Phononic bandgaps protect superconducting qubits from phonon mediated decay

In an ideal superconducting quantum computer, the underlying quantum bits 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 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.

Similar to previous works that introduced electromagnetic bandgaps around the qubit operating frequency to improve coherence of superconducting circuits, Rosen et al. examined the use of phononic bandgaps to prevent vibrations at frequencies emitted by defects.

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.

Source: “Protecting superconducting qubits from phonon mediated decay,” by Yaniv J. Rosen, Matthew A. Horsley, Sara E. Harrison, Eric T. Holland, Allan S. Chang, Tiziana Bond, and Jonathan L DuBois, Applied Physics Letters (2019). The article can be accessed at https://doi.org/10.1063/1.5096182 .