Bifluxon: Fluxon-Parity-Protected Superconducting Qubit*

We have developed and characterized a symmetry-protected superconducting qubit that offers simultaneous exponential suppression of energy decay from both the charge and flux noise, and dephasing from flux noise [1]. The qubit is implemented as a superconducting loop formed by a Cooper-pair box (CPB) and a superinductor. Provided the offset charge on the CPB island is an odd number of electrons, the qubit potential corresponds to that of a cos (φ/2) Josephson element, preserving the parity of fluxons in the loop via Aharonov-Casher interference. Importantly, the protection can be turned on and off by controlling the CPB charge. In the protected state, the logical-state wavefunctions reside in disjoint regions of phase space, thereby ensuring protection against energy decay. By turning the protection on, we observed a ten-fold increase of the decay time, up to 100 μs. We will discuss strategies for mitigation of the charge noise effects by designing small arrays of cos (φ/2) elements.

1. K. Kalashnikov et al., Bifluxon: Fluxon-Parity-Protected Superconducting Qubit, arXiv:1910.03769.