Bifluxon: Fluxon-Parity-Protected Superconducting Qubit

Understand

We have developed and characterized a symmetry-protected superconducting qubit that offers simultaneous exponential suppression of energy decay from charge and flux noise, and dephasing from flux noise.

• The qubit consists of a Cooper-pair box (CPB) shunted by a superinductor, thus 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 $\cos \phi / 2$ 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 phase space, thereby ensuring the protection against energy decay.