Flip chip implementation for generalized flux qubits

Superconducting flux qubits are a versatile and promising platform to implement coherent and tunable qubits with high anharmonicity. In this work, we investigate a generalized flux qubit consisting of a single Josephson junction (JJ) shunted by a capacitance and a granular aluminum inductor. When biased at the flux degeneracy point, the potential landscape can be widely engineered by exploring the parameter space of the flux qubit, which includes the loop inductance, the Josephson energy of the JJ and the total capacitance across the latter. We demonstrate a high engineerability of the qubit frequency, yielding flux qubits in the range of 150 MHz to 7.6 GHz. Dispersive readout of the qubit state is performed via an embedded harmonic mode that is inductively coupled through an asymmetry of the qubit loop. The readout mode is capacitively coupled to a control chip, which is used to excite, read out and flux bias the qubit. This flip chip approach allows very well isolated qubits to be tested in a modular architecture and enables coupling to two distinct coupler chips, effectively creating a unit cell that can be scaled up to an array of coupled qubits.