Investigating loss and dephasing mechanisms in constriction-based phase slip qubits

A superconducting constriction-based junction provides an alternative source of nonlinearity to the ubiquitous Al/AlOx/Al junctions used in superconducting circuits that promises new types of protected qubits and the possibility of high-temperature and high-frequency operation. Superconducting constrictions have previously been incorporated into qubits but these qubits have typically had very short lifetimes and coherence times – both on the order of ns. We recently demonstrated a phase slip qubit based on a titanium nitride constriction with relaxation times of ~70 μs, but short coherence times of 10-20 ns. Here we investigate the loss and dephasing mechanisms in multiple phase slip qubits and their dependence on phase slip energy, inductive energy and constriction size. For our best devices, we observe relaxation times of ~100 μs and coherence times of ~4 μs, which significantly improves upon the state-of-the-art qubits using large-gap superconductors. Our analysis indicate that the qubit lifetime is limited by inductive loss, while the coherence time is constrained by multiple noise sources including flux and thermal noise. Our results indicate a path forward for using superconducting constrictions in qubits and establishes constriction-based qubits as a promising platform.