Exotic quantum vacuum phenomena are predicted in cavity quantum
electrodynamics (QED) systems with ultrastrong light-matter interactions. Their
ground states are predicted to be vacuum squeezed states with suppressed
quantum fluctuations. The source of such phenomena are antiresonant terms in
the Hamiltonian, yet antiresonant interactions are typically negligible
compared to resonant interactions in light-matter systems. We report an unusual
coupled matter-matter system of magnons that can simulate a unique cavity QED
Hamiltonian with coupling strengths that are easily tunable into the
ultrastrong coupling regime and with dominant antiresonant terms. We found a
novel regime where vacuum Bloch-Siegert shifts, the hallmark of antiresonant
interactions, greatly exceed analogous frequency shifts from resonant
interactions. Further, we theoretically explored the system's ground state and
calculated up to 5.9 dB of quantum fluctuation suppression. These observations
demonstrate that magnonic systems provide an ideal platform for simulating
exotic quantum vacuum phenomena predicted in ultrastrongly coupled light-matter
systems.