Towards a 4-local coupler for superconducting flux qubits – Part 1: Quantum landscape engineering of the ground state*

The response of a superconducting circuit's ground state energy to external flux is a versatile resource for qubit architecture design. For instance, it can be engineered to mediate interactions between flux qubits or assist in qubit readout. Here we propose a methodology for adding higher-order polynomial terms into the flux dependence of the circuit ground state by strongly coupling a series of rf SQUIDs, thereby landscaping the spectrum at the fluxon level. The operation principle of such a circuit module is demonstrated experimentally. We find that it has the desired spectral property and does not adversely affect the lifetime of a connected flux qubit. It is thus suitable as a quantum coupling or readout module. In addition, we numerically show that the spectral landscaping concept can be scaled by systematically adding circuit loops to the module.