Quantum strategies to overcome classical multiplexing limits
Tzula B. Propp, Jeroen Grimbergen, Emil R. Hellebek, Junior R. Gonzales-Ureta, Janice van Dam, Joshua A. Slater, Anders Søndberg Sørensen, Stephanie Wehner
Near-term quantum networks face a bottleneck due to low quantum communication rates. This degrades performance both by lowering operating speeds and increasing qubit storage time in noisy memories, making some quantum internet applications infeasible. One way to circumvent this bottleneck is multiplexing: combining multiple signals into a single signal to improve the overall rate. Standard multiplexing techniques are classical in that they do not make use of coherence between quantum channels nor account for decoherence rates that vary during a protocol's execution. In this paper, we first derive semiclassical limits to multiplexing for many-qubit protocols and then introduce two techniques: single click quantum multiplexing and multiserver multiplexing. These can enable beyond-classical multiplexing advantages. We illustrate these techniques through three example applications: (1) entanglement generation between two asymetric quantum network nodes (i.e., repeaters or quantum servers with unequal memories), (2) remote state preparation between many end user devices and a single quantum node, and (3) remote state preparation between one end user device and many internetworked quantum nodes. By utilizing many noisy internetworked quantum devices instead of fewer low-noise devices, our multiplexing strategies enable new paths towards achieving high-speed many-qubit quantum network applications.
Near-term quantum networks face a bottleneck due to low quantum communication rates. This degrades performance both by lowering operating speeds and increasing qubit storage time in noisy memories, making some quantum internet applications infeasible. One way to circumvent this bottleneck is multiplexing: combining multiple signals into a single signal to improve the overall rate. Standard multiplexing techniques are classical in that they do not make use of coherence between quantum channels nor account for decoherence rates that vary during a protocol's execution. In this paper, we first derive semiclassical limits to multiplexing for many-qubit protocols and then introduce two techniques: single click quantum multiplexing and multiserver multiplexing. These can enable beyond-classical multiplexing advantages. We illustrate these techniques through three example applications: (1) entanglement generation between two asymetric quantum network nodes (i.e., repeaters or quantum servers with unequal memories), (2) remote state preparation between many end user devices and a single quantum node, and (3) remote state preparation between one end user device and many internetworked quantum nodes. By utilizing many noisy internetworked quantum devices instead of fewer low-noise devices, our multiplexing strategies enable new paths towards achieving high-speed many-qubit quantum network applications.
Abstract Remote quantum entanglement can enable numerous applications including distributed quantum computation, secure communication, and precision sensing. We consider how a quantum network—nodes equipped with limited quantum processing capabilities connected via lossy optical links—can distribute high-rate entanglement simultaneously between multiple pairs of users. We develop protocols for such quantum “repeater” nodes, which enable a pair of users to achieve large gains in entanglement rates over using a linear chain of quantum repeaters, by exploiting the diversity of multiple paths in the network. Additionally, we develop repeater protocols that enable multiple user pairs to generate entanglement simultaneously at rates that can far exceed what is possible with repeaters time sharing among assisting individual entanglement flows. Our results suggest that the early-stage development of quantum memories with short coherence times and implementations of probabilistic Bell-state measurements can have a much more profound impact on quantum networks than may be apparent from analyzing linear repeater chains. This framework should spur the development of a general quantum network theory, bringing together quantum memory physics, quantum information theory, quantum error correction, and computer network theory.
Abstract Quantum networks distributed over distances greater than a few kilometres will be limited by the time required for information to propagate between nodes. We analyse protocols that are able to circumvent this bottleneck by employing multi-qubit nodes and multiplexing. For each protocol, we investigate the key network parameters that determine its performance. We model achievable entangling rates based on the anticipated near-term performance of nitrogen-vacancy centres and other promising network platforms. This analysis allows us to compare the potential of the proposed multiplexed protocols in different regimes. Moreover, by identifying the gains that may be achieved by improving particular network parameters, our analysis suggests the most promising avenues for research and development of prototype quantum networks.
The Internet has revolutionized the way we live. It has enabled applications far beyond what it was originally built for, and it will continue to exceed our expectations for the future. Quantum computers, and the network that will connect them—the Quantum Internet— are likely going to follow the same path. Differently from normal computers, quantum computers can share a property called entanglement, which allows the qubits (quantum bits) to be connected at a much more fundamental level. This property enables a range of new applications that span secure communication to enhanced metrology. Over the past fifteen years, significant progress has been made in connecting rudimentary quantum network nodes via long-distance entanglement. Several quantum platforms have demonstrated entanglement generation between two physically separated qubits. In this thesis we take a significant step forward, both in terms of experimental complexity achieved, and in the abstraction of said complexity for future developments. Moving past two-node experiments required a fundamental redesign of our experimental apparatus, as well as developing the capabilities to control simultaneously the additional node. The first result of this thesis is building a three-node entanglement-based quantum network. We demonstrated distribution of Greenberger-Horne-Zeilinger states over the network, as well as a building block for larger networks: entanglement swapping. Differently from previous multi-node demonstrations, which relied on post-selection, our network is able to perform the entanglement distribution in a heralded fashion: a signal will notify the users that the protocol was successful, and that the state is ready to be used. The second result builds on the first, by adding control over a fifth qubit, improving the quality of the entanglement, and introducing a novel repetitive readout technique, to achieve quantum teleportation of a qubit from the third node to the first—nodes that do not share a direct entanglement channel. The third and final result is the demonstration of entanglement delivery using a quantum network stack. The Internet is built using a plethora of physical platforms: optical fibers, Ethernet cables, Wi-Fi, satellite signals etc. To abstract their functionality, and make applications work regardless of the underlying platform, a layered approach was developed in the 1970s (the Internet protocol). Taking inspiration from classical network stacks, we demonstrate the first two layers of a quantum network stack, the physical layer (where the qubits, lasers and signal generators live), and the link layer, which abstracts the concepts of qubit and entanglement generation such that they can be used by applications at the higher-layers, hiding the complexity of the quantum platform being used.
We consider quantum and private communications assisted by repeaters, from the basic scenario of a single repeater chain to the general case of an arbitrarily-complex quantum network, where systems may be routed through single or multiple paths. In this context, we investigate the ultimate rates at which two end-parties may transmit quantum information, distribute entanglement, or generate secret keys. These end-to-end capacities are defined by optimizing over the most general adaptive protocols that are allowed by quantum mechanics. Combining techniques from quantum information and classical network theory, we derive single-letter upper bounds for the end-to-end capacities in repeater chains and quantum networks connected by arbitrary quantum channels, establishing exact formulas under basic decoherence models, including bosonic lossy channels, quantum-limited amplifiers, dephasing and erasure channels. For the converse part, we adopt a teleportation-inspired simulation of a quantum network which leads to upper bounds in terms of the relative entropy of entanglement. For the lower bounds we combine point-to-point quantum protocols with classical network algorithms. Depending on the type of routing (single or multiple), optimal strategies corresponds to finding the widest path or the maximum flow in the quantum network. Our theory can also be extended to simultaneous quantum communication between multiple senders and receivers.
The scalability of quantum communication networks is crucial for enabling secure information exchange and distributed quantum computing across global infrastructures. However, achieving scalability remains a significant challenge due to the effects of decoherence, photon loss, and fidelity degradation over extended distances. This paper performs a critical investigation into the scalability limits of quantum communication networks, focusing on the interplay between entanglement generation rates, end-to-end fidelity, and network parameters such as node separation, noise levels, and total distance. Leveraging the SeQUeNCe simulation framework, we model adaptable trapped-ion repeater networks to uncover fundamental insights into both homogeneous and heterogeneous configurations. Our findings reveal transformative insights that help reshape system designs for near term quantum networks. We identify maximum node separation thresholds critical for practical implementations, illuminate the non-linear degradation of fidelity with increasing distance, and expose how routing and swapping protocols influence entanglement success rates at scale. Most notably, we demonstrate that while additional repeaters can mitigate distance-induced failures, excessive node counts introduce compounding fidelity losses that threaten network viability under certain architectures. These results provide a roadmap for revolutionizing quantum networks, with actionable design strategies such as optimized node placement, advanced routing algorithms, and the adoption of cutting-edge qubit technologies like topological qubits. By addressing key bottlenecks in quantum communication scalability, our work lays the foundation for the next generation of global quantum networks.
Quantum networking allows the transmission of information in ways unavailable in the classical world. The information encoded in a single qubit can now be split and transmitted in a coherent way over different routes. This aggregation allows information to be transmitted in a fault tolerant way between different parts of the quantum network (or the future internet), even when that is not achievable with a single path approach. It is a quantum phenomenon not available in conventional telecommunication networks either. We show how the multiplexing of independent quantum channels allows a distributed form of quantum error correction to protect the transmission of quantum information between nodes or users of a quantum network. Combined with spatial-temporal single-photon multiplexing, we observe a significant drop in network resources required to transmit that quantum signal, even when only two channels are involved. This work goes far beyond the concepts of channel capacities and shows how quantum networking may operate in the future.