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# 6 types of quantum network topologies

## Quantum networks require efficient architectures to optimize communication and enable large-scale quantum information processing. Learn about the main quantum network topologies.

It's critical to implement an appropriate network topology to design efficient architectures for enterprise-level quantum networks. A relevant quantum networking topology enables quantum infrastructure to optimize the multinode network and process large-scale quantum information.

## Quantum vs. classical network topologies

In classical communication, a network topology is a physical or virtual local arrangement of networking devices to form a network for intercommunication and data flow. Similarly, a quantum networking topology is an arrangement of spatially independent or interconnected nodes that exchange quantum information -- mostly entangled states -- within a quantum network infrastructure.

Quantum network topologies use various arrangements to deploy spatially separated or interconnected nodes in a quantum network. These nodes exchange entangled quantum bits (qubits) and perform quantum computing tasks. The most important function of a quantum networking topology is to provide the path for transmitting and receiving quantum information.

Hypothetically, an efficient quantum network processes large-scale quantum information through multiple nodes located at various points over large distances. The local arrangement of the quantum network must cover long distances to overcome the limitations of classical communication. Practical quantum networks are much smaller.

Unlike classical networking, quantum networking establishes the state of entanglement, which is when two quantum particles interact and become linked. These particles exhibit excited states -- higher energy capable of more efficient computation -- and distribute the states between two or more nodes. A node in a quantum network can be a quantum processor, repeater, sensor, transceiver, gateway or memory. These nodes perform the function of storing, extracting, handling, processing and distributing qubits.

## Different quantum network topologies

To exchange quantum information, two nodes must be coupled. A quantum network topology can have N number of connected nodes, but only permissible nodes are coupled together to exchange quantum information.

Quantum networking uses similar topologies as networking topologies. A few of them are listed below with their effect on the performance of the quantum networks.

### 1. Linear chain network

A linear chain quantum network resembles a chain of coupled nodes because it consists of linearly and sequentially coupled nodes. Each quantum node is coupled to the two nearest neighboring nodes: the predecessor and successor. A linear chain quantum network enables unidimensional data flow from one point to another point.

Implementing a linear chain network topology is among the simplest ways to transfer excitations between two nodes. The sequential order of the linear chain quantum network offers a low rate and limits its scalability to short distances.

### 2. Tree tensor network

A quantum tree tensor network (TTN) represents a branched connection where each node has its own hierarchical tensor. The parent node bifurcates into two or more child nodes, while each secondary node is coupled to two or more tertiary nodes and so on.

Tree tensor topologies are best suited for multiflow quantum infrastructures and have a better framework to represent quantum states. The hierarchical arrangement in tree tensors eliminates the probability of network congestion. But a high number of branches can load the TTN and increase computational cost in the quantum infrastructure.

### 3. Star network

A star quantum network couples a single node to all the other nodes, offering long-range quantum communication. But it's important to note that all the other nodes are not coupled to each other. In a star quantum network, nodes can transfer excited states beyond the local nodes.

Experimental studies have shown that a five-node star quantum network with four entanglement generation sources violates the nonlocality principle, which states that quantum particles can know the states of other particles and match those states. The violation of nonlocality makes the network susceptible to eavesdroppers.

### 4. Ring network

A ring quantum network resembles a ringlike structure where all the nodes are placed in a circular arrangement. Each node is coupled to the preceding and successive nodes, and adding more nodes in the ring makes them scalable in quantum infrastructure. A ring quantum network is also sometimes called a *closed-loop quantum network*.

Quantum cryptography protocols, like quantum key distribution, can be implemented using the ring quantum network topology. But some drawbacks of ring quantum networks include limited capabilities to overcome quantum networking bottlenecks and reduced network efficiency that can result from single-node failure.

### 5. Mesh network

In a mesh quantum network, a single node is coupled to several other nodes. Each node can connect to two or more nodes to offer multipath quantum communication, but the interconnected nodes aren't necessarily neighbors.

A mesh quantum network is a nonhierarchical topology. It tends to reduce hop-by-hop delays in quantum network infrastructure. In case of node failure, quantum information can be transmitted or received through other nodes without hindering quantum communication.

Increasing node numbers and making more internodal connections can enhance the scalability of quantum networks. But a quantum mesh topology can be slightly complex for routing protocols.

### 6. Fully connected network

A fully connected quantum network topology couples every node together. A single node performs the functions of the sender and the receiver to maintain the bidirectional flow of quantum information. This topology forms a highly interconnected, weblike quantum network for efficient communication.

The presence of multiple data paths to send or receive qubits improves network performance. But, because each node is interconnected, a fully connected quantum network can experience congestion, increasing the need to deploy a reliable decongestion protocol. Intersecting routes might predominantly introduce propagation delay and latency.

## Strategies to achieve quantum network efficiency

Fault tolerance, redundancy, scalability and hybrid topologies can all help make quantum networks more efficient.

### Employ fault tolerance

A quantum network is efficient if it continues its operation in faulty conditions due to environmental factors, such as decoherence, interference, signal loss and node failure. The ability of a quantum network to operate during times of unreliable conditions, error and failure is called *fault tolerance*.

Below are some critical steps to succeed with a quantum network:

- Deploy efficient hardware.
- Upgrade software regularly.
- Optimize quantum networking protocols.
- Measure entanglement fidelity.
- Test and troubleshoot.

### Introduce redundancy

In quantum networking, redundancy is the backup plan for quantum information. No cloning theorem states that the information cannot be reproduced. Using various operators and quantum logic gates to manipulate quantum information is crucial for enterprise network efficiency. It's possible to introduce redundancy in error correction codes at the time of processing. Higher redundancy in the quantum network is a performance metric indicator.

### Enhance scalability

It's possible to scale a quantum network by adding repeaters over vast distances. However, some simple topologies cannot provide long-distance quantum communication. Choose bidirectional network topologies to distribute entangled states to increase efficiency and reduce the risk of failure due to node malfunction. Quantum information can easily take another path from the source to reach the destination.

### Implement hybrid network topology

A hybrid quantum network is a mixture of two or more network topologies to customize quantum communication per enterprise requirements. For example, a hybrid quantum network might implement linear chain, tree tensor and star network topologies together. Implementing a hybrid network can seamlessly distribute entanglement in the quantum infrastructure.

*Venus Kohli is an electronics and telecommunications engineer, having completed her engineering degree from Bharati Vidyapeeth College of Engineering at Mumbai University in 2019. Kohli works as a technical writer for electronics, electrical, networking and various other technological categories.*