Flatsonium: Charge and flux insensitive tunable superconducting qubit

Superconducting qubits with in-situ tunable properties are important capabilities for constructing quantum computer. But, tunability often comes at the expense of increased noise sensitivity for the qubits. Here, we propose a flux-tunable superconducting qubit that minimizes the dephasing due to the global flux-noise by engineering controllable „flux sweet spots“ at frequencies of interest. This is realized by using SQUID with asymmetric junctions shunted by an superconductor formed from array of Josephson junctions. When the main contribution to the magnetic flux noise comes from the global fluctuations of the magnetic field, it is possible to achieve several orders of magnitude improvement in dephasing time. The proposed qubit can be used to realize fast, high-fidelity two-qubit gates in large scale quantum processors, a key ingredient for implementing fault-tolerant quantum computers.