High-performance transmon qubits with an epitaxially grown TiN film on Si (100) substrate*

The transmon qubit is a fundamental superconducting quantum element, and its performance directly affects the performance of integrated quantum circuits. The energy decay time of a transmon is known to be sensitive to the surface condition of the superconducting electrodes, and there have been many attempts to improve the performance of transmons by changing the materials and surface treatments. Here we present the properties of our transmons consisting of a shadow-evaporated Al Josephson junction and TiN electrodes grown on Si (100) substrate with domain-matching epitaxy at high temperature. The energy decay time of 250 μs is obtained for a transmon with the transition frequency of 4.5 GHz. We evaluate the loss mechanism of our transmons by changing their designs.