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Physical Review A

Absorption of magnons in dispersively coupled hybrid quantum systems

Zeng‐Xing Liu, Hao Xiong, Muying Wu, Yong-qing Li

Published Jun 2, 2021• 24 citations
Publisher

Abstract

Phenomena involved in magnons have attracted extensive attention for several decades because of their challenging physical properties, powerful capabilities, and potential applications in signal processing in the microwave frequency range. In this work study we study the steady-state dynamic processes of a hybrid quantum system based on magnonics, where the magnon mode couples dispersively to a superconducting qubit. We find that the dispersive coupling between the qubit and magnons causes the system to be bistable and we discuss in detail an interesting phenomenon of dispersive-coupling-induced absorption of magnons. Furthermore, a feasible scheme to measure the qubit-magnon dispersive shift by employing the absorption curves of the probe field is proposed. The results presented here are of great significance to the investigation of the steady-state dynamic behaviors of quantum magnonics in a strong-dispersive regime and may provide a theoretical reference for the realization of quantum sensing of magnons.

DOI
10.1103/physreva.103.063702

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Phenomena involved in magnons have attracted extensive attention for several decades because of their challenging physical properties, powerful capabilities, and potential applications in signal processing in the microwave frequency range. In this work study we study the steady-state dynamic processes of a hybrid quantum system based on magnonics, where the magnon mode couples dispersively to a superconducting qubit. We find that the dispersive coupling between the qubit and magnons causes the system to be bistable and we discuss in detail an interesting phenomenon of dispersive-coupling-induced absorption of magnons. Furthermore, a feasible scheme to measure the qubit-magnon dispersive shift by employing the absorption curves of the probe field is proposed. The results presented here are of great significance to the investigation of the steady-state dynamic behaviors of quantum magnonics in a strong-dispersive regime and may provide a theoretical reference for the realization of quantum sensing of magnons.
We theoretically study the non-linear behavior of a hybrid quantum magnomechanical system, where the system consisting of a superconducting qubit couples dispersively to a magnon–phonon mode. The magnetic excitations (magnons) can interact with the mechanical vibrations of the system via a magnomechanical interaction, which results in the formation of magnomechanically induced transparency in the output of the system. We investigate the system’s switching behavior in the dressed qubit anharmonicity of the system and shows the sharp bistable frequency switching of the magnon–excitons. The proposed model also shows a realistic scheme to measure the magnon-qubit dispersive shift in the absorption spectra of the probe field. The absorption spectrum induced by the dispersive coupling shows a series of asymmetric double Fano line shapes whose positions can be determined by the magnon-qubit driving field. Our results will provide a theoretical approach to understand the complex and dynamic non-linear interactions and may come up with great significance in the realization of quantum sensing applications of magnonic systems.
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.
Magnons, the quanta of collective spin excitations in magnetic materials, may enable functionalities, such as nonreciprocity and transduction in hybrid quantum devices. To assess the potential of such applications, it is necessary to understand magnon dynamics beyond the simple harmonic oscillator regime, where theory predicts effects like population-dependent damping and quantum fluctuations in the form of magnon shot noise. Probing these phenomena requires sensors with high sensitivity and the ability to resolve magnon properties across different excitation regimes. Here, we demonstrate accurate and sensitive detection of magnon population and decay over a wide range of occupation numbers. We use a superconducting qubit to probe magnons in a ferrimagnet over approximately 2000 excitations. Using qubit control and parametrically induced qubit-magnon interactions, we demonstrate few-excitation sensitive detection of magnons with a dynamic range of approximately 30 dB, and are able to accurately resolve their decay with few-ns sensitivity. These capabilities offer a powerful and practical technique for probing magnon dynamics in or beyond the linear regime over a wide range of excitations.
In general, a magnon does not interact directly with an atom. Here we present a scheme to generate coherent coupling between two magnons and a superconducting artificial atom via exchange of virtual photons in dispersive cavities. By probing into the induced magnon-atom interactions, we identify the two-magnon process, which causes almost perfect two-mode squeezing and entanglement of the two magnons for specific conditions. The quantum entanglement of the magnon modes in two massive ferrimagnets survives at the steady state and represents genuinely macroscopic quantum state. Our work offers an interesting alternative in designing experiments for generating the coherent coupling between the magnons and the atom. It finds broad applications in the investigation of macroscopic quantum effects and the preparation of quantum devices.
Rigidity of an ordered phase in condensed matter results in collective excitation modes spatially extending in macroscopic dimensions. Magnon is a quantum of an elementary excitation in the ordered spin system, such as ferromagnet. Being low dissipative, dynamics of magnons in ferromagnetic insulators has been extensively studied and widely applied for decades in the contexts of ferromagnetic resonance, and more recently of Bose-Einstein condensation as well as spintronics. Moreover, towards hybrid systems for quantum memories and transducers, coupling of magnons and microwave photons in a resonator have been investigated. However, quantum-state manipulation at the single-magnon level has remained elusive because of the lack of anharmonic element in the system. Here we demonstrate coherent coupling between a magnon excitation in a millimetre-sized ferromagnetic sphere and a superconducting qubit, where the interaction is mediated by the virtual photon excitation in a microwave cavity. We obtain the coupling strength far exceeding the damping rates, thus bringing the hybrid system into the strong coupling regime. Furthermore, we find a tunable magnon-qubit coupling scheme utilising a parametric drive with a microwave. Our approach provides a versatile tool for quantum control and measurement of the magnon excitations and thus opens a new discipline of quantum magnonics.
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.