  We investigate the ultrastrong tunable coupler for coupling of
superconducting resonators. Obtained coupling constant exceeds 1 GHz, and the
wide range tunability is achieved both antiferromagnetics and ferromagnetics
from $-1086$ MHz to 604 MHz. The ultrastrong coupler is composed of rf-SQUID
and dc-SQUID as tunable junctions, which connected to resonators via shared
aluminum thin film meander lines enabling such a huge coupling constant. The
spectrum of the coupler obviously shows the breaking of the rotating wave
approximation, and our circuit model treating the Josephson junction as a
tunable inductance reproduces the experimental results well. The ultrastrong
coupler is expected to be utilized in quantum annealing circuits and/or NISQ
devices with dense connections between qubits.
