Pt-Symmetric Spins Enable Long-Distance Quantum Entanglement

Ahuja and colleagues at Harish-Chandra Research Institute and Homi Bhabha National Institute demonstrate a method for dynamically establishing long-distance entanglement via engineered non-Hermitian systems. The research reveals that by using parity-time (PT)-symmetric models, entanglement can not only be generated from initially separated states but also ‘frozen’ in time near exceptional points, offering a pathway towards stronger and more persistent quantum links within future quantum networks. This freezing effect, unattainable in traditional Hermitian systems, represents a key advance in maintaining entanglement fidelity.

Su-Schrieffer-Heeger models and non-Hermitian elements enable long-range quantum entanglement

A carefully constructed model of interacting spin systems mediated entanglement between distant nodes, utilising the Su-Schrieffer-Heeger (SSH) model. The SSH model, originally developed to understand the electronic properties of trans-polyacetylene, provides a simplified yet powerful framework for describing the behaviour of electrons moving along a one-dimensional chain of atoms. It represents a lattice where the hopping integral, the quantum mechanical probability of an electron moving between adjacent atoms, alternates in strength. This alternation creates a unique band structure with topological properties, crucial for robust quantum information transfer. In this context, the SSH model formed the ‘bulk’ of the system, a central component designed to enable connections between the distant nodes, or ‘links’. Non-Hermitian elements were introduced, effectively creating a system where energy gains and losses are balanced, akin to a perfectly balanced seesaw with weight constantly being added and removed. This departure from the standard Hermitian framework, which assumes energy conservation, allows for the exploration of novel quantum phenomena. Specifically, weak couplings and imaginary magnetic fields were chosen to examine how these dynamics could create and sustain long-distance entanglement, something difficult to achieve in standard symmetrical systems. The strength of these couplings, at approximately 1, and the carefully chosen imaginary magnetic fields are critical to the observed effects. This approach moves beyond the limitations of traditional, symmetrical quantum systems by actively manipulating the system’s energy flow.

Sustained near-unit entanglement via balanced gain and loss in a non-Hermitian spin system

The researchers from Harish-Chandra Research Institute and the Homi Bhabha National Institute have achieved near-unit time-averaged entanglement, a measure of quantum connection, between links, representing a substantial improvement over previous systems. This breakthrough hinges on exploiting non-Hermitian systems, which deviate from standard quantum symmetry rules, allowing entanglement to ‘freeze’ near exceptional points, unique conditions within the system where quantum behaviour dramatically changes. Exceptional points are eigenvalues of the non-Hermitian Hamiltonian where two or more eigenstates coalesce, leading to a breakdown of the traditional eigenvalue-eigenstate correspondence and enhanced sensitivity to perturbations. By carefully balancing energy gains and losses within their model, high entanglement could be sustained over time, a key step towards building strong quantum networks. Analytical calculations pinpointed the exceptional points, where this effect is strongest, and tuning the strength of these imaginary fields to be weak, alongside the connections between the system’s components, sustained near-unit time-averaged entanglement for extended periods. Furthermore, extending beyond nearest-neighbour interactions to include longer-range connections boosted the average optimised long-distance entanglement, an enhancement consistent even when starting with completely separate quantum states. The researchers found that incorporating interactions beyond immediate neighbours, extending the network’s reach, further improved entanglement fidelity. The entanglement persisted when examining the system’s long-term, stationary behaviour, exceeding levels found in standard, Hermitian systems. This long-term stability is crucial for practical applications, as it reduces the need for constant error correction and maintenance of the quantum link.

Dissipation-driven entanglement generation in non-Hermitian quantum networks

Engineered dissipation, once considered detrimental to maintaining delicate quantum states, now appears as a surprisingly effective tool for building distributed quantum networks. The team has shown that carefully constructed non-Hermitian systems, those defying conventional symmetry rules, can dynamically generate entanglement, a key quantum connection, between distant nodes starting from a completely disconnected state. This is achieved by leveraging the unique properties of PT-symmetric systems, where balanced gain and loss can compensate for the natural decay of quantum coherence. However, this approach relies heavily on specific models, in particular the Su-Schrieffer-Heeger model and XX bulk systems, raising questions about how widely applicable these findings are to other network designs. The XX model, a specific type of spin interaction, was used to define the interactions within the bulk of the system, complementing the SSH model’s structural properties.

Acknowledging that these encouraging results depend on specific quantum system designs, the Su-Schrieffer-Heeger model and XX bulk systems, does not diminish their significance. These findings demonstrate that engineered dissipation, previously viewed as a hindrance, can actively support entanglement, a vital resource for quantum communication and computation. The ability to dynamically create connections between previously isolated quantum nodes offers a novel pathway towards building more durable and effective quantum networks, even if broader applicability requires further investigation into diverse architectures. The implications extend to quantum key distribution, secure communication protocols, and distributed quantum sensing, where robust and long-lived entanglement is paramount. Future research will focus on exploring the robustness of this approach to noise and imperfections, and on adapting the model to more complex network topologies.

Previously considered detrimental to quantum systems, engineered dissipation actively supports entanglement between distant quantum nodes. The team demonstrated that carefully designed non-Hermitian systems, deviating from standard symmetry rules, can dynamically generate these connections, even beginning with completely unconnected states. This process relies on ‘exceptional points’, unique conditions within the system where quantum behaviour changes dramatically, allowing entanglement to stabilise. The ability to harness dissipation, rather than suppress it, opens up new avenues for designing and controlling quantum networks, potentially leading to more efficient and resilient quantum technologies. The observed ‘freezing’ of entanglement near exceptional points offers a mechanism for protecting quantum information from decoherence, a major challenge in building practical quantum devices.

Engineered dissipation actively supported entanglement between distant quantum nodes, as researchers dynamically generated connections beginning with unconnected states. This was achieved through carefully designed non-Hermitian systems and the exploitation of ‘exceptional points’ where quantum behaviour changes. The resulting entanglement stabilised and, under weak imaginary magnetic fields, exhibited near-unit time-averaged entanglement between the links. The team intends to explore the robustness of this approach to noise and adapt the model to more complex network topologies.