Acoustically shielded TLS exceeding 100us T1 time

State-of-the-art superconducting (SC) transmon qubits are limited in energy relaxation time by microscopic two-level state (TLS) defects living at the amorphous material interfaces making up the qubits [1]. Although the microscopic origin is yet to be confirmed, TLS defects are believed to be tunneling states between two configurations in disordered materials, and they respond coherently to both electric and acoustic fields. The short relaxation time of TLSs is theoretically attributed to decay into the phonon bath of the bulk material. Based on this model (known as the standard tunneling model (STM) developed for glassy materials [2]), we engineered the phonon bath such that isolated TLS defects at microwave frequencies, resonant with transmon qubits, could become long-lived and a useful quantum resource.

We introduce the design, fabrication and test of a hybrid system incorporating SC circuits and acoustic metamaterials. The acoustic metamaterial features an acoustic bandgap which forbids propagation of phonons around 4-6GHz, and hence suppresses the resonant decay of TLSs into the phonon bath. Consequently, TLSs inside the acoustic bandgap are long-lived as probed by the auxiliary SC qubit. We will present data on the performance of our acoustically shielded TLSs, with energy relaxation time T1 exceeding 100us, representing a three-orders-of-magnitude improvement over similar TLSs interacting with an unstructured phonon bath. We will discuss prospects for better understanding the STM through these measurements, and translating the long TLS relaxation time to improved transmon T1 time.

[1] C. Wang et al., Appl. Phys. Lett. 107, 162601 (2015)

[2] W. A. Phillips, Rep. Prog. Phys. 50, 1657 (1987); P. W. Anderson et al., Philos. Mag. 25, 1 (1972)