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arXiv:2507.19066 · Physical review. B./Physical review. B

Quantum sensing magnonic number states using a bosonic mode as the probe

Anonymous, Sonu Verma, Mathias Weiler, Akashdeep Kamra

Published Jul 22, 2026• 0 citations
arXivHTMLPDF

Abstract

Sensing number states of a magnonic mode has been accomplished using a superconducting qubit by realizing an effective dispersive interaction between the two systems. Here, we theoretically demonstrate that a seemingly classical bosonic mode can be utilized as a probe for resolving the number states of a magnon mode, while outperforming a qubit in various regards as the sensor. Considering another magnon mode in an antiferromagnet as the probe mode, we delineate the required dispersive coupling emerging directly from antiferromagnetic exchange interaction. When a phonon is used as the probe mode, we derive the effective dispersive coupling emerging from the lowest-order nonlinear magnon-phonon interactions. Our two considered examples provide the general design principles for identifying and utilizing a bosonic probe mode for sensing magnonic superpositions in a physical platform of interest.

DOI
10.1103/f8jq-1cxh

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N\'eel ordered antiferromagnets exhibit two-mode squeezing such that their ground state is a nonclassical superposition of magnon Fock states. Here we theoretically demonstrate that antiferromagnets can couple to spin qubits via direct dispersive interaction stemming from, e.g., interfacial exchange. We demonstrate that this kind of coupling induces a magnon number-dependent level splitting of the excited state resulting in multiple system excitation energies. This series of level splittings manifests itself as nontrivial excitation peaks in qubit spectroscopy thereby revealing the underlying nonclassical magnon composition of the antiferromagnetic quantum state. By appropriately choosing the drive or excitation energy, the magnonic state can be controlled via the qubit, suggesting that Fock states of magnon pairs can be generated deterministically. This enables achieving states useful for quantum computing and quantum information science protocols.
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Ordered magnets harbor intrinsically squeezed ground states and magnonic excitations characterized by entanglement between spins and nonclassical magnon number composition. A pathway to detecting the superpositions of noneigenmode magnon number states underlying these nonclassical magnetic ground states has recently been demonstrated by utilizing a qubit coupled to the magnon mode via a direct dispersive interaction. Here, we theoretically develop this qubit spectroscopy further delineating the capabilities and limitations of this qubit spectroscopy for sensing the quantum superpositions that underlie the excited states. We demonstrate that the spectroscopy lends itself naturally to unraveling the superpositions that underlie the various quantized squeezed-magnon number states. However, excited states comprising superpositions of multiple squeezed Fock states become increasingly hard due to the large number of possible transitions, and resulting peaks, in the qubit spectroscopy thereby requiring qubits with narrower linewidths. Along the same lines, we theoretically demonstrate the qubit spectroscopy of a low amplitude coherent squeezed-magnon state analyzing the tradeoff between frequency crowding due to multiple transitions and peak linewidths. Our work lays the groundwork and design equations for deploying high-quality qubits towards sensing the composite nature of spin excitations in magnetic systems.
Hybrid quantum devices expand the tools and techniques available for quantum sensing in various fields. Here, we experimentally demonstrate quantum sensing of a steady-state magnon population in a magnetostatic mode of a ferrimagnetic crystal. Dispersively coupling the magnetostatic mode to a superconducting qubit allows for the detection of magnons using Ramsey interferometry with a sensitivity on the order of 10^{-3} magnons/sqrt[Hz]. The protocol is based on dissipation as dephasing via fluctuations in the magnetostatic mode reduces the qubit coherence proportionally to the number of magnons.
We investigate the magnon blockade effect in a quantum magnonic system operating in the strong dispersive regime, where a superconducting qubit interacts dispersively with a magnonic mode in a yttrium-iron-garnet sphere. By solving the quantum master equation, we demonstrate that the magnon blockade, characterized by the second-order correlation function g(2)(0)→0.04 , emerges under optimal dispersive coupling and driving detuning. The mechanism is attributed to suppressed two-magnon transitions as a result of qubit-induced anharmonicity. Notably, our study identifies the critical role of dispersive interaction strength and environmental temperature, showing that magnon blockade remains observable under experimentally achievable cryogenic conditions. This work extends the magnon blockade effect into the dispersive regime, offering a robust platform for single-magnon manipulation and advancing applications in quantum sensing and information processing.
We investigate the magnon blockade effect in a quantum magnonic system operating in the strong dispersive regime, where a superconducting qubit interacts dispersively with a magnonic mode in a yttrium-iron-garnet sphere.By solving the quantum master equation, we demonstrate that the magnon blockade, characterized by the second-order correlation function g(2)(0)→0.04g^{(2)}(0) \rightarrow 0.04g(2)(0)→0.04, emerges under optimal dispersive coupling and driving detuning.The mechanism is attributed to suppressed two-magnon transitions as a result of qubit-induced anharmonicity. Notably, our study identifies the critical role of dispersive interaction strength and environmental temperature, showing that magnon blockade remains observable under experimentally achievable cryogenic conditions.This work extends the magnon blockade effect into the dispersive regime, offering a robust platform for single-magnon manipulation and advancing applications in quantum sensing and information processing.