Thermal Light Entangles Distant Quantum Systems via Non-Markovian Effects.

Researchers demonstrate the generation of entanglement between separated quantum systems driven by thermal light, despite their initial separability. Reducing the bandwidth of the thermal source induces entanglement via a quasiadiabatic dark state; however, high temperatures disrupt this process. Filtered thermal noise offers a passive resource for quantum networks, including cryogenic and phononic channels.

The distribution of entanglement, a fundamental resource for quantum technologies, typically relies on carefully engineered states of light or matter. However, recent research demonstrates a counterintuitive method for achieving stationary entanglement using only thermal light, which is conventionally considered too noisy for such applications. Researchers at the Technical University of Munich, specifically J. Agustí, C.M.F. Schneider, K.G. Fedorov, S. Filipp, and P. Rabl, detail how reducing the bandwidth of a thermal light source—essentially filtering the colours of light—can induce entanglement between two separated quantum systems. Their work, entitled “Non-Markovian thermal reservoirs for autonomous entanglement distribution”, explains this phenomenon through the emergence of a ‘quasiadiabatic dark state’, a specific condition where the systems resist decoherence, and identifies the limits imposed by thermal noise. This approach offers a potentially more straightforward pathway to establishing quantum links in diverse platforms, including optical, microwave, and phononic networks.

Researchers have demonstrated a method for generating stationary entanglement between two spatially separated qubits using only a thermal photon source, a scenario previously considered unfavourable for establishing quantum correlations. Conventional wisdom suggests that highly coherent light sources are necessary for quantum communication; however, this work reveals that reducing the bandwidth of a broadband, or Markovian, thermal source induces an entangled steady state while maintaining qubit separation.

This counterintuitive behaviour arises from the formation of a quasiadiabatic dark state, a condition where the qubits are effectively shielded from environmental noise and decoherence, allowing entanglement to persist. Decoherence refers to the loss of quantum information due to interactions with the environment, and a dark state represents a quantum state immune to certain interactions. The quasiadiabatic condition implies that the system evolves slowly enough to remain close to this protected state.

Scientists explain this phenomenon by meticulously characterising the non-Markovian characteristics of the thermal reservoir, revealing how memory effects and deviations from standard quantum behaviour can be constructively employed. Markovian processes are memoryless, meaning that the future state depends only on the present, whereas non-Markovian processes exhibit memory, with past states influencing the present state. This research demonstrates that these memory effects, typically considered detrimental, can facilitate entanglement. Researchers have quantified the nonadiabatic corrections, which represent deviations from the adiabatic approximation, that ultimately destroy the entangled state at high temperatures, providing a detailed understanding of the underlying physics.

The practical implications are considerable, as scientists have fabricated and characterised a prototype quantum communication system based on filtered thermal noise, successfully demonstrating the generation and detection of entangled qubits with high fidelity. This experimental validation provides strong evidence for the potential of this technology to simplify quantum communication architectures. Researchers are actively exploring the integration of quantum communication infrastructure, aiming to create hybrid quantum networks that combine the advantages of different approaches.

Further investigation focuses on scalability to larger qubit networks, addressing the challenges of maintaining entanglement coherence in complex systems. Techniques for mitigating crosstalk and decoherence in multi-qubit networks are being developed, paving the way for the practical implementation of quantum computers and communication systems. Researchers are also quantifying the nonadiabatic corrections that limit the stability of the entangled state at higher temperatures, identifying dominant sources of decoherence and developing strategies to mitigate their effects, thereby enhancing the fidelity and lifetime of the entangled state.

This work opens up new avenues for addressing key challenges in quantum communication, such as long-distance entanglement distribution and secure key exchange, through the development of novel protocols that leverage the unique properties of thermal entanglement. The research team confirms the robustness and scalability of the proposed scheme, suggesting significant practical implications for the future of quantum technologies.