Engineering the phonon bath for TLS in a superconducting qubit*

Superconducting (SC) qubits are one of the leading platforms for fault-tolerant quantum computation. However, state-of-the-art SC qubits have a relatively short T1 lifetime and T2* coherence time compared to their atomic counterparts. One of the major limitations in the lifetimes of the SC qubits are the presence of various microscopic forms of two-level state (TLS) defects in the amorphous surface and/or bulk of the materials making up the qubits. Qubit decoherence is believed to come from a two step dissipation process in which: (i) the qubit state interacts first with the bath of TLS, and (ii) the TLS then interacts with the phonon bath of the bulk material. Previous work has focused mainly on mitigating the first step of the dissipation process by studying qubit-TLS interactions, and suppression of TLS effects via circuit design and new materials.

Here, we look further down the dissipation channel, and study the second step in which the TLS couple to the phonon bath. We introduce the design and fabrication of a qubit embedded in an acoustic bandgap material that forbids phonon propagation around the qubit frequency, and suppresses the resonant decay of TLS into the phonon bath. Consequently, TLS inside the acoustic bandgap are expected to be long-lived, which in the case of discrete TLS interacting with the SC qubit, should suppress the decoherence of the SC qubit. We will present data on the performance of our acoustically shielded SC qubit, along with prospects for using the TLS themselves as a form of long-lived quantum memory for the SC qubit.