  Strong interactions between spins in many-body solid-state quantum system is
a crucial resource for exploring and applying non-classical states. In
particular, electronic spins associated with defects in diamond system are a
leading platform for the study of collective quantum phenomena and for quantum
technology applications. While such solid-state quantum defect systems have the
advantage of scalability and operation under ambient conditions, they face the
key challenge of controlling interactions between the defects spins, since the
defects are spatially fixed inside the host lattice with relative positions
that cannot be well controlled during fabrication. In this work, we present a
dressed-state approach to control the effective dipolar coupling between
solid-state spins; and then demonstrate this scheme experimentally using two
strongly-coupled nitrogen vacancy (NV) centers in diamond. Including Rabi
driving terms between the m$_s$ = 0 and $\pm$1 states in the NV spin
Hamiltonian allows us to turn on and off or tune the effective dipolar coupling
between two NV spins. Through Ramsey spectroscopy, we detect the change of the
effective dipolar field generated by the control NV spin prepared in different
dressed states. To observe the change of interaction dynamics, we then deploy
spin-lock-based polarization transfer measurements via a Hartmann-Hahn matching
condition between two NV spins in different dressed states. We perform
simulations that indicate the promise for this robust scheme to control the
distribution of interaction strengths in strongly-interacting spin systems,
including interaction strength homogenization in a spin ensemble, which can be
a valuable tool for studying non-equilibrium quantum phases and generating high
fidelity multi-spin correlated states for quantum-enhanced sensing.
