Bifluxon: Fluxon-Parity-Protected Superconducting Qubit
Konstantin Kalashnikov, Wen Ting Hsieh, Wenyuan Zhang, Wen-Sen Lu, Plamen Kamenov, Agustin Di Paolo, Alexandre Blais, Michael E. Gershenson, Matthew Bell
Abstract
We develop and characterize a symmetry-protected superconducting qubit that offers simultaneous exponential suppression of energy decay from charge and flux noises, and dephasing from flux noise. The qubit consists of a Cooper-pair box (CPB) shunted by a superinductor, forming a superconducting loop. Provided the offset charge on the CPB island is an odd number of electrons, the qubit potential corresponds to that of a cos (/2) Josephson element, preserving the parity of fluxons in the loop via Aharonov-Casher interference. In this regime, the logical-state wavefunctions reside in disjoint regions of Hilbert space, thereby ensuring protection against energy decay. By switching the protection on, we observe a tenfold increase of the decay time, reaching up to 100 s. Though the qubit is sensitive to charge noise, the sensitivity is much reduced in comparison with the charge qubit, and the charge-noise-induced dephasing time of the current device exceeds 1 s. Implementation of full dephasing protection can be achieved in the next-generation devices by combining several cos(/2) Josephson elements in a small array.