Quantum Communications is the field concerned with transmitting, processing and distributing information using quantum states and quantum information principles. Unlike conventional communications, where information is represented as classical electromagnetic signals and can generally be copied or amplified without fundamentally changing its logical content, quantum communication must account for measurement disturbance, no-cloning, entanglement, probabilistic measurement and the fragility of quantum states.
Photons are the most common physical carriers because they can propagate through optical fibre and free space, although other quantum systems can serve as memories, interfaces or network nodes. Quantum Communications includes Quantum Key Distribution, quantum teleportation, entanglement distribution, quantum state transfer, quantum repeaters and emerging quantum-network architectures. It does not imply that all communications become faster than conventional communications, and quantum mechanics does not permit usable information to be transmitted faster than light. The distinctive value is instead the ability to transmit or manipulate quantum information and to create security or networking capabilities that depend on quantum physical laws.
Quantum communication channels are usually accompanied by classical communication channels. A quantum state may travel through fibre, free space or another physical medium, while classical messages coordinate measurements, authentication, error handling and protocol decisions. The quantum channel cannot generally be treated like an ordinary optical data link because unknown quantum states cannot be copied perfectly and conventional optical amplification introduces limitations that do not arise in the same way for classical signals.
Photon loss is particularly important because attenuation reduces the probability that a transmitted quantum state reaches its destination. Detectors also have finite efficiency and produce dark counts and other errors. Consequently, quantum communication engineering focuses heavily on source quality, transmission loss, detector performance, timing synchronisation, environmental isolation and statistical security analysis.
Quantum entanglement is a major resource in Quantum Communications. Two or more quantum systems can share a joint state whose correlations cannot be represented as independent local states. Entanglement can be distributed between distant locations and subsequently used for protocols such as quantum teleportation, entanglement-based key distribution and future quantum-network operations. Quantum teleportation does not transport matter or information instantaneously.
Instead, an unknown quantum state can be transferred from one location to another using a shared entangled state together with classical communication, with the receiver applying a correction determined by the classical measurement result. The original state is destroyed during the protocol, consistent with the no-cloning theorem. Teleportation therefore illustrates how quantum information can be transferred without physically transporting the original quantum system itself, while still respecting relativistic limits on communication.
Quantum Key Distribution is the most developed application associated with Quantum Communications. Protocols such as BB84 use quantum states whose measurement properties allow communicating parties to detect certain forms of interception and establish a shared secret key after classical post-processing. Other approaches use entanglement or continuous-variable optical states. QKD is distinct from general-purpose quantum networking because its objective is cryptographic key establishment rather than arbitrary quantum-state communication. It also differs from post-quantum cryptography, which uses classical computational algorithms designed to resist attacks by quantum computers. Quantum Communications can therefore encompass security applications, networking applications and quantum-state-transfer applications, each with different physical requirements and security models.
Transmission loss is one of the fundamental obstacles to long-distance Quantum Communications. In optical fibre, the probability of successful transmission declines as distance increases, while free-space systems face atmospheric absorption, turbulence, background light and pointing requirements. Classical repeaters cannot simply solve the problem by copying and amplifying an unknown quantum state because quantum information cannot be perfectly cloned.
Quantum repeaters are designed to address this limitation by dividing long links into shorter segments, generating entanglement over those segments and using entanglement swapping and quantum memories to extend correlations over larger distances. Practical repeaters require high-quality memories, efficient interfaces, reliable entanglement generation and low-error operations. Large-scale quantum communication networks therefore depend on progress in several technologies simultaneously rather than simply installing more optical fibre.
Quantum communication security depends on both protocol mathematics and physical implementation. A theoretically secure protocol can be undermined by detector vulnerabilities, source imperfections, side channels, compromised classical authentication or flawed key management. Security proofs specify assumptions about the adversary and derive bounds on information leakage or failure probability. Device-independent and measurement-device-independent approaches seek to reduce dependence on trusting particular hardware components, but they introduce more demanding experimental requirements. Quantum Communications therefore cannot be described accurately as automatically secure merely because photons are involved. The security or reliability of a system must be evaluated from the complete architecture, including state preparation, transmission, detection, classical control, authentication, software and endpoint security.
Future Quantum Communications is often described in terms of quantum networks linking quantum processors, sensors and memories. A quantum network would need to support the generation, distribution, storage, routing and manipulation of quantum states or entanglement. Quantum memories would temporarily store quantum information while other network operations complete, and quantum interfaces could translate information between different physical platforms. Network architectures may contain metropolitan links, long-distance fibre, free-space links and potentially satellite components. The required protocols differ from conventional Internet protocols because quantum information cannot simply be duplicated for backup or broadcast to arbitrary recipients. Network management must therefore account for entanglement generation probabilities, fidelity, memory lifetime, synchronisation and probabilistic operations.
The significance of Quantum Communications lies in extending quantum information processing beyond an isolated device. It provides the physical basis for secure key distribution, entanglement-based protocols and eventual interconnected quantum processors and sensors. Its development combines quantum optics, information theory, photonic engineering, cryogenics, communications engineering, cryptography and network architecture. Major practical challenges include photon loss, detector inefficiency, quantum-memory limitations, heterogeneous hardware interfaces, error accumulation and the cost of specialised infrastructure.
The field should therefore be understood not as a replacement for conventional communications but as a new communication layer for tasks in which quantum states themselves are valuable. Classical networking will remain essential because quantum channels require classical coordination and because most applications ultimately interact with classical information systems.
