Quantum entanglement is usually considered a fragile quantity and decoherence through coupling to an external environment, such as a thermal reservoir, can quickly destroy the entanglement resource. This doesn't have to be the case and the environment can be engineered to assist in the formation of entanglement. We investigate a system of qubits and higher dimensional spins interacting only through their mutual coupling to a reservoir. We explore the entanglement of multipartite and multidimensional system as mediated by the bath and show that at low temperatures and intermediate coupling strengths multipartite entanglement may form between qubits and between higher spins, i.e., qudits. We characterise the multipartite entanglement using an entanglement witness based upon the structure factor and demonstrate its validity versus the directly calculated entanglement of formation, suggesting possible experiments for its measure.Quantum entanglement is usually considered a fragile quantity and decoherence through coupling to an external environment, such as a thermal reservoir, can quickly destroy the entanglement resource. This doesn't have to be the case and the environment can be engineered to assist in the formation of entanglement. We investigate a system of qubits and higher dimensional spins interacting only through their mutual coupling to a reservoir. We explore the entanglement of multipartite and multidimensional system as mediated by the bath and show that at low temperatures and intermediate coupling strengths multipartite entanglement may form between qubits and between higher spins, i.e., qudits. We characterise the multipartite entanglement using an entanglement witness based upon the structure factor and demonstrate its validity versus the directly calculated entanglement of formation, suggesting possible experiments for its measure.

Scientific ReportsJun 24, 20198 citations

Reservoir-mediated spin entanglement in the mean-force Gibbs state

L. A. Williamson, W. McEniery, F. Cerisola +1 more

Two qubits strongly coupled to a common bosonic reservoir can become entangled with each other, despite having no direct interaction. In equilibrium, such coupling-induced coherences can be described by the mean-force Gibbs state. Here we derive approximate, analytic expressions for the two-qubit mean-force Gibbs state, and use these to characterize equilibrium qubit-qubit entanglement mediated by a thermal reservoir. Entanglement, which is highest at lowest temperatures, is a non-monotonic function of the system-reservoir coupling strength. Moreover, we find that broadening the reservoir spectral density beyond a single mode, as is realistic for typical baths, can enhance the qubit entanglement. Our results provide a comprehensive understanding of reservoir-mediated two-qubit entanglement in thermal equilibrium and provide a benchmark to compare with numerical methods, as well as demonstrating the utility of strong system-reservoir coupling as a resource.Two qubits strongly coupled to a common bosonic reservoir can become entangled with each other, despite having no direct interaction. In equilibrium, such coupling-induced coherences can be described by the mean-force Gibbs state. Here we derive approximate, analytic expressions for the two-qubit mean-force Gibbs state, and use these to characterize equilibrium qubit-qubit entanglement mediated by a thermal reservoir. Entanglement, which is highest at lowest temperatures, is a non-monotonic function of the system-reservoir coupling strength. Moreover, we find that broadening the reservoir spectral density beyond a single mode, as is realistic for typical baths, can enhance the qubit entanglement. Our results provide a comprehensive understanding of reservoir-mediated two-qubit entanglement in thermal equilibrium and provide a benchmark to compare with numerical methods, as well as demonstrating the utility of strong system-reservoir coupling as a resource.

quant-pharXiv:2604.26562Apr 30, 20260 citations

Microscopic QED origin of spin entanglement

Moslem Zarei

We study effective spin interactions arising from quantum electrodynamics (QED) scattering between localized fermionic spins. By integrating out photon and mediator fields, the dynamics reduce to an effective spin Hamiltonian. For two qubits in the nonrelativistic regime, the resulting interaction takes a tensor dipolar form with an asymptotic decay proportional to $R^{-3}$ . We obtain analytical expressions for the entanglement negativity, highlighting its dependence on coupling strength and spatial configuration. We then examine a setup in which two bath spins interact via a sequential exchange with an intermediate fermionic mediator. At the perturbative order considered, the mediator remains unentangled and induces an effective bath--bath interaction with stronger spatial suppression than in the photon-mediated case. Extending the construction to an $N$ -spin setting yields an effective interaction network mediated by virtual exchange processes, which can support the generation of multipartite entanglement across the system.We study effective spin interactions arising from quantum electrodynamics (QED) scattering between localized fermionic spins. By integrating out photon and mediator fields, the dynamics reduce to an effective spin Hamiltonian. For two qubits in the nonrelativistic regime, the resulting interaction takes a tensor dipolar form with an asymptotic decay proportional to $R^{-3}$. We obtain analytical expressions for the entanglement negativity, highlighting its dependence on coupling strength and spatial configuration. We then examine a setup in which two bath spins interact via a sequential exchange with an intermediate fermionic mediator. At the perturbative order considered, the mediator remains unentangled and induces an effective bath--bath interaction with stronger spatial suppression than in the photon-mediated case. Extending the construction to an $N$-spin setting yields an effective interaction network mediated by virtual exchange processes, which can support the generation of multipartite entanglement across the system.

quant-pharXiv:2608.09670Aug 11, 20260 citations

Dephasing-Induced distribution of entanglement in tripartite quantum systems

Sovik Roy, Md. Manirul Ali, Abhijit Mandal +1 more

We investigate the distribution of entanglement in tripartite quantum systems under the influence of noisy environment. Entanglement distribution is an important concept to understand the robustness of entanglement in multipartite systems, while preserving multipartite entanglement amid decoherence remains a major challenge. Measuring multipartite entanglement in mixed states under decoherence is challenging. We quantify tripartite entanglement using relative entropy due to its applicability to both pure and mixed states. In this work, we explore how entanglement is distributed among the qubits of multipartite states when the three qubits are exposed to multipartite dephasing setting. In our study we use various pure and mixed tripartite states subjected to both Markovian and non-Markovian Local/Common bosonic reservoir at finite temperature. We show that the robustness of tripartite systems to decoherence depends on the distribution of entanglement and its interaction with various configurations and parameters of the bath.We investigate the distribution of entanglement in tripartite quantum systems under the influence of noisy environment. Entanglement distribution is an important concept to understand the robustness of entanglement in multipartite systems, while preserving multipartite entanglement amid decoherence remains a major challenge. Measuring multipartite entanglement in mixed states under decoherence is challenging. We quantify tripartite entanglement using relative entropy due to its applicability to both pure and mixed states. In this work, we explore how entanglement is distributed among the qubits of multipartite states when the three qubits are exposed to multipartite dephasing setting. In our study we use various pure and mixed tripartite states subjected to both Markovian and non-Markovian Local/Common bosonic reservoir at finite temperature. We show that the robustness of tripartite systems to decoherence depends on the distribution of entanglement and its interaction with various configurations and parameters of the bath.

arXiv:2408.09801Modern Physics Letters AMay 21, 20251 citation

Quantum entanglement dynamics based on composite quantum collision model*

Xiaoming Li, Yong-Xu Chen, Yun‐Jie Xia +2 more

By means of composite quantum collision models, we study the entanglement dynamics of a bipartite system, i.e., two qubits S 1 and S 2 interacting directly with an intermediate auxiliary qubit S A, while S A is in turn coupled to a thermal reservoir. We are concerned with how the intracollisions of the reservoir qubits influence the entanglement dynamics. We show that even if the system is initially in the separated state, their entanglement can be generated due to the interaction between the qubits. In the long-time limit, the steady-state entanglement can be generated depending on the initial state of S 1 and S 2 and the environment temperature. We also study the dynamics of tripartite entanglement of the three qubits S 1, S 2, and S A when they are initially prepared in the GHZ state and separated state, respectively. For the GHZ initial state, the tripartite entanglement can be maintained for a long time when the collision strength between the environment qubits is sufficiently large.By means of composite quantum collision models, we study the entanglement dynamics of a bipartite system, i.e., two qubits S 1 and S 2 interacting directly with an intermediate auxiliary qubit S A, while S A is in turn coupled to a thermal reservoir. We are concerned with how the intracollisions of the reservoir qubits influence the entanglement dynamics. We show that even if the system is initially in the separated state, their entanglement can be generated due to the interaction between the qubits. In the long-time limit, the steady-state entanglement can be generated depending on the initial state of S 1 and S 2 and the environment temperature. We also study the dynamics of tripartite entanglement of the three qubits S 1, S 2, and S A when they are initially prepared in the GHZ state and separated state, respectively. For the GHZ initial state, the tripartite entanglement can be maintained for a long time when the collision strength between the environment qubits is sufficiently large.

Chinese Physics BMar 31, 20209 citations

Qubit-environment entanglement in pure dephasing dynamics for mixed initial states

Małgorzata Strzałka, Nico Peters, Laura Ares +2 more

Decoherence of qubits is one main limitation in quantum information processing, where the qubits themselves can be mixed because of decoherence and additional decoherence may generated through interactions with a bath. Here, we present a complete study of qubit-environment entanglement in the case of interactions that lead to pure dephasing of the qubit by allowing the initial state of the qubit to be mixed, exceeding previous restrictions. We find that the criteria for separability that hold for pure initial states of the qubit are reduced to witness status. Yet, the evolution of the witnesses reflects the array of features that entanglement can exhibit in the mixed case but are forbidden in the pure-qubit case. Most notably, this includes sudden birth of entanglement as the initial phase of the evolution is typically separable. Moreover, there is a stark numerical advantage to the study of entanglement for large environments when using the witness rather than the traditional measures available. We include the study of entanglement generated between a spin qubit and a sparse spin environment to illustrate the peculiarities of the evolution of entanglement in this case, as well as to emphasize the level of reliability of the entanglement witness.Decoherence of qubits is one main limitation in quantum information processing, where the qubits themselves can be mixed because of decoherence and additional decoherence may generated through interactions with a bath. Here, we present a complete study of qubit-environment entanglement in the case of interactions that lead to pure dephasing of the qubit by allowing the initial state of the qubit to be mixed, exceeding previous restrictions. We find that the criteria for separability that hold for pure initial states of the qubit are reduced to witness status. Yet, the evolution of the witnesses reflects the array of features that entanglement can exhibit in the mixed case but are forbidden in the pure-qubit case. Most notably, this includes sudden birth of entanglement as the initial phase of the evolution is typically separable. Moreover, there is a stark numerical advantage to the study of entanglement for large environments when using the witness rather than the traditional measures available. We include the study of entanglement generated between a spin qubit and a sparse spin environment to illustrate the peculiarities of the evolution of entanglement in this case, as well as to emphasize the level of reliability of the entanglement witness.

quant-pharXiv:2610.36936Oct 28, 20260 citations

R^{-3}

. We obtain analytical expressions for the entanglement negativity, highlighting its dependence on coupling strength and spatial configuration. We then examine a setup in which two bath spins interact via a sequential exchange with an intermediate fermionic mediator. At the perturbative order considered, the mediator remains unentangled and induces an effective bath--bath interaction with stronger spatial suppression than in the photon-mediated case. Extending the construction to an

N

-spin setting yields an effective interaction network mediated by virtual exchange processes, which can support the generation of multipartite entanglement across the system.