High-coherence fluxonium qubit

We report coherence measurement of a 3D fluxonium circuit capacitively coupled to a box resonator. The circuit was fabricated by Al evaporation on an untreated silicon substrate. Energy relaxation time (T1) varies from a few microseconds at integer flux to over 100 microseconds at half-integer flux sweet spot, and coherence time (T2) approaches 2T1. Interestingly, T1 is limited at the sweet spot by dielectric loss with an effective loss tangent an order of magnitude higher than a recently reported value in transmon devices [1, 2]. Therefore, we expect that our fluxonium design has the potential to reach millisecond coherence once state-of-the-art fabrication techniques are employed. We discuss possible mechanisms that may limit T2 at the millisecond scale and ways to circumvent them.


[1] Y. Chu, C. Axline, C. Wang, T. Brecht, Y. Gao, L. Frunzio and R. Schoelkopf, Applied Physics Letters 109, (2016).
[2] C. Wang, C. Axline, Y. Gao, T. Brecht, Y. Chu, L. Frunzio, M. Devoret and R. Schoelkopf, Applied Physics Letters 107, (2015).