Quasiparticles and qubits: history and recent developments

Gianluigi Catelani

Amin Hosseinkhani

Bogoliubov quasiparticle excitations are detrimental for the operation of many superconducting devices. In superconducting qubits, quasiparticles interact with the qubit degree of freedom when tunneling through a Josephson junction, and this interaction can lead to qubit relaxation. At millikelvin temperatures, there is substantial evidence of nonequilibrium quasi- particles. While there is no agreed upon explanation for the origin of these excess quasiparticles, it is nevertheless possible to limit the quasiparticle-induced relaxation by steering quasiparticles away from qubit active elements. In this thesis, we study quasiparticle traps that are formed by a normal-metal in tunnel contact with the superconducting electrode of a qubit. We develop a model to explain how a trap can influence the dynamics of the excess quasiparticles injected in a transmontype qubit. This model makes it possible to find the time it takes to evacuate the injected quasiparticles from the transmon as a function of trap parameters. We show when the trap size is increased, the evacuation time decreases monotonically and saturates at a level that depends on the quasiparticles diffusion constant and the qubit geometry. We find the characteristic trap size needed for the evacuation time to approach the saturation value. It turns out that the bottleneck limiting the trapping rate is the slow quasiparticle energy relaxation inside the normal-metal trap, a quantity that is very hard to control. In order to optimize normal-metal quasiparticle trapping, we study the effects of trap size, number, and placement. These factors become important when the trap size increases beyond the characteristic length. We discuss for some experimentally relevant examples how to shorten the evacuation time of the excess quasiparticle density. Moreover, we show that a trap in the vicinity of a Josephson junction can significantly suppress the steady-state quasiparticle density near that junction and reduce the impact of fluctuations in the generation rate of quasiparticles. When such normal-metal elements are connected to a superconducting material, Cooper- pairs can leak into the normal-metal trap. This modifies the superconductor properties and, in turn, affects the qubit coherence. Using the Usadel formalism, we first revisit the proximity effect in uniform NS bilayers; despite the long history of this problem, we present novel findings for the density of states. We then extend our results to describe a non-uniform system in the vicinity of a trap edge. Using these results together with the previously developed model for the suppression of the quasiparticle density due to the trap, we find in a transmon qubit an optimum trap-junction distance at which the qubit relaxation rate is minimized. This optimum distance, of the order of 4 to 20 coherence lengths, originates from the competition between proximity effect and quasiparticle density suppression. We conclude that the harmful influence of the proximity effect can be avoided so long as the trap is farther away from the junction than this optimum.

Jaan Männik

Gianluigi Catelani, Jukka P Pekola

Abstract The fundamental excitations in superconductors—Bogoliubov quasiparticles—can be either a resource or a liability in superconducting devices: they are what enables photon detection in microwave kinetic inductance detectors, but they are a source of errors in qubits and electron pumps. To improve operation of the latter devices, ways to mitigate quasiparticle effects have been devised; in particular, combining different materials quasiparticles can be trapped where they do no harm and their generation can be impeded. We review recent developments in these mitigation efforts and discuss open questions.

Gianluigi Catelani

Kazunori Ohumi

Modern physics has achieved remarkable success by describing reality in terms of particles, states, and information bits. Yet recent developments in quantum matter, unconventional superconductivity, and topological computation increasingly suggest that a purely particle-centric framework may not be the most natural organizing description, but rather a convenient approximation. This work proposes an ontological reframing—from It from Bit to It from Wave—in which physical reality is understood as persistent, collective wave processes characterized by phase continuity, nonlocal coherence, and history-bearing relations. Within this framework, quasiparticles—including magnons, phonons, plasmons, and anyons—are reinterpreted not as entities, but as stable collective excitation modes: enduring patterns of interacting waves sustained by a medium. Building on this shift, localization is reinterpreted as the breakdown of wave connectivity, while delocalization corresponds to the restoration of global phase coherence. Superconductivity is reformulated not as a special property of paired particles, but as a phase-continuous, delocalized regime in which dissipation is not eliminated but reorganized into collective phase dynamics—naturally suggesting disorder-tolerant and non-BCS-like superconducting behavior. Extending the same ontology to computation, we argue that quantum computation is fundamentally history-based rather than state-based. Information is encoded not primarily in localized bits or qubits, but in the topological and phase-coherent histories of wave evolution, where robustness arises from nonlocal distribution rather than active error suppression. Crucially, this framework motivates previously under-emphasized physical observables—such as phase continuity length, history coherence time, degree of nonlocality, and dissipation-reorganized coherence—as primary descriptors of relational persistence. Rather than introducing new equations, this work reorganizes interpretation across domains and specifies concrete experimental and theoretical directions by asking which wave relations persist, under what constraints, and for how long. This paper constitutes the ontological foundation of the “It from Wave” research program. A complementary interpretive study examines the historical and conceptual shift from particle transport to phase propagation in electrical conduction, while a minimal physical model demonstrates how information and conduction can arise from phase-coupled dynamics without particle motion. Together, these works articulate a unified framework in which phase relations, history, and nonlocal coherence are treated as physically primary. This paper forms part of a conceptual trilogy exploring the transition from a Machine OS to a Life OS through an It from Wave perspective. The trilogy consists of:(i) a foundational conceptual work reframing information, existence, and generation beyond the It from Bit paradigm;(ii) a minimal theoretical study addressing nonlinear generative time, resonance, and feedback dynamics; and(iii) an applied interpretation engaging recent cosmological observations, including extreme intracluster heating in the early universe. While each paper is self-contained, together they articulate a unified framework in which reality is understood as a resonant, generative process rather than a sequential, machine-like assembly.