Bifluxon Qubit with Reduce Sensitivity to Charge Noise*

The Bifluxon qubit is a symmetry-protected qubit whose states are encoded in the parity of fluxons in a superconducting loop [1]. Protection of fluxon-parity in the Bifluxon qubit is preserved through Aharonov-Casher destructive interference of single-fluxon tunneling events. Charge noise detrimentally affects parity protection by increasing the probability of single-fluxon tunneling. With the aim of protection improvement, we propose a modified design of the Bifluxon qubit which consists of two parallel split-Cooper-pair boxes shunted by a superinductor arranged in a superconducting loop. We show that the addition of a second Cooper-pair box suppresses the effect of charge noise and results in improved protection against energy relaxation. We will present preliminary results on the implementation of this Bifluxon qubit with the gate control of induced charges on two superconducting islands; the data demonstrate exceptional charge stability and suppressed quasi-particle poisoning due to the gap engineering in circuit elements. The spectroscopy data will be compared with numerical diagonalization of circuit Hamiltonian.

[1] K. Kalashnikov et. al. PRX Quantum 1, 010307 (2020).