Nonlocal Magnonic Cat States in Hybrid Magnon-Qubit Architectures

This paper investigates a method for generating nonlocal quantum entanglement between magnonic modes—collective spin excitations in magnetic materials—housed in spatially separated hybrid architectures. The researchers propose an entanglement-swapping protocol that utilizes superconducting transmon qubits coupled to yttrium iron garnet (YIG) spheres. By first entangling each magnon with its local transmon qubit and subsequently performing a joint Bell-state measurement on the two qubits, the entanglement is transferred to the remote magnons, resulting in a bipartite magnonic cat state.

The study demonstrates that the proposed scheme can effectively generate four distinct types of bipartite magnonic cat states. The authors model the system using a Lindblad master equation to account for realistic dissipation channels, including magnon decay and qubit relaxation. Their numerical simulations show that while the fidelity of the generated states is high under ideal conditions, it is susceptible to the decoherence of the individual components. Specifically, the fidelity of the resulting cat states is shown to be sensitive to the timing of the measurement and the specific decay rates of the magnons and qubits.

To verify the presence of these nonclassical states, the authors propose using joint quantum state tomography based on Wigner function reconstruction. By performing joint displaced parity measurements, researchers can map the quasi-probability distribution of the bipartite system, which is essential for confirming the presence of intermode quantum correlations. This work provides a theoretical framework for long-distance quantum communication and distributed quantum computation, leveraging the long lifetimes and high spin densities of magnonic systems in hybrid quantum networks.