Remote nonreciprocal magnon magnon entanglement via the Barnett effect with coherent feedback

arXiv:2610.11607 · quant-ph · Submitted 2026-10-08 · Read on arXiv

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Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: Today's paper: "Remote nonreciprocal magnon magnon entanglement via the Barnett effect with coherent feedback".

Mira: The gist:

Kai: First, who's behind it and why it matters.

Paper summary: Kai: So looking at "Remote nonreciprocal magnon magnon entanglement via the Barnett effect with coherent feedback," what does this paper really tell us about building these kinds of systems? It seems like they've laid out a path to getting controllable remote quantum correlations in cavity magnonic setups >

Mira: They’re demonstrating how the combination of the Barnett effect, cavity detuning, and feedback can allow you to switch that entanglement on or off depending on the direction of the magnetic field you apply >

Lev: From an error correction angle, I think this control over nonreciprocity is crucial because if you can flip it like that, it opens up possibilities for more robust quantum protocols >

Kai: It shows a controllable approach to preparing these kinds of nonreciprocal quantum correlations in cavity magnonic systems >

Mira: The paper really highlights the interplay between the input squeezing and the feedback loop as the primary control mechanisms for these remote connections >

Conclusion: Kai: So we’re wrapping up on this paper about remote nonreciprocal magnon entanglement, titled "Remote nonreciprocal magnon magnon entanglement via the Barnett effect with coherent feedback."

Mira: It basically shows how you can link two separate microwave cavities together, using a spinning magnetic material in one cavity and a stationary one in the other to create correlations between their spin excitations.

Lev: The authors are talking about using that Barnett effect—basically reversing the bias field changes the direction of entanglement—and they're controlling it with feedback and detuning settings.

Kai: So, what does this actually mean for us? It means we can build a system where you can decide which way the quantum information flows between two distant points in a chip or even across a lab setup.

Mira: They are showing that you get complete nonreciprocity by tuning the detuning and feedback phase so that entanglement only exists in one magnetic field direction and vanishes for the other.

Lev: From an error correction standpoint, that control is significant because it gives you a switch to turn those correlations on or off precisely when needed.

Kai: The numbers they’re throwing around show how coherent feedback boosts the entanglement at a certain level of reflectivity, but if you push the feedback too hard, it actually reduces the amount.

Mira: And they also found that using unequal squeezing factors for the input fields helps improve entanglement even when you keep the total number of photons in both cavities fixed.

Lev: That's interesting because it suggests we don't just need perfect symmetry in our input states to get good results in this setup.

Kai: So, to sum up, they’ve given us a controllable recipe for creating these remote quantum links using cavity magnonics and the Barnett effect.

Mira: It really moves the discussion past just getting entanglement and into how you can precisely engineer its directionality and robustness.

Lev: We’re going to look at some of those specific control parameters next, like how much feedback is actually needed to make this work reliably in a real device.

School of Physics and Astronomy, Beijing Normal University

quant-ph

Submitted: 2026-10-08

Updated: 2026-10-08

Comments: 8 Pages, 8 figures

License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/

Importance score: 92/100

The gist: The gist: A scheme for generating and controlling remote nonreciprocal magnon–magnon entanglement in a cavity magnonic system with coherent feedback was proposed using two microwave cavities driven

Key concepts

Barnett Effect: This effect describes how reversing the direction of a static magnetic field changes the frequency of a rotating YIG sphere. In this system, this change in magnon frequency is used to introduce nonreciprocity, meaning the physical outcome depends on whether the bias field is applied in one direction or the opposite.
Nonreciprocal Entanglement: This refers to quantum correlations between magnons that are different when the physical setup (like the magnetic field direction) is reversed. The scheme achieves this by using the Barnett effect; entanglement exists for one magnetic field orientation but disappears for the other, demonstrating a directional dependence in quantum correlation.
Coherent Feedback: This involves sending a portion of the cavity's output back into it to influence its behavior. In this context, coherent feedback is used to enhance the magnon-magnon entanglement at an optimal level when tuned correctly, providing a mechanism for controlling the strength of the quantum correlation.
Squeezed Vacuum Fields: These are specialized microwave fields that have reduced noise in one quadrature (like amplitude or phase) at the expense of increased noise in another. Using independent single-mode squeezed vacuum fields drives the two cavities, and their relative squeezing phases offer an additional layer of control over how quantum correlations are transferred.

Terminology

Entanglement — 116× in this paper · explained in 4 episodes
Nonreciprocity — 6× in this paper · explained in 2 episodes
Steering — 6× in this paper · explained in 2 episodes
Robustness — 4× in this paper · explained in 5 episodes

Summary

The gist: A scheme for generating and controlling remote nonreciprocal magnon–magnon entanglement in a cavity magnonic system with coherent feedback was proposed using two microwave cavities driven by independent single-mode squeezed vacuum fields and utilizing the Barnett effect to introduce nonreciprocity, which is then controlled by tuning cavity detuning and feedback phase.

System Setup

The system consists of two spatially separated microwave cavities connected by a coaxial cable, with a rotating YIG sphere in the left cavity and a stationary one in the right cavity Each cavity supports a microwave photon mode with frequency ωaj, while its embedded YIG sphere supports a magnon mode representing collective spin excitations The bare resonance frequency of the jth magnon mode is given by ωmj = γHj, where Hj is the magnitude of the corresponding bias magnetic field and γ is the gyromagnetic ratio (j = 1, 2) Each magnon mode couples to the photon mode through the magnetic dipole interaction

Mechanism for Nonreciprocity and Entanglement

Nonreciprocal entanglement is achieved by exploiting the Barnett effect, where reversing the bias magnetic field changes the sign of the Barnett shift In a rotating YIG sphere, this effect produces a shift of the magnon frequency For a fixed rotation direction, reversing the bias magnetic field changes the sign of the Barnett shift, allowing quantum correlations to depend on the magnetic field direction By tuning cavity detuning and feedback phase, entanglement can be generated for one direction of the bias magnetic field while it vanishes for the opposite direction, yielding complete nonreciprocity

Role of Coherent Feedback and Control Parameters

Coherent feedback enhances the entanglement at an appropriate reflectivity, although this enhancement does not necessarily increase its normalized contrast By tuning the cavity detuning and feedback phase, entanglement can exist for one magnetic field direction and vanish for the opposite direction The relative squeezing phase and the two squeezing factors provide further control Finally, with the squeezed inputs kept fixed, moderate feedback also increases the critical temperature for magnon–magnon entanglement

Controllable Features of Entanglement

The study demonstrates several controllable aspects of the entanglement:

**: The relative squeezing phase provides additional control. The two squeezing factors provide additional control. An appropriate choice of unequal squeezing factors improves the entanglement at fixed total input photon occupation. Moderate feedback increases the critical temperature for magnon–magnon entanglement with fixed squeezed inputs. The relative phase between the two squeezed input fields affects the transfer of quantum correlations from the two squeezed fields to the magnon modes. Increasing the squeezing factors, while keeping N1+N2 fixed, leads to a maximum entanglement when both inputs are squeezed. The optimal squeezing factors depend on both rb and the sign of the Barnett shift. The feedback phase can be tuned so that reversing the rotation direction switches magnon–magnon entanglement on and off. The cavity detuning can be used to select which rotation direction supports magnon–magnon entanglement. Increasing rb from 0 to 0.3 raises the peak entanglement in both cases, while narrowing the interval of cavity detunings with nonzero entanglement. The critical temperatures are affected by feedback reflectivity, showing that stronger feedback does not necessarily improve thermal robustness. The entanglement decreases with increasing temperature and vanishes above a critical temperature. For rb = 0.3, temperatures between these two thresholds yield entanglement only for ΔB < 0, corresponding to E = 1. The gray regions in panel (c) indicate vanishing entanglement for both rotation directions, where E is undefined and the gray regions correspond to vanishing entanglement for both signs at rb = 0.8, where E is undefined. The optimal phases are different when rb = 0.8 compared with theta = 0. The relative phase affects the transfer of quantum correlations from the two squeezed fields to the magnon modes. Increasing rb from 0 to 0.3 enhances EN for both signs, while E decreases from approximately 0.40 to 0.35. Entanglement persists over a wider phase interval for ΔB < 0 when the relative squeezing phase is varied. The optimal squeezing factors depend on both rb and the sign of the Barnett shift. Increasing rb from 0 to 0.3 raises the critical temperature from approximately 324 mK to 369 mK for the positive Barnett shift, and from 513 mK to 558 mK for the negative shift. The entanglement decreases with increasing temperature and vanishes above a critical temperature. For rb = 0.3, temperatures between these two thresholds yield entanglement only for ΔB < 0, corresponding to E = 1. The gray regions in panel (c) indicate undefined contrast at rb = 0.8, where entanglement vanishes for both signs. Entanglement survives at higher temperatures for ΔB 0. The critical temperatures decrease when rb is further increased to 0.8, showing that stronger feedback does not necessarily improve thermal robustness. The results provide a controllable scheme for preparing remote nonreciprocal quantum correlations in cavity magnonic systems. This work provides a J. Jia et al.: Preprint submitted to Elsevier Page 8 of 8> is the final sentence.">J. Jia et al.: Preprint submitted to Elsevier Page 8 of 8The results provide a controllable scheme for preparing remote nonreciprocal quantum correlations in cavity magnonic systems.This work provides a J. Jia et al.: Preprint submitted to Elsevier Page 8 of 8The results provide a controllable scheme for preparing remote nonreciprocal quantum correlations in cavity magnonic systems.This work provides a J. Jia et al.: Preprint submitted to Elsevier Page 8 of 8The results provide a controllable scheme for preparing remote nonreciprocal quantum correlations in cavity magnonic systems.This work provides a J. Jia et al.: Preprint submitted to Elsevier Page 8 of 8The results provide a controllable scheme for preparing remote nonreciprocal quantum correlations in cavity magnonic systems.This work provides a J. Jia et al.: Preprint submitted to Elsevier Page 8 of 8The results provide a controllable scheme for preparing remote nonreciprocal quantum correlations in cavity magnonic systems.This work provides a J. Jia et al.: Preprint submitted to Elsevier Page 8 of 8The results provide a controllable scheme for preparing remote nonreciprocal quantum correlations in cavity magnonic systems.This work provides a J. Jia et al.

Improvements for AI systems

The improved system can generate remote nonreciprocal magnon–magnon entanglement in cavity magnonic systems by exploiting the Barnett effect enables remote nonreciprocal magnon-magnon entanglement and controlling it via cavity detuning and feedback phase enable complete nonreciprocity.
The AI system can achieve complete nonreciprocity by tuning parameters such that entanglement can be generated for one direction of the bias magnetic field while it vanishes for the opposite direction.
The system can enhance entanglement without increasing its nonreciprocity, as suggested by the finding that Coherent feedback enhances the entanglement at an appropriate reflectivity, whereas stronger feedback can reduce it.
The AI can improve entanglement at fixed total input photon occupation by utilizing unequal squeezing factors, specifically noting that an appropriate choice of unequal squeezing factors improves the entanglement at fixed total input photon occupation.
The system can increase the critical temperature for magnon–magnon entanglement under thermal conditions by employing moderate feedback, as stated: With the squeezed inputs kept fixed, moderate feedback also increases the critical temperature for magnon–magnon entanglement.
The AI can perform nonreciprocal quantum correlation steering by selecting a rotation direction based on cavity detuning, as demonstrated by the result that entanglement exists only for the positive Barnett shift when tuned to specific detunings.
The system can switch magnon–magnon entanglement on and off by tuning the feedback phase, as shown by the finding that the feedback phase can be tuned so that reversing the rotation direction switches magnon–magnon entanglement on and off.