Coherent Josephson Qubit Suitable for Scalable Quantum Integrated Circuits

TLDR

The qubit’s design minimizes radiative loss and material‑defect coupling, and defect physics are clarified by geometry variation and model analysis. We demonstrate a planar, tunable superconducting qubit with energy relaxation times up to 44 μs, observe a fine‑structure in lifetime versus frequency revealing sparse incoherent two‑level defects, and show that the Xmon qubit’s fabrication simplicity, connectivity, fast control, and long coherence make it a viable route to chip‑based quantum computers.

Abstract

We demonstrate a planar, tunable superconducting qubit with energy relaxation times up to $44\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$. This is achieved by using a geometry designed to both minimize radiative loss and reduce coupling to materials-related defects. At these levels of coherence, we find a fine structure in the qubit energy lifetime as a function of frequency, indicating the presence of a sparse population of incoherent, weakly coupled two-level defects. We elucidate this defect physics by experimentally varying the geometry and by a model analysis. Our ``Xmon'' qubit combines facile fabrication, straightforward connectivity, fast control, and long coherence, opening a viable route to constructing a chip-based quantum computer.