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arXiv:2607.19533

High-purity entanglement mediated by magnons despite weak coupling

Sanchar Sharma

quant-phcond-mat.mes-hallSubmitted Jul 21, 2026• Announced Jul 23, 2026• 0 citations
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Abstract

Entangling distant spins via a shared magnonic bus typically faces a tradeoff: stronger spin-magnon coupling increases the entanglement fidelity, but also the spin decay rate. We propose a protocol that breaks this tradeoff. The magnons couple only to a transition outside the computational basis, in which the Bell state is created. Our protocol is probabilistic, and weak coupling reduces the success probability but not the fidelity. We analyze a setup where the spins are two nitrogen-vacancy (NV) centers near a magnetic wire. In the absence of NV dephasing, the protocol reaches unit fidelity with a maximally entangled state, for arbitrarily weak coupling. In our simulations via the Monte-Carlo wavefunction approach, the NV-magnon coupling is taken to be one-third of the magnon linewidth. Considering finite NV dephasing, we predict a fidelity of >0.99 for a state-of-the-art rate, and an optimal fidelity of 0.91 for a moderate rate, both at 0.6% success probability.

Comments
9 pages of main content with 9 pages of appendix

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Entangling distant spins via a shared magnonic bus typically faces a tradeoff: stronger spin-magnon coupling increases the entanglement fidelity, but also the spin decay rate. We propose a protocol that breaks this tradeoff. The magnons couple only to a transition outside the computational basis, in which the Bell state is created. Our protocol is probabilistic, and weak coupling reduces the success probability but not the fidelity. We analyze a setup where the spins are two nitrogen-vacancy (NV) centers near a magnetic wire. In the absence of NV dephasing, the protocol reaches unit fidelity with a maximally entangled state, for arbitrarily weak coupling. In our simulations via the Monte-Carlo wavefunction approach, the NV-magnon coupling is taken to be one-third of the magnon linewidth. Considering finite NV dephasing, we predict a fidelity of >0.99 for a state-of-the-art rate, and an optimal fidelity of 0.91 for a moderate rate, both at 0.6% success probability.
We propose a hybrid quantum system in which a magnet supporting nonreciprocal magnons, chiral magnons, or both mediates the dissipative and unidirectional coupling of spin qubits. By driving the qubits, the steady state of this qubit-qubit coupling scheme becomes the maximally entangled Bell state. We devise a protocol where the system converges to this entangled state and benchmark it including qubit decay and dephasing. The protocol is numerically tested on a hybrid system consisting of nitrogen-vacancy (NV) centers coupled to magnon surface modes of a yttrium iron garnet (YIG) film. We show that the dephasing time of the NV centers forms the bottleneck for achieving the entanglement of NV centers separated by a distance within the magnon coherence length. Our findings identify the key technological requirements and demonstrate a viable route toward steady-state entanglement of solid-state spins over distances of several microns using magnonic quantum networks, expanding the toolbox of magnonics for quantum information purposes.
We propose a protocol where chiral magnons mediate a state transfer between two distant spin qubits. The protocol is implemented by varying the coupling between the spin qubits and the magnons in time, such that an arbitrary state is transferred from one qubit to the other. The modulation of the coupling is performed such that the two-spin-qubit state is kept as a dark state of the magnon bath, bypassing the associated losses. We show that the protocol can be realized on a hybrid system composed of two nitrogen-vacancy (NV) centers coupled to the nonreciprocal and chiral magnon modes of an yttrium iron garnet (YIG) stripe. We propose two methods to achieve the time modulation of the NV-magnon coupling: i) the NV-magnet distance of both NV centers is varied; ii) the external magnetic field and the NV-magnet distance of one NV center are varied. We evaluate the implementability of both methods numerically, including the constraints on the temperature, and the dephasing time and minimal lifetime of the spin qubits required for high-fidelity state transfer. We find that using realistic experimental parameters, a state transfer between NV centers at a distance of several microns can be achieved with a fidelity ≳0.95\gtrsim 0.95≳0.95. Our findings expand the toolbox of magnonics for quantum information purposes.
We propose an efficient two-stage protocol for generating distant entanglement in a magnon-mediated hybrid quantum system, where magnons serve dual roles as both interaction mediators and qubits. This integrated design reduces the physical component count while leveraging the inherent advantages of magnons, such as their strong coupling via magnetic dipole interactions, low dissipation, and high integrability. In our setup, a superconducting resonator interfaces between a local superconducting qubit (SQ) and a local magnonic system (QM1), which is waveguide-coupled to a remote magnonic system (QM2). The protocol comprises two stages: (i) deterministic Bell-state generation between the SQ and QM1 using shortcuts to adiabaticity, and (ii) coherent state transfer to QM2 via engineered Hamiltonian dynamics. This adiabatic characteristic enhances robustness against environmental dissipation. Numerical simulations under realistic noise conditions confirm strong resilience to decoherence, achieving fidelity F>0.90F > 0.90F>0.90 and negativity N2>0.40\mathcal{N}_2 > 0.40N2​>0.40. These results establish the protocol as a scalable and practical building block for distributed quantum networks.
An ensemble of nitrogen-vacancy (NV) centers coupled to a circuit QED device is shown to enable an efficient, high-fidelity generation of high-N00N states. Instead of first creating entanglement and then increasing the number of entangled particles NNN, our source of high-N00N states first prepares a high-NNN Fock state in one of the NV ensembles and then entangles it to the rest of the system. With such a strategy, high-NNN N00N states can be generated in just a few operational steps with an extraordinary fidelity. Once prepared, such a state can be stored over a longer period of time due to the remarkably long coherence time of NV centers.
We present a hybrid entanglement purification and concentration protocol for nuclear spin entangled states of nitrogen-vacancy (N-V) centers resorting to the ancillary photons and electrons. In the entanglement purification protocol, the mixed entangled states of the N-V centers can be purified with high fidelity using two pairs of entangled photons to interact with each electron in N-V centers. By performing local measurement on the electrons, the original entangled state can be obtained between the two remote parities. In the entanglement concentration protocol, the partially entangled states of the nuclear spin in N-V centers can be concentrated to the maximally entangled state by using an ancillary photon on one local user. The fidelity of the entangled state with different excited state decay rate and the relation between the yield and different decay rate are numerically simulated.
We investigate steady state entanglement (SSE) between two nitrogen-vacancy (NV) center defects in a diamond host on an ultrathin yttrium iron garnet (YIG) strip. We determine the dephasing and dissipative interactions of the qubits with the quanta of spin waves (magnon bath) in the YIG depending on the qubit positions on the strip. We show that the magnon's dephasing effect can be eliminated, and we can transform the bath into a multimode displaced thermal state using external magnetic fields. Entanglement dynamics of the qubits in such a displaced thermal bath have been analyzed by deriving and solving the master equation. An additional electric field is considered to engineer the magnon dispersion relation at the band edge to control the Markovian character of the open system dynamics. We determine the optimum geometrical parameters of the system of distant qubits and the YIG strip to get SSE. Furthermore, parameter regimes for which the shared displaced magnon bath can sustain significant SSE against the local dephasing and decoherence of NV centers to their nuclear spin environments have been determined. Along with SSE, we investigate the steady state coherence (SSC) and explain the physical mechanism of how delayed SSE appears following a rapid generation and sudden death of entanglement using the interplay of decoherence-free subspace states, system geometry, displacement of the thermal bath, and enhancement of the qubit dissipation near the magnon band edge. A nonmonotonic relation between bath coherence and SSE is found, and critical coherence for maximum SSE is determined. Our results illuminate the efficient use of system geometry, band edge in bath spectrum, and reservoir coherence to engineer system-reservoir interactions for robust SSE and SSC.