This paper analyzes fundamental physical constraints on magnon-based quantum memory systems. We examine collective-mode behavior and magnetohydrodynamic (MHD) limits that govern the scalability of photon-magnon quantum state transfer. Key findings: - Spin wave propagation limits spatial addressing resolution - Magnetic Reynolds number R_m ≈ 10⁻¹² confirms diffusion-dominated regime - Maximum achievable channels: N_max ~ 10³-10⁴ via frequency-based addressing - Channel crosstalk analysis provides design guidelines This work complements the theoretical framework (Paper I) and experimental proposal (Paper II) in a three-paper series on magnetic quantum memory. Keywords: quantum memory, magnons, spin waves, magnetohydrodynamics, photon-magnon coupling
This paper analyzes fundamental physical constraints on magnon-based quantum memory systems. We examine collective-mode behavior and magnetohydrodynamic (MHD) limits that govern the scalability of photon-magnon quantum state transfer. Key findings: - Spin wave propagation limits spatial addressing resolution - Magnetic Reynolds number R_m ≈ 10⁻¹² confirms diffusion-dominated regime - Maximum achievable channels: N_max ~ 10³-10⁴ via frequency-based addressing - Channel crosstalk analysis provides design guidelines This work complements the theoretical framework (Paper I) and experimental proposal (Paper II) in a three-paper series on magnetic quantum memory. Keywords: quantum memory, magnons, spin waves, magnetohydrodynamics, photon-magnon coupling
We propose a theoretical framework for quantum data storage based on magnon modes in magnetic media, with frequency-based addressing implemented through quantum superposition states. Unlike conventional memory architectures that rely on fixed spatial positions, the proposed approach encodes logical addresses in mode structure, reducing dependence on absolute spatial coordinates. We analyze the system within a mode-resolved Hamiltonian formalism and show that the number of independently addressable channels is fundamentally limited by spectral linewidth (Γ) and available bandwidth. Coherent signal accumulation exhibits an ideal N² scaling in the fully phase-locked limit, though practical implementations are expected to be linewidth- and decoherence-limited. Photon–magnon coupling provides a physically grounded mechanism for state transfer between optical and magnetic degrees of freedom. Additionally, recently reported p-wave magnetic structures in NiI₂ may offer a potential platform for exploring mode-defined frequency channels, although coherent storage in such materials remains experimentally unverified. This work defines quantitative operating limits and proposes falsifiable criteria for experimental validation. Keywords: quantum memory, magnon modes, linewidth-limited addressing, photon–magnon coupling, mode-based storage
We propose a minimal experimental test to validate photon-magnon quantum state transfer for memory applications. Using established YIG-cavity technology, we demonstrate single-channel quantum addressing with one bit of information. Success criteria: coherence > 1 microsecond, fidelity > 50%. Estimated cost: $50-100k with existing infrastructure. This proposal is a companion to: "Towards Magnetic Field Quantum Memory" (Glamngiw, 2026), DOI: 10.5281/zenodo.18451271 Keywords: photon-magnon coupling, quantum memory, experimental proposal, YIG cavity, coherenc
The addressability of parallel spatially multimode quantum memory for light allows one to control independent collective spin waves within the same cold atomic ensemble. Generally speaking, there are transverse and longitudinal degrees of freedom of the memory that one can address by a proper choice of the pump (control) field spatial pattern. Here we concentrate on the mutual evolution and transformation of quantum states of the longitudinal modes of collective spin coherence in the cavity-based memory scheme. We assume that these modes are coherently controlled by the pump waves of the on-demand transverse profile, that is, by the superpositions of waves propagating in the directions close to orthogonal to the cavity axis. By the write-in, this allows one to couple a time sequence of the incoming quantized signals to a given set of superpositions of orthogonal spin waves. By the readout, one can retrieve quantum states of the collective spin waves that are controllable superpositions of the initial ones and are coupled on demand to the output signal sequence. In a general case, the memory is able to operate as a controllable delayed multi-port beamsplitter, capable of transformation of the delays, the durations and time shapes of signals in the sequence. We elaborate the theory of such light-matter interface for the spatially multivariate cavity-based off-resonant Raman-type quantum memory. Since, in order to speed up the manipulation of complex signals in multivariate memories, it might be of interest to store relatively short light pulses of a given time shape, we also address some issues of the cavity-based memory operation beyond the bad cavity limit.
Magnons, as quasiparticles arising from spin wave excitations in magnetic materials, have demonstrated significant application potential in quantum information technology, spintronics, and microwave engineering in recent years. The cavity magnon optomechanical system, serving as a key platform for investigating magneto-optical interactions, has advanced the exploration of nonlinear dynamical behaviors and the innovative design of quantum devices through strong coupling between magnons, photons, and phonons. However, traditional single-cavity systems face limitations in terms of tunability, long-range interactions, and nonlinear enhancement, making them insufficient for complex quantum control requirements. In recent years, dual-cavity systems have become a research hotspot due to their multidimensional control capabilities achieved through inter-cavity coupling, such as photon mode splitting and enhanced nonlinear Kerr effects. Meanwhile, semiconductor quantum dots, provide a novel pathway for regulating magnon dynamics due to their tunable nonlinear response characteristics. In this work, we construct a novel coupled quantum system by integrating quantum dots and a dual-cavity architecture, and investigate the bistable phenomena under both forward and backward driving field inputs. By comparing the third-order nonlinear equations governing magnon populations in the two scenarios, we derive the impedance matching condition. When this condition is satisfied, the magnon responses induced by forward driving field and backward driving field are identical. Conversely, under impedance mismatch, the magnon responses exhibit different behaviors. Specifically, when the impedance matching condition is violated, the dual-cavity magnon optomechanical system incorporating three-level quantum dot molecules exhibits a lower bistability threshold than its counterpart without quantum dots. This allows for a transition from low steady state to high steady state while reducing the driving field strength, thereby achieving switching functionality at lower input power. Furthermore, we establish a multiparameter cooperative control model, revealing a three-dimensional parameter space formed by tunneling coupling, cavity-quantum dot coupling, and inter-cavity coupling. By adjusting these coupling strengths, the bistability threshold and hysteresis loop width can be effectively controlled, thereby modulating the driving field intensity required for bistability. This system is expected to experimentally observe the magnonic bistability through the vector network analyzer-based detection of abrupt changes in transmission or absorption windows in reflection spectra. Such capabilities can advance data signal transmission, switching devices, and memory technologies, and has the potential to serve as components of large-scale quantum information processing units. Additionally, this research may find important applications in the field of magnetic spintronics.
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.
Magnons, the quanta of spin waves, have significant potential for use in modern technologies, especially when strongly coupled to another mode for read-out and control. However, while magnons strongly interact with microwave photons via the magnetic-dipole interaction to form hybrid cavity-magnon polariton modes, the weak magnetostrictive magnon-phonon interaction, together with large polariton linewidths dominated by magnon dissipation, has so far restricted magnonic-spheres to the weak-coupling regime. The material-limited magnon dissipation rate in particular has been regarded as an unavoidable limitation in these systems. Here, we surpass this long-standing limitation by implementing an active microwave feedback loop to suppress the linewidth of cavity-magnon polaritons and strongly suppress their effective decay rate below the magnon-limited linewidth, thereby enhancing the polariton-phonon cooperativity from C=1 to C=150. As a key milestone, we achieve normal-mode splitting between a cavity-magnon polariton and a mechanical mode, providing direct evidence of three-mode hybridization among photons, magnons, and phonons. Our results establish feedback as a general route to accessing strong-coupling regimes in systems previously thought to be limited by material properties and hence open new opportunities for coherent control in hybrid quantum systems.