Deterministic delivery of remote entanglement on a quantum network (1712.07567v2)

Abstract: Large-scale quantum networks promise to enable secure communication, distributed quantum computing, enhanced sensing and fundamental tests of quantum mechanics through the distribution of entanglement across nodes. Moving beyond current two-node networks requires the rate of entanglement generation between nodes to exceed their decoherence rates. Beyond this critical threshold, intrinsically probabilistic entangling protocols can be subsumed into a powerful building block that deterministically provides remote entangled links at pre-specified times. Here we surpass this threshold using diamond spin qubit nodes separated by 2 metres. We realise a fully heralded single-photon entanglement protocol that achieves entangling rates up to 39 Hz, three orders of magnitude higher than previously demonstrated two-photon protocols on this platform. At the same time, we suppress the decoherence rate of remote entangled states to 5 Hz by dynamical decoupling. By combining these results with efficient charge-state control and mitigation of spectral diffusion, we are able to deterministically deliver a fresh remote state with average entanglement fidelity exceeding 0.5 at every clock cycle of $\sim$100 ms without any pre- or post-selection. These results demonstrate a key building block for extended quantum networks and open the door to entanglement distribution across multiple remote nodes.

• The paper introduces a single-photon entanglement protocol that achieves a 39 Hz rate, offering a three-order magnitude improvement over traditional two-photon methods.
• It implements dynamic decoupling to suppress decoherence to 5 Hz, maintaining entangled state coherence for up to 200 ms.
• The deterministic delivery protocol leverages high link efficiency (~8) to reliably distribute entanglement, paving the way for scalable quantum networks.

Overview of Deterministic Delivery of Remote Entanglement on a Quantum Network

The paper entitled "Deterministic delivery of remote entanglement on a quantum network" presents significant advancements in the development of quantum networks, specifically focusing on the efficient and deterministic entanglement of remote nodes. The challenge addressed is the transition from probabilistic generation and preservation of entanglement to deterministic delivery, essential for the practical implementation of quantum networks. The researchers utilize nitrogen-vacancy (NV) centers in diamond as qubits to achieve a high quantum link efficiency exceeding unity, a critical parameter governing the ability to maintain entangled states over significant timescales.

Major Contributions and Methodology

The paper exhibits several key contributions:

1. Single-Photon Entanglement Protocol: An essential novelty of the methodology is the realization of a single-photon entanglement protocol that supersedes the traditional two-photon methods. The authors harness a single-photon entanglement generation scheme to drastically enhance entangling rates to 39 Hz, a stark three-order magnitude improvement over the existing two-photon protocols on similar NV platforms. The experiment achieves a high balance between entanglement rate and fidelity by tuning the bright-state population parameter $\alpha$ to appropriate values, reinforcing the single-photon scheme's efficacy.
2. Suppression of Decoherence: The researchers notably suppress the decoherence rate of remote entangled states to 5 Hz through dynamical decoupling. This process is vital for extending the coherence time of the entangled states up to 200 ms, efficiently maintaining high-fidelity quantum states over the time required for remote operations.
3. Deterministic Entanglement Delivery: The demonstrated deterministic delivery involves a protocol utilizing heralded success that ensures entangled states are reliably delivered at specified intervals. This approach is made feasible by the high quantum link efficiency of ~8, surpassing the critical threshold for deterministic quantum operations. This enables NV centers to be reliably used for deterministic entanglement delivery without post-selection or pre-selection, opening avenues for robust multi-node quantum networks.

Implications and Future Directions

The results from this paper indicate substantial progress in constructing scalable quantum networks. By establishing deterministic remote entanglement with high link efficiency, the authors set a precedent for future works on many-body quantum state distribution across extended quantum networks. The improvements in entanglement generation and preservation showcased herein promise enhancements in secure communications and distributed quantum computing.

Practical scalability prospects suggest immediate extensions by integrating improved photon detection efficiencies and refining the classical control systems. Furthermore, the potential use of nuclear spins as memory nodes with even longer coherence lifetimes is anticipated to further augment quantum link efficiencies into the magnitude of 100 or more.

These findings pave the way toward functional metropolitan-scale quantum networks, with possible applications expanding to more complex quantum tasks like entanglement routing. Additionally, cross-node phase stabilization displays promise for networks extending over tens of kilometers, potentially expanding the operational geographical boundaries of future quantum networks.

The comprehensive approach, combining single-photon entanglement with dynamic noise suppression, constitutes a pivotal step in proving the feasibility of large-scale, fully functional quantum networks. Subsequent works will likely build upon these results to explore sophisticated multi-node configurations and intricate quantum network protocols.