWS in California this October. Congratulations and best of luck !

This fall, we are welcoming three new members in the group. Gustave Coulombe (left) is starting his Master’s after completing his Bachelor’s in Engineering Physics at Université Laval. Julien-Pierre Houle (center) is also beginning his Master’s after completing his Bachelor’s in Physics at Université de Sherbrooke. His research will be co-supervised with Professor Cunlu Zhou from the Computer Science Department. Finally, Émile Baril (right) is starting his PhD after completing a Bachelor’s in Physics at Université Laval and a Master’s in Physics at Université de Montréal. Welcome to all three!