Mapping photonic entanglement into and out of a quantum memory (0712.3571v2)

Abstract: Recent developments of quantum information science critically rely on entanglement, an intriguing aspect of quantum mechanics where parts of a composite system can exhibit correlations stronger than any classical counterpart. In particular, scalable quantum networks require capabilities to create, store, and distribute entanglement among distant matter nodes via photonic channels. Atomic ensembles can play the role of such nodes. So far, in the photon counting regime, heralded entanglement between atomic ensembles has been successfully demonstrated via probabilistic protocols. However, an inherent drawback of this approach is the compromise between the amount of entanglement and its preparation probability, leading intrinsically to low count rate for high entanglement. Here we report a protocol where entanglement between two atomic ensembles is created by coherent mapping of an entangled state of light. By splitting a single-photon and subsequent state transfer, we separate the generation of entanglement and its storage. After a programmable delay, the stored entanglement is mapped back into photonic modes with overall efficiency of 17 %. Improvements of single-photon sources together with our protocol will enable "on demand" entanglement of atomic ensembles, a powerful resource for quantum networking.

• The paper introduces a deterministic protocol that decouples entanglement generation from storage to map photonic states into atomic ensembles.
• The experiment using cesium ensembles and EIT achieved a 17% efficiency with approximately 91% interference visibility.
• The work advances quantum network scalability by enabling on-demand retrieval and suggesting improvements in optical depth and memory lifetime.

Mapping Photonic Entanglement into and out of a Quantum Memory

The paper "Mapping Photonic Entanglement into and out of a Quantum Memory" presents an innovative protocol for the generation, storage, and retrieval of entangled photonic states with atomic ensembles serving as quantum memories. The research addresses fundamental challenges in quantum network scalability, specifically targeting the low success rates associated with probabilistic entanglement creation methods.

Summary of the Protocol and Experimental Setup

The core contribution of this work lies in a coherent protocol that separates entanglement generation from its storage. The outlined methodology employs a single photon, from which an entangled state of light is generated by splitting it into two spatially separated and orthogonally polarized modes. This entangled photonic state is then adiabatically mapped into two distinct atomic ensembles, acting as quantum memories, through Electromagnetically Induced Transparency (EIT). The stored atomic states can be retrieved as photonic states on demand after a controlled delay.

This protocol is demonstrated primarily with cesium atomic ensembles housed in a magneto-optical trap. The precision control of the optical fields and the atomic ensemble states allows for reversibly mapping entangled photonic states into matter states and vice versa. The experimental setup, as illustrated in the provided figures, incorporates state-of-the-art single-photon sources and control techniques to facilitate the mapping process with high efficiency.

Numerical Results and Entanglement Analysis

The authors report an overall storage and retrieval efficiency of 17%. The efficiency is constrained by the interplay between optical depth and the length of the atomic ensemble, resulting in significant losses in storage due to residual signal field leakage. Despite these constraints, the experiment achieves a concurrence of approximately 0.019 for the retrieved entanglement, consistent with theoretical predictions.

Moreover, the entanglement transfer from photonic modes $L_{in}, R_{in}$ to $L_{out}, R_{out}$ managed to preserve high visibility for interference fringes (maintaining approximately 91%), suggesting minimal degradation in entanglement quality through the storage process. The protocol allows for deterministic entanglement generation, distinct from traditional DLCZ schemes where entanglement generation and storage are intertwined probabilistically.

Implications and Future Directions

The implications of this research are considerable for the field of quantum information science, particularly in facilitating scalable and efficient quantum networks. With improved single-photon sources and storage efficiencies, the protocol could allow for the reliable distribution of entanglement over long distances, overcoming the constraints of probabilistic approaches.

For future work, enhancements in single-photon source quality and larger optical depths are suggested as methods to improve the entanglement degree and transfer efficiency. Additionally, extending memory lifetimes beyond the present limit of 8 µs through refined control over magnetic fields and optomechanical trapping techniques is considered crucial for practical quantum networking.

In conclusion, this paper provides a robust framework for addressing some of the core bottlenecks in the deployment of quantum communication networks, with potential for significant advances in both theoretical models and practical applications. The integration of deterministic entanglement generation with on-demand retrieval represents a promising avenue for further research in quantum information technologies.