Excitation of quasiparticle pairs in superconducting nanodevices by 1/f noise

Abstract

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.

Figures (3)

• Figure 1: Sum of Lorentzian spectra yields a 1/f noise spectrum.
• Figure 2: Schematic of energy levels of the superconducting Hamiltonian, Eq. \ref{['eq:Hamiltonian']}. a: Schematic of energy levels of the superconducting Hamiltonian, Eq. \ref{['eq:Hamiltonian']}. In the absence of noise at zero temperature, the chemical potential $\mu=E_F$, where $E_F$ is the Fermi energy, and the eigenstates of the Hamiltonian at the Fermi level are equal mixtures of electrons and holes. For each $k$ the ground state is a Cooper pair in the superconducting condensate, while the excited state is a quasiparticle pair. Changing $\mu$ changes the relative energy cost of holes and electrons. b: Illustration of the change in the wavefunction when the chemical potential is changed at frequencies exceeding the superconducting gap frequency. Quasiparticles must be excited because the relative amounts of electrons and holes in the condensate wavefunction change.
• Figure 3: Schematic illustrating the possibility of obtaining information about the orientation of TLF dipoles by measuring excitation of quasiparticle pairs. All the dipoles are in the substrate (lighter gray); the blue dipoles are under the superconductor, and are vertically oriented because their fluctuations have a greater effect on the chemical potential of the superconductor. The fluctuations pink dipoles, which are not directly under the superconductor, have a larger effect when they are horizontally oriented.