As quantum coherence times of superconducting circuits have increased from nanoseconds to hundreds of microseconds, they are currently one of the leading platforms for quantum information processing. However, coherence needs to further improve by orders of magnitude to reduce the prohibitive hardware overhead of current error correction schemes. Reaching this goal hinges on reducing the density of broken Cooper pairs, so-called quasiparticles. Here, we show that environmental radioactivity is a significant source of nonequilibrium quasiparticles. Moreover, ionizing radiation introduces time-correlated quasiparticle bursts in resonators on the same chip, further complicating quantum error correction. Operating in a deep-underground lead-shielded cryostat decreases the quasiparticle burst rate by a factor thirty and reduces dissipation up to a factor four, showcasing the importance of radiation abatement in future solid-state quantum hardware.

Out of equilibrium quasiparticles (QPs) are one of the main sources of decoherence in superconducting quantum circuits, and are particularly detrimental in devices with high kinetic inductance, such as high impedance resonators, qubits, and detectors. Despite significant progress in the understanding of QP dynamics, pinpointing their origin and decreasing their density remain outstanding tasks. The cyclic process of recombination and generation of QPs implies the exchange of phonons between the superconducting thin film and the underlying substrate. Reducing the number of substrate phonons with frequencies exceeding the spectral gap of the superconductor should result in a reduction of QPs. Indeed, we demonstrate that surrounding high impedance resonators made of granular aluminum (grAl) with lower gapped thin film aluminum islands increases the internal quality factors of the resonators in the single photon regime, suppresses the noise, and reduces the rate of observed QP bursts. The aluminum islands are positioned far enough from the resonators to be electromagnetically decoupled, thus not changing the resonator frequency, nor the loading. We therefore attribute the improvements observed in grAl resonators to phonon trapping at frequencies close to the spectral gap of aluminum, well below the grAl gap.Superconducting circuits play a central role in a variety of research and application areas, such as solid state quantum optics 1 , metrology 2,3 , and low temperature detectors 4,5 . In particular, the field of superconducting qubits has grown impressively during the last decade 6,7 . In these devices quantum states can live for up to tens of microseconds, while gate times can be as short as tens of nanoseconds [8][9][10][11] . Nevertheless, coherence times need to be further improved by orders of magnitude in order to be able to perform quantum error correction 12,13 with an affordable hardware overhead.One of the main sources of decoherence in superconducting devices at millikelvin temperatures are out of equilibrium quasiparticles (QPs) [14][15][16][17][18][19][20][21][22] , which can be viewed as broken Cooper pairs (CPs). Quasiparticles can be particularly damaging in high kinetic inductance circuits [23][24][25][26][27] , which are a promising avenue for protected qubits 28 and hybrid superconductingsemiconducting devices [29][30][31] . Proposed mechanisms for CP breaking include stray infrared radiation 32,33 , direct microwave drive 34,35 , and high energy phonons in the device substrate created by environmental or cosmic radioactivity [36][37][38] . The latter is particularly damaging because it gives rise to correlated QP bursts in multiple devices on the same chip 36,39 , possibly resulting in a) Both authors contributed equally b) Electronic

Non-equilibrium quasiparticles can deteriorate the performance of superconducting qubits by reducing their coherence. We are investigating a source of quasiparticles that has been too long neglected, namely radioactivity: cosmic rays, environmental radioactivity and contaminants in the materials can all generate phonons of energy sufficient to break Cooper pairs and thus increase the number of quasiparticles. In this contribution, we describe the status of the project and its perspectives.

The Ornstein-Uhlenbeck process can be seen as a paradigm of a finite-variance and statistically stationary rough random walk. Furthermore, it is defined as the unique solution of a Markovian stochastic dynamics and shares the same local regularity as the one of the Brownian motion. Based on previous works, we propose to include in the framework of one of its generalization, the so-called fractional Ornstein-Uhlenbeck process, some Multifractal corrections, using a Gaussian Multiplicative Chaos. The aforementioned process, called a Multifractal fractional Ornstein-Uhlenbeck process, is a statistically stationary finite-variance process. Its underlying dynamics is non-Markovian, although nonanticipating and causal. The numerical scheme and theoretical approach are based on a regularization procedure, that gives a meaning to this dynamical evolution, which unique solution converges towards a well-behaved stochastic process.