Calibration of a Double-Loop Galvanic Coupler between Tunable Fluxoniums

Abstract

Analog quantum computing requires independent, in-situ control over every term of the implemented Hamiltonian, including the qubit-qubit couplings. One issue of commonly used galvanic couplers between fluxoniums is the couplerinduced nonlinear crosstalk. The presence of a current circulating in the coupler main loop (the z-loop) generates a flux that shifts, nonlinearly, the degeneracy point of the fluxonium. For precise qubit control, such a crosstalk needs to be compensated. While for a single qubit this can be done accurately, this becomes a highly non-trivial task when multiple qubits are involved. Here we perform preliminary measurements on a double-loop galvanic coupler. Besides the z-loop, we include a DC SQUID as an x-loop. The DC SQUID enables us to tune the coupler susceptibility while keeping the net circulating current in the coupler at zero, therefore eliminating the nonlinear crosstalk. Furthermore, we show that the coupler susceptibility can be tuned through the ferromagnetic, zero and antiferromagnetic coupling regimes using only the SQUID flux, keeping the coupler at its sweet spot and therefore leaving the qubit sweet spots unperturbed. Finally, we measure and fit the spectrum of each qubit with the coupling switched off in order to extract the qubits’ energy parameters, and report a preliminary measurement of the two-qubit spectrum with the coupling turned on