Disentangling radioactive backgrounds in superconducting qubits with event-by-event detection

In the past twenty years, the greatest advances towards the realization of fault-tolerant quantum computers have been achieved by employing quantum bits, or qubits, based on superconducting circuits. However, the quantum states of superconducting qubits remain inherently fragile, as they are affected by several noise mechanisms due to unwanted couplings with the surrounding environment. Recently, increasing concerns for the impact of environmental radioactivity on quantum circuits have arisen: hitting particles, in fact, produce phonons in the chip substrate that can disturb multiple qubits simultaneously, complicating the implementation of error correction algorithms. This thesis deeply investigates how radiation sources affect superconducting qubits. The first step of this project was the realization of a Monte Carlo simulation to understand which radiation sources produce the highest rate of impacts in qubit chips and should, therefore, be taken into account for the implementation of mitigation strategies. Following that, two studies were performed, respectively on a fluxonium and on a transmon qubit. The first study aimed at understanding the impact of environmental radioactivity on the global decay time of the fluxonium qubit, concluding that other decay mechanisms dominate. The second study enabled the detection of individual particle interactions on a transmon chip, demonstrating that gamma radiation can induce qubit decays and establishing the transmon as a real-time radiation detector. Building on this result, a new firmware for the control and readout electronics was developed to enable multiplexed operation and the simultaneous measurement of up to four qubits. Having demonstrated real-time radiation detection with a single qubit, the multiplexed architecture allows the study of correlated errors across multiple qubits, a crucial step toward understanding radiation-induced error bursts in quantum processors. Initial tests of this firmware, performed by illuminating a chip with an LED, revealed correlated errors in three transmon qubits, paving the way for systematic investigations of radiation-induced correlated events. More broadly, by characterizing the transmon as a sensitive probe of substrate phonons, this work opens the door toward a new paradigm of phonon-mediated detection with superconducting qubits, a possibility that had previously been largely theoretical.