Improving the coherence of superconducting circuits
In an ideal superconducting quantum computer, the underlying quantum bits (qubits) are isolated from the noisy environment with no energy loss to mechanical or vibrational modes. However, in the real world, amorphous materials and material interfaces have defect states that cause qubits to lose their energy through vibrations and interactions with the surrounding environment.
A new article by LLNL researchers reports a method to enhance the coherence of superconducting circuits by introducing a phononic bandgap around the system’s operating frequency. Phononic bandgaps can be created using meta-materials with shapes designed to manipulate vibrations within a system. In theory, a meta-material with the right shape can provide a phononic bandgap capable of blocking resonant decay of defect states in these superconducting circuits.
The new paper describes a theoretical framework for implementing phononic bandgaps in practice. The T1 energy decay process in qubits, dominated by vibrating defects, is defined as the amount of time a qubit can stay in the 1 state. The paper created a multi-scale model that can predict the decrease in the density of states due to the bandgap, as well as the resulting increase in T1 times.
Depending on the specific material parameters, their theoretical model shows that a phononic bandgap should decrease qubit decay rates, or at least delay energy decay to the environment such that information can be recovered. Overall, the fabrication of devices with integrated phononic bandgap structures should increase the quality factors of superconducting resonators and qubits.
This paper was featured as a “scilight” by the journal. The research was supported by the Laboratory Directed Research and Development Program (18-FS-036).
[Y.J. Rosen, M.A. Horsley, S.E. Harrison, E.T. Holland, A.S. Chang, T. Bond, and J.L. DuBois, Protecting superconducting qubits from phonon mediated decay, Appl. Phys. Lett. 114, 202601 (2019), doi: 10.1063/1.5096182.]
