Analysis of Operating Points in the Fluxonium Molecule

An ideal qubit architecture should support fast, reliable, one and two qubit gates while maintaining coherence over long times. In the fluxonium qubit, inductive shunting of a Josephson junction eliminates low frequency charge noise, while careful tuning of a flux bias yields first-order insensitivity to low-frequency flux noise. Recent efforts have demonstrated fluxonium with T1 on the order of 1 millisecond and two-qubit gate fidelities of 99.9% [1].  An extension of this circuit is the fluxonium molecule [2], which is composed of two fluxonium circuits with independent flux bias controls and coupled through a strong shared inductive element. The two flux biases must be simultaneously tuned to sweet spots in order to suppress low-frequency flux noise, but the remaining space of possible operating points is considerably larger. Importantly, the choice of operating point will determine the dominant mechanism of decoherence and constrain available control schemes. We present a theoretical analysis of this system and the results of preliminary measurements.  We examine the dependence of the spectrum and decoherence mechanisms on the flux bias and circuit symmetry. 

This material is based upon work supported under Air Force Contract No. FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the U.S. governement or the U.S. Air Force. 

 

[1] L. Ding, et al., High Fidelity, Frequency-Flexible Two-Qubit Fluxonium Gates with a Transmon Coupler, arXiv:2304.06087v1 

[2] A. Kou, et al., A fluxonium-based artificial molecule with a tunable magnetic moment, Phys. Rev. X 7, 031037 (2017)