Quantum sensing composite excitations in an anisotropic ferromagnet via a qubit

This paper investigates whether a qubit can act as a quantum sensor to resolve the internal, non-classical composition of squeezed-magnon excitations in an anisotropic ferromagnet. While previous work established that qubits can sense the ground state of such magnets, this study extends the theoretical framework to characterize excited states and their arbitrary superpositions.

The researchers model an anisotropic ferromagnet where spin non-conserving interactions naturally generate squeezed magnons—quasiparticles that are themselves quantum superpositions of multiple Fock magnon number states. They employ a dispersive interaction between these magnons and a superconducting qubit. By driving the qubit with an external microwave signal and performing spectroscopy, they map the qubit's steady-state population as a function of drive frequency. The resulting peaks in the spectrum correspond to transitions between different squeezed-magnon levels, effectively acting as a fingerprint of the underlying quantum superposition.

The study confirms that the qubit spectroscopy naturally unravels the superpositions within quantized squeezed-magnon number states. Each state produces a distinct set of spectral peaks, with peak heights proportional to the occupation probabilities of the constituent number states. The authors also analyze the generation and sensing of coherent states of squeezed magnons, demonstrating that the protocol remains viable but faces challenges from spectral crowding as the number of possible transitions increases.

This work provides the theoretical design equations necessary for using high-quality qubits to probe the composite nature of spin excitations. By enabling the readout of internal quantum information within quasiparticles, this sensing protocol offers a potential pathway for detecting exotic excitations, such as fractionalized states in quantum spin liquids, and contributes to the broader goal of integrating magnonic systems into quantum computing architectures.