What Are Quantum Networks?

Quantum networks are communication systems that transmit and distribute quantum information, typically encoded in photonic qubits, between nodes equipped with quantum processors, quantum memories, and classical control electronics. Unlike classical networks, which copy and amplify digital signals, quantum networks must work around the no-cloning theorem, which prohibits creating identical copies of an arbitrary unknown quantum state. Security, scalability, and distance are therefore governed by different physical constraints from those of fiber-optic data networks. The term encompasses everything from metropolitan-scale testbed deployments using fiber optics to proposed satellite constellations for intercontinental quantum key distribution and, at the frontier of the field, the quantum internet, a global infrastructure for distributing entanglement on demand.

The concept gained practical momentum in the 1990s as quantum key distribution protocols such as BB84 and E91 moved from theory to experiment, demonstrating that quantum channels could provide information-theoretically secure key exchange. Since then, the field has expanded to include multipartite entanglement distribution, quantum teleportation over networks, and distributed quantum computing, where quantum processors at separate nodes are linked by entangled channels to solve problems too large for a single device.

Quantum Repeaters

Transmitting quantum states across distances greater than roughly 100 to 200 kilometers in optical fiber is limited by photon loss: a signal that would simply be re-amplified on a classical network cannot be amplified without disturbing the quantum information. Quantum repeaters solve this by dividing the total link distance into shorter segments, establishing entanglement across each segment, and then connecting adjacent segments through a procedure called entanglement swapping, in which a Bell measurement at an intermediate node joins two shorter entangled links into a single longer one. Quantum memories at each repeater node store the entangled states for the duration needed to coordinate the swapping operations. The NIST quantum communications and networks program is actively developing the measurement science needed to certify repeater performance and network protocols.

Entanglement Distribution and Network Architecture

A quantum network consists of nodes, which generate or store entangled states, connected by quantum channels, which transmit photons while preserving quantum coherence. In a link-layer protocol, adjacent nodes establish entangled pairs through photon pair emission and coincidence detection. A network-layer protocol then routes entanglement across multiple hops by chaining swapping operations, analogous in function to IP routing but requiring fundamentally different algorithms because quantum resources cannot be buffered or copied freely. Multipartite entanglement, distributed among three or more nodes simultaneously, enables applications such as quantum conference key agreement and distributed quantum sensing. A detailed treatment of these architectures appears in the arXiv review of quantum internet technologies and protocols.

Protocols and Security

Quantum network protocols operate on two layers simultaneously: the quantum channel, which distributes entangled states or transmits qubits, and a classical authenticated channel, which carries reconciliation messages, timing signals, and error correction data. Security of quantum key distribution over a network depends on correctly bounding information leakage at each node and link. Device-independent protocols, which certify security through Bell inequality violations without trusting the hardware, are the most secure but also the most demanding in terms of detection efficiency and loss tolerance. An accessible survey of quantum communication protocols is available in arXiv literature on quantum communication.

Applications

Quantum networks have applications in a range of fields, including:

• Quantum key distribution over metropolitan and long-haul optical fiber
• Satellite-based global quantum key distribution
• Distributed quantum computing using entangled remote processors
• Quantum clock synchronization and distributed quantum sensing
• Secure multiparty computation with information-theoretic guarantees