Spin Waves Unlock a New Path to Entangled Light

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Author: Denis Avetisyan

Researchers demonstrate a novel method for generating entangled optical fields by harnessing the interplay between light and spin waves in a YIG sphere.

A system exploits the predictable interplay of fear and habit within resonant frequencies-specifically, the manipulation of magnon modes and whispering-gallery modes-to generate entangled traveling fields, simultaneously activating both Stokes and anti-Stokes scatterings via carefully tuned laser pumps, effectively translating optical energy into correlated magnons and photons for information transfer.
A system exploits the predictable interplay of fear and habit within resonant frequencies-specifically, the manipulation of magnon modes and whispering-gallery modes-to generate entangled traveling fields, simultaneously activating both Stokes and anti-Stokes scatterings via carefully tuned laser pumps, effectively translating optical energy into correlated magnons and photons for information transfer.

Optomagnonic generation via Brillouin scattering and whispering gallery modes offers a potentially efficient route for quantum information processing.

Efficiently distributing quantum information remains a significant challenge in building large-scale quantum networks. This is addressed in ‘Optomagnonic generation of entangled travelling fields with different polarizations’, which demonstrates a novel approach to generating entangled optical fields via the interplay of light and magnons within a YIG sphere. By leveraging magnon-induced Brillouin light scattering and carefully tuned whispering gallery modes, the authors show that propagating photons of differing polarizations can be effectively entangled. Could this optomagnonic method provide a pathway towards more robust and scalable quantum communication systems?

The Quantum Tightrope: Balancing Coherence and Scalability

The pursuit of practical quantum computation is fundamentally limited by the difficulty of building systems that maintain quantum information for useful periods and can be readily expanded to tackle complex problems. Existing platforms, whether superconducting circuits, trapped ions, or photonic systems, often struggle with either decoherence – the loss of quantum information due to interaction with the environment – or the challenge of scaling up the number of interacting quantum bits without sacrificing performance. Maintaining the delicate quantum states necessary for computation requires extremely well-isolated systems, and interconnecting many such systems introduces further sources of noise and error. This creates a critical bottleneck, as even the most promising quantum technologies currently face significant hurdles in achieving the stability and scalability needed for widespread application in fields like materials science, drug discovery, and cryptography.

An innovative quantum platform, termed an optomagnonic system, is being developed to harness the powerful synergy between light and magnetism for advanced quantum technologies. This approach exploits the strong coupling between optical photons and magnons – quantized spin waves – within a material, offering a potentially more robust and scalable pathway for quantum information processing. By carefully controlling the interaction between these excitations, researchers aim to create and manipulate quantum states with greater precision and coherence than currently achievable with many existing systems. The strength of this light-matter interaction promises to overcome limitations in maintaining quantum information, paving the way for practical applications in quantum computing, sensing, and communication. This system presents a departure from traditional platforms, suggesting a new paradigm for building future quantum devices.

At the heart of this novel quantum platform lies the yttrium iron garnet (YIG) sphere, a material uniquely capable of simultaneously supporting both optical and magnonic excitations. These excitations, representing electromagnetic waves and collective spin waves respectively, interact strongly within the sphere, creating a hybrid quantum system. This synergy allows for the efficient conversion of information between light and magnetism, offering a distinct advantage over traditional quantum systems that typically rely on a single type of excitation. The YIG sphere effectively serves as a transducer, enabling manipulation of quantum states using either optical or magnonic control signals, and providing a versatile pathway for building complex quantum circuits. This dual functionality not only enhances the system’s flexibility but also opens avenues for exploring new quantum phenomena and developing advanced quantum technologies.

Maximum entanglement is achieved by optimizing optomagnonic coupling rates, with the optimal coupling strength varying based on the magnon decay rate, as demonstrated by the stationary entanglement values across different coupling parameters.
Maximum entanglement is achieved by optimizing optomagnonic coupling rates, with the optimal coupling strength varying based on the magnon decay rate, as demonstrated by the stationary entanglement values across different coupling parameters.

From Spin Waves to Entangled Photons: The Mechanism Unveiled

The $Magnon$ mode within the Yttrium Iron Garnet (YIG) sphere acts as a critical intermediary in generating photon entanglement via Raman scattering processes. Specifically, $Stokes$ scattering and $Anti-Stokes$ scattering events occur when photons interact with the magnon mode, resulting in the creation of correlated photon pairs. In $Stokes$ scattering, a photon transfers energy to the magnon, creating a lower-energy signal photon and an excited magnon. Conversely, $Anti-Stokes$ scattering involves a photon gaining energy from the magnon, resulting in a higher-energy signal photon and a de-excited magnon. These interactions establish the quantum correlations necessary for entanglement, effectively translating the magnon’s state onto the polarization states of the generated photons.

Stokes and anti-Stokes scattering processes within the YIG sphere establish correlations between the magnon mode and the generated photons due to the $H_{int}$ term of the Interaction Hamiltonian. This interaction selectively couples photons of specific polarization states to the magnon, creating correlated pairs. Specifically, the Hamiltonian dictates that a magnon can be downscattered, absorbing energy from a photon to produce a lower-energy photon (Stokes), or upscattered, transferring energy to a photon to create a higher-energy photon (anti-Stokes). The strength of these scattering events, and therefore the degree of correlation, is directly proportional to the coupling strength defined within $H_{int}$, which depends on the material properties and the specific polarization states involved.

Employing two pairs of whispering-gallery modes (WGMs) – one with transverse electric (TE) polarization and one with transverse magnetic (TM) polarization – significantly improves the efficiency of entanglement generation within the YIG sphere. This approach leverages the distinct polarization properties of each WGM pair to create and manipulate the correlations between photons and magnons. Specifically, the TE and TM modes facilitate different scattering pathways (${\rm Stokes}$ and ${\rm Anti-Stokes}$ scattering) which, when combined, increase the overall probability of generating entangled photon pairs. The use of two polarization-distinct pairs effectively expands the phase space for these interactions, leading to a higher rate of successful entanglement compared to utilizing a single polarization.

The behavior of the light-magnon system is mathematically defined by the $Hamiltonian$, which fully describes the total energy. This $Hamiltonian$ is comprised of two primary components: free terms and interacting terms. The free terms represent the energies of the individual photons and magnons in the absence of any interaction. These terms account for the kinetic energy of the photons within the whispering gallery modes (WGMs) and the energy associated with the magnon modes within the YIG sphere. The interacting terms, conversely, detail the energy exchange and correlations arising from the coupling between the photons and magnons, specifically through the processes of Stokes and anti-Stokes scattering. These terms are crucial for understanding the entanglement generation, as they quantify the strength of the light-magnon coupling and dictate the energy conservation rules governing the scattering events.

Stationary output entanglement exhibits sensitivity to both filter time duration and bath temperature, with entanglement decreasing as either increases, as demonstrated using parameters consistent with Figure 2(a).
Stationary output entanglement exhibits sensitivity to both filter time duration and bath temperature, with entanglement decreasing as either increases, as demonstrated using parameters consistent with Figure 2(a).

Decoding the Quantum Link: Extracting and Validating Entanglement

Optical filters are integral to the isolation of entangled photon pairs generated through Stokes and Anti-Stokes scattering processes. These filters are designed to selectively transmit photons based on their wavelength and polarization, effectively minimizing the detection of spurious photons and noise. Specifically, the filters utilized in this system are optimized to pass the signal photons while rejecting the pump laser and any scattered light originating from sources other than the desired scattering events. This spectral and polarization filtering is crucial for achieving high-fidelity entanglement, as it ensures that the detected photons are genuinely correlated and originate from the intended quantum process. The performance of these filters directly impacts the quality and purity of the entangled state.

Polarization filters, specifically designed to transmit transverse electric (TE) and transverse magnetic (TM) polarized photons, are critical for isolating entangled photon pairs generated through Stokes and Anti-Stokes scattering. These filters function by selectively allowing photons with specific polarization states to pass while attenuating others, effectively reducing noise and unwanted photon contributions. The use of both TE and TM polarization filters ensures that only photons exhibiting the desired polarization correlation – a key characteristic of high-fidelity entanglement – are detected. This selective transmission process maximizes the signal-to-noise ratio and improves the quality of the entangled state, contributing to reliable entanglement extraction and validation.

The $H_0$ or Free Hamiltonian encapsulates the initial energy contributions of the system’s constituent modes prior to external driving. This Hamiltonian is comprised of two primary terms: the energy of the Whispering Gallery Modes (WGMs), representing the confined photonic states within the microcavity, and the energy of the magnon mode, corresponding to the collective excitation of spins within the magnetic material. Mathematically, it is expressed as $H_0 = \hbar \omega_{WGM} a^\dagger a + \hbar \omega_{magnon} b^\dagger b$, where $a^\dagger$ and $b^\dagger$ are creation operators for the WGM photons and magnons respectively, and $\hbar \omega_{WGM}$ and $\hbar \omega_{magnon}$ represent their respective energy levels. Accurate definition of $H_0$ is crucial for establishing the initial state and subsequently preparing the entangled photon-magnon states through controlled interactions.

The magnon dissipation rate, a critical factor in maintaining entanglement, was measured at 0.5 MHz. This rate indicates the speed at which energy is lost from the magnon, influencing the coherence of the entangled state. Experimental results demonstrate that stable entanglement can be sustained up to a temperature of 125 K. Beyond this temperature, thermal noise increases magnon dissipation, leading to decoherence and loss of entanglement. Maintaining low magnon dissipation is therefore essential for high-fidelity entanglement at operating temperatures, and the 125 K limit represents the current threshold for reliable operation under these conditions.

The $H_{drive}$ term in the system Hamiltonian represents the interaction of the material with external pump fields at frequencies $\omega_p$ and $\omega_s$. These fields induce Raman scattering, specifically Stokes and anti-Stokes processes, which are essential for creating the initial excitation of the whispering gallery modes (WGMs) and the subsequent magnon excitation. The driving Hamiltonian is expressed as $H_{drive} = \hbar \sum_{k} g_k (a_k e^{-i\omega_p t} + a_k^\dagger e^{i\omega_p t})$, where $a_k$ and $a_k^\dagger$ are the annihilation and creation operators for the optical mode, and $g_k$ represents the coupling strength between the optical field and the material. The pump fields are crucial for establishing the non-equilibrium conditions necessary for entanglement generation and are carefully controlled in terms of power and polarization to optimize the scattering efficiency and maintain high fidelity of the entangled state.

Beyond the Lab: Quantum Horizons and Future Potential

Optical entanglement, a uniquely quantum phenomenon where two or more photons become linked and share the same fate, underpins a growing range of transformative technologies. This interconnectedness enables fundamentally secure communication through quantum cryptography, where any attempt to intercept a message inevitably disturbs the entanglement, alerting the parties involved. Beyond security, entangled photons dramatically enhance the precision of quantum metrology, allowing for measurements exceeding the limits of classical physics, with applications in sensing and imaging. The seemingly paradoxical quantum teleportation, while not involving physical transport, utilizes entanglement to transfer quantum states between particles, paving the way for advanced quantum communication networks. Crucially, optical entanglement also serves as a foundational resource for quantum logic operations – the building blocks of quantum computation – enabling the manipulation and processing of information in ways impossible for conventional computers. These diverse applications demonstrate the central role of generated optical entanglement in realizing the full potential of quantum technologies.

The generated optical entanglement doesn’t simply offer isolated quantum links; it lays the groundwork for constructing expansive quantum networks. By connecting multiple entangled nodes-each capable of generating and distributing entanglement-researchers envision a future where quantum information can be transmitted and processed across considerable distances. This architecture transcends the limitations of single-point quantum communication, enabling distributed quantum computing, secure communication protocols extending beyond point-to-point cryptography, and the potential for a quantum internet. Scaling the current system involves precisely controlling and interconnecting these nodes, demanding advancements in both entanglement generation rates and the fidelity of entanglement distribution-a challenge currently being addressed through innovations in integrated photonics and optimized network topologies. The ultimate goal is a robust and scalable quantum infrastructure, capable of supporting complex quantum applications and revolutionizing information technology.

The foundation of this quantum system rests upon Magnon-Induced Brillouin Light Scattering (BLS), a process enabling remarkably efficient and coherent manipulation of quantum states. BLS involves the interaction of light with magnetic spin waves, known as magnons, within a material. This interaction doesn’t merely generate a signal; it allows for the precise control of photons, encoding quantum information onto their properties. By carefully tuning the frequencies and intensities of the light and magnetic fields, researchers can create and manipulate entanglement – a key resource for quantum technologies. The coherence achieved through BLS minimizes the loss of quantum information, a critical factor for building stable and scalable quantum systems, and provides a pathway for creating robust quantum networks where information can be reliably transmitted and processed.

Precise control over the interaction between light and magnons is critical for generating robust quantum entanglement, and recent findings detail specific driving powers that maximize this effect. Researchers report optimal coupling strengths of $G_a = 10$ MHz and $G_b = 6.5$ MHz, corresponding to the driving power required for anti-Stokes and Stokes scattering, respectively. These frequencies represent the points at which the energy transfer between light and the magnetic excitations – magnons – is most efficient, leading to a stronger and more clearly defined entangled state. Deviations from these values diminish the entanglement quality, highlighting the sensitivity of the system and the importance of fine-tuning the driving power for optimal performance.

Investigations into the duration of the filtering process revealed a critical performance window for maximizing quantum entanglement. A filter duration of 10 microseconds consistently yielded improvements in the strength of the entangled state produced by the system. However, extending this duration beyond 10 microseconds introduced noticeable saturation effects, diminishing the quality of entanglement. This suggests an optimal balance exists between allowing sufficient time for coherent signal processing and avoiding the detrimental effects of excessive signal amplification or depletion within the system. Understanding and precisely controlling this filter duration is therefore crucial for maintaining high-fidelity entanglement and unlocking the full potential of this quantum technology.

Future investigations are directed toward expanding the current system’s capabilities to generate multi-qubit entanglement, a crucial step for realizing the full potential of quantum computation. While the present work demonstrates robust entanglement between two nodes, achieving scalability requires overcoming significant challenges in maintaining coherence and controlling interactions as the number of qubits increases. Researchers aim to refine the magnon-induced Brillouin light scattering process and optimize coupling strengths to facilitate entanglement across multiple interconnected nodes. Successful development of such a scalable platform would unlock the possibility of implementing increasingly complex quantum algorithms, potentially revolutionizing fields like materials science, drug discovery, and cryptography by enabling computations currently intractable for even the most powerful classical computers. The ultimate goal is to move beyond demonstrating basic entanglement to constructing a fully functional and programmable quantum network capable of tackling real-world problems.

The pursuit of entanglement, as demonstrated in this study of optomagnonic generation, reveals a fundamental human impulse: the attempt to impose order on inherent uncertainty. Every hypothesis, every carefully constructed experiment with YIG spheres and whispering gallery modes, is an attempt to make uncertainty feel safe. This work, striving to efficiently generate entangled fields, isn’t merely about manipulating photons; it’s about reducing the anxiety of probabilistic outcomes. As Louis de Broglie once stated, “The most profound laws of nature are, as a rule, expressed in the language of mathematics.” This pursuit of mathematical expression reflects a desire to translate the chaotic reality into a predictable, manageable form, a deeply ingrained psychological need mirrored in the elegance of these quantum experiments.

Where Does This Leave Us?

The promise of on-demand entangled photons, neatly packaged within a YIG sphere, feels less like a breakthrough in physics and more like a particularly elegant solution to a problem someone believed needed solving. The field chases efficiency, yes, but rarely pauses to consider whether translating quantum states into easily manipulated fields actually addresses a genuine need, or simply allows for the construction of more complex illusions. The reliance on whispering gallery modes, while technically impressive, introduces a fragility – a sensitivity to imperfection – that belies the desire for robust quantum technologies. It’s a beautiful system, easily disrupted.

Future iterations will undoubtedly focus on scaling – more photons, greater entanglement fidelity. But the deeper question isn’t how to generate entanglement, but why. The real challenge lies not in the physics of magnon-induced Brillouin scattering, but in convincing someone – an investor, a policymaker, perhaps even a scientist – that this particular manifestation of quantum weirdness offers something genuinely new.

One suspects the next stage will involve attempts to camouflage the inherent limitations – to present this system not as a delicate dance on the edge of chaos, but as a reliable building block. The illusion of control, after all, is often more valuable than control itself. The human tendency to extrapolate order from noise is, predictably, the most reliable phenomenon in the entire field.

Original article: https://arxiv.org/pdf/2512.10338.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

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2025-12-14 05:38