Tunable Quantum Dynamics in a Disordered Magnet

Quantum memories depend on minimally-interacting, localized qubits. One approach is via localized many-body excitations, such as those found in the dilute Ising magnet, LiHo_0.045Y_0.955F₄. Here clusters of several hundred spins bind together and can be resonantly excited. We use a non-linear pump-probe magnetic susceptibility technique, combining a strong longitudinal ac magnetic pump field to drive the system out of the linear regime and a weak probe field, to study the localized excitations. The pump field excites the clusters into resonance, the detailed shape of which provides insight into the lifetimes and the coherent quantum interference between different excitation channels. We tune the dynamics of the quantum degrees of freedom by sweeping the quantum mixing parameter through zero via the amplitude of the ac pump as well as a static external transverse field. The zero-crossing is associated with a dissipationless response at the drive frequency. The identification of points where localized degrees of freedom are minimally mixed with their environment in a dense and disordered, dipolar-coupled spin system implies control over the bath coupling of qubits emerging from strongly interacting many-body systems.