Experimental demonstration of memory-enhanced quantum communication (1909.01323v1)

Abstract: The ability to communicate quantum information over long distances is of central importance in quantum science and engineering. For example, it enables secure quantum key distribution (QKD) relying on fundamental principles that prohibit the "cloning" of unknown quantum states. While QKD is being successfully deployed, its range is currently limited by photon losses and cannot be extended using straightforward measure-and-repeat strategies without compromising its unconditional security. Alternatively, quantum repeaters, which utilize intermediate quantum memory nodes and error correction techniques, can extend the range of quantum channels. However, their implementation remains an outstanding challenge, requiring a combination of efficient and high-fidelity quantum memories, gate operations, and measurements. Here we report the experimental realization of memory-enhanced quantum communication. We use a single solid-state spin memory integrated in a nanophotonic diamond resonator to implement asynchronous Bell-state measurements. This enables a four-fold increase in the secret key rate of measurement device independent (MDI)-QKD over the loss-equivalent direct-transmission method while operating megahertz clock rates. Our results represent a significant step towards practical quantum repeaters and large-scale quantum networks.

• The paper introduces a breakthrough in quantum communication by integrating solid-state SiV memory nodes to perform asynchronous Bell-state measurements for QKD.
• It reports a four-fold increase in secret key rates and achieves a QBER as low as 0.097, outperforming traditional direct transmission methods.
• The study paves the way for scalable quantum networks by highlighting the feasibility of memory-assisted protocols to overcome photon loss constraints.

Experimental Demonstration of Memory-Enhanced Quantum Communication

The paper presented by Bhaskar et al. introduces a significant advancement in quantum communication through the experimental implementation of memory-enhanced quantum communication utilizing solid-state spin memories. This paper focuses on overcoming the constraints of photon losses in quantum key distribution (QKD) protocols by leveraging quantum repeaters, which utilize intermediate quantum memory nodes.

Key Technical Insights

The authors describe the use of a silicon-vacancy (SiV) color-center in a nanophotonic diamond cavity, which serves as a robust quantum memory node enabling photon-photon Bell-state measurements (BSM). The system achieves a high cooperativity of $C = 105 \pm 11$, which is essential for efficient interaction between single photons and the cavity. Notably, the design facilitates asynchronous BSM, a critical capability for QKD protocols, particularly the measurement device independent QKD (MDI-QKD).

Numerical Results and Claims

The performance of the described system is benchmarked against traditional direct transmission methods. The most striking result is the four-fold increase in the secret key rate over the loss-equivalent direct-transmission method at megahertz clock rates. This enhancement is attributed to the capability of the quantum memory to store and processing incoming photonic qubits more efficiently, allowing for the operation over extended distances with reduced photon loss.

Furthermore, the BSM using quantum memory exceeds the theoretic maximum secret key rates achievable with linear optics in repeaterless communication scenarios. The experiment achieved a QBER as low as 0.097, within the thresholds necessary for secure quantum communication.

Theoretical and Practical Implications

This research underscores the potential of integrated quantum memory nodes in scaling quantum networks, a fundamental requirement for practical quantum communication. The successful implementation illustrates not only the feasibility of memory-assisted QKD but also its scalability to large network systems. The protocol developed by the authors addresses one of the critical hurdles for practical implementations of quantum repeaters, paving the way for secure communication across expansive networks.

Speculation on Future Developments

In the future, these findings suggest potential developments towards integration with long-distance telecommunications infrastructure. Noteworthy is the potential for nuclear spin qubits to extend memory storage times and improve coherence, leading to further improvements in BSM success rates. Additionally, adaptations in the device to incorporate multi-photon gate operations akin to cluster-state entangled photons illustrate avenues toward rapid quantum communication and scalable computations.

Conclusion

By demonstrating quantum communication that transcends the limits of repeaterless systems, this paper marks an instrumental step in the journey toward the realization of large-scale quantum networks. The implementation of memory-enhanced protocols not only optimizes current quantum key distribution systems but establishes a framework for diverse applications extending from cryptography to modular quantum computing architectures. As improvements continue and integration into classical and quantum hybrid networks advances, the promise of ubiquitous quantum communication draws progressively nearer to fruition.