Ab initio modeling of nonequilibrium dynamics in superconducting detectors and qubits

Abstract

Nonequilibrium quasiparticle (QP) and phonon dynamics are central to the operation of superconducting devices. Superconducting detectors, such as the superconducting nanowire single-photon detector, perform best when a large QP population is generated in response to small perturbations. Conversely, for superconducting qubits and topologically protected Majorana fermions, even relatively small QP densities can lead to significant performance degradation, and thus, ideal materials are less susceptible to QP poisoning. However, existing models of these devices lack a rigorous description of the QP and phonon dynamics, relying on approximations and phenomenology. In this article, we combine kinetic equations with density functional theory to model the nonequilibrium dynamics of a superconducting film ab initio. To demonstrate the universality of our model, we illustrate two examples: (1) we develop a model for the detection of single photons in superconducting nanowires, and (2) we calculate the energy-relaxation rate of a transmon qubit due to the presence of excess QPs. Our examples demonstrate from first principles that NbN is well-suited for single-photon detection and that Ta transmon qubits possess reduced sensitivity to QP poisoning relative to other materials, which is likely in part responsible for their longer coherence times. In contrast to previous models, our ab initio approach makes these predictions without experimental input and thus can be used to accelerate progress in device development. Moreover, by considering the full-bandwidth electron-phonon coupling, our approach can incorporate strong-coupling effects. Our methods effectively integrate ab initio materials modeling with nonequilibrium theory of superconductivity to perform practical modeling of superconducting devices, providing a comprehensive approach that connects fundamental theory with device applications.