Generating magnon Bell states via parity measurement

We propose a scheme to entangle two magnon modes based on parity measurements. In particular, we consider a system that two yttrium-iron-garnet spheres are coupled to a V-type superconducting qutrit through the indirect interactions mediated by cavity modes. An effective parity-measurement operator that can project the two macroscopic spin systems to the desired subspace emerges when the ancillary qutrit is projected onto the ground state. Consequently, conventional and multi-excitation magnon Bell states can be generated from any separable states with a nonvanishing population in the desired subspace. The target state can be distilled with a near-to-unit fidelity only by several rounds of measurements and can be stabilized in the presence of the measurement imperfection and environmental decoherence. In addition, a single-shot version of our scheme is obtained by shaping the detuning between magnon and qutrit in the time domain. Our scheme that does not rely on any nonlinear Hamiltonian brings insights into the entangled-state generation in massive ferrimagnetic materials via quantum measurements.