Progress Towards a Protected Qubit Subspace within a Fluxonium Molecule*

 

Protected qubits possess inherent immunity to both depolarization and pure dephasing processes. This noise protection is owed to both circuit design and parameter choices for which wavefunctions of logical states simultaneously exhibit disjoint support as well as sweet spots. The former suppresses spontaneous qubit transitions, the latter renders the qubit energy splitting first or second order insensitive to noise thus reducing pure dephasing. Here, we present recent work towards improving noise protection beyond the level demonstrated with the “soft” 0- qubit [1], instead employing a fluxonium molecule circuit [2]. We discuss our results and the general competing constraints encountered in engineering protected qubits.

 

[1] A. Gyenis, P. S. Mundada, A. Di Paolo, T. Hazard, X. You, D. Schuster, J. Koch, A. Blais and A. A. Houck, PRX Quantum 2, 010339 (2021).

[2] A. Kou, W. C. Smith, U. Vool, R. T. Brierley, H. Meier, L. Frunzio, S. M. Girvin, L. I. Glazman, and M. H. Devoret, Phys. Rev. X 7, 031037 (2017)