Quantum repeaters based on atomic ensembles and linear optics (0906.2699v2)

Abstract: The distribution of quantum states over long distances is limited by photon loss. Straightforward amplification as in classical telecommunications is not an option in quantum communication because of the no-cloning theorem. This problem could be overcome by implementing quantum repeater protocols, which create long-distance entanglement from shorter-distance entanglement via entanglement swapping. Such protocols require the capacity to create entanglement in a heralded fashion, to store it in quantum memories, and to swap it. One attractive general strategy for realizing quantum repeaters is based on the use of atomic ensembles as quantum memories, in combination with linear optical techniques and photon counting to perform all required operations. Here we review the theoretical and experimental status quo of this very active field. We compare the potential of different approaches quantitatively, with a focus on the most immediate goal of outperforming the direct transmission of photons.

• The paper analyzes a quantum repeater architecture using atomic ensembles as quantum memories and linear optics to extend quantum communication distance.
• Key operations detailed include heralded entanglement generation, using atomic ensembles as quantum memories, and entanglement swapping to link distant nodes.
• Quantitative insights compare performance against direct transmission, identifying challenges and future improvements for practical quantum network implementation.

Quantum Repeaters Based on Atomic Ensembles and Linear Optics

The paper "Quantum repeaters based on atomic ensembles and linear optics" provides a comprehensive analysis and assessment of a quantum repeater architecture essential for overcoming the distance limitations in quantum communication. The authors elucidate a promising strategy that employs atomic ensembles as quantum memories combined with linear optical techniques for practical large-scale quantum networks.

The principal obstacle in transmitting quantum states over extended distances lies in photon loss. Unlike classical telecommunications, where information can be amplified to mitigate signal loss, quantum communication can't employ straightforward amplification due to the no-cloning theorem. This is where quantum repeaters become essential. They enable long-distance entanglement by using short-distance entanglement swapping, thus overcoming the limitations of direct quantum transmission.

Atomic Ensembles and Linear Optics

In devising quantum repeaters, atomic ensembles serve as quantum memories, leveraging their capacity for efficient interaction with light, thus supporting the storage and retrieval of quantum information. The use of linear optics for manipulation, combined with photon counting, forms the core methodology for executing required quantum operations.

The paper emphasizes several crucial operations in this architecture:

1. Entanglement Generation: The creation of heralded entanglement between distant atomic ensembles is the foundational task, achieved when detection of specific photon emissions heralds successful entanglement, leveraging collective interaction within atomic ensembles.
2. Quantum Memories: Efficient quantum memories are pertinent for holding entangled states until distributed across long distances. The use of atomic ensembles as memories is explored for their collective interaction benefits, allowing reversible photon storage and retrieval.
3. Swapping Entanglement: By performing entanglement swapping—a technique originally from quantum teleportation—new connections between distant quantum memories are formed, effectively extending entanglement over greater distances with a hierarchical series of operations.

Comparative Analysis and Numerical Insights

The authors provide quantitative insights into several proposed repeater protocols in terms of their ability to outperform direct photon transmission across significant distances. The performance assessment considers factors such as success probabilities of various operations, entanglement fidelity, memory time, and protocol complexity.

Critically, the paper evaluates numerical results central to affirming the viability and advantages of quantum repeaters. It identifies constraints like the probability of multiphoton emissions and their contributions to errors in state fidelity, offering systematic comparisons to gauge success over existing direct-transmission methods quantitatively.

Implications and Future Directions

The implications of this research bridge theoretical foundations with experimental feasibility, catalyzing advancements in both quantum communication technology and the broader objectives of quantum networking. Achieving practical scalability remains contingent on technological evolution, specifically in photon detection reliability, memory efficiency, and loss mitigation in channels.

Future work may pivot towards refining these foundations, exploring potential enhancements in multiphoton management, or hybridizing with other quantum systems promising synergistic improvements in performance and robustness. Continued exploration into ancillary technologies, such as advanced photon detectors and multiplexing techniques, will also be pivotal in transitioning from theoretical constructs to real-world implementations.

In conclusion, the authors offer a thorough exposition of quantum repeaters based on atomic ensembles and linear optics, laying the groundwork for extending quantum communication across expansive networks. This paper delineates a dense landscape of challenges and opportunities guiding practical implementations of robust quantum networks critical to the future of quantum information science.