Quasiparticle Excitations

Péter R. Śurján

Kyle Serniak, Max Hays, G. de Lange +6 more

Nonequilibrium quasiparticle excitations degrade the performance of a variety of superconducting circuits. Understanding the energy distribution of these quasiparticles will yield insight into their generation mechanisms, the limitations they impose on superconducting devices, and how to efficiently mitigate quasiparticle-induced qubit decoherence. To probe this energy distribution, we systematically correlate qubit relaxation and excitation with charge-parity switches in an offset-charge-sensitive transmon qubit, and find that quasiparticle-induced excitation events are the dominant mechanism behind the residual excited-state population in our samples. By itself, the observed quasiparticle distribution would limit T_{1} to ≈200 μs, which indicates that quasiparticle loss in our devices is on equal footing with all other loss mechanisms. Furthermore, the measured rate of quasiparticle-induced excitation events is greater than that of relaxation events, which signifies that the quasiparticles are more energetic than would be predicted from a thermal distribution describing their apparent density.

Jiaju Zhang, M. A. Rajabpour

Abstract We investigate the subsystem Schatten distance, trace distance and fidelity between the quasiparticle excited states of the free and the nearest-neighbor coupled fermionic and bosonic chains and the ferromagnetic phase of the spin-1/2 XXX chain. The results support the scenario that in the scaling limit when one excited quasiparticle has a large energy it decouples from the ground state and when two excited quasiparticles have a large momentum difference they decouple from each other. From the quasiparticle picture, we get the universal subsystem distances that are valid when both the large energy condition and the large momentum difference condition are satisfied, by which we mean each of the excited quasiparticles has a large energy and the momentum difference of each pair of the excited quasiparticles is large. In the free fermionic and bosonic chains, we use the subsystem mode method and get efficiently the subsystem distances, which are also valid in the coupled fermionic and bosonic chains if the large energy condition is satisfied. Moreover, under certain limit the subsystem distances from the subsystem mode method are even valid in the XXX chain. We expect that the results can be also generalized for other integrable models.

Orlando Luongo

Clare C. Yu, M. C. Goffage, Wang, Yifan +3 more

Superconducting nanodevices such as qubits, resonators, and photodetectors, have revolutionized our capabilities for probing and controlling quantum phenomena. Nonequilibrium quasiparticles, which are broken Cooper pairs that cause decoherence and energy loss, can limit their performance. The number of these quasiparticles is often tens of orders of magnitude greater than would be present in thermal equilibrium. Background radiation has been shown to excite quasiparticles, but quasiparticles are observed even when the devices are carefully shielded. Here we show that the high-frequency components of 1/f noise can excite quasiparticle pairs and that this mechanism is consistent with previously unexplained experimental results. We also propose new experiments that exploit this quasiparticle excitation mechanism to non-invasively characterize high-frequency charge noise as well as the locations and nature of the defects producing the noise. The proposed experiments would also investigate how this noise changes as the defects that give rise to it evolve towards thermal equilibrium.