Quantum channel

Quantum channel is the physical medium or transmission path used to convey quantum states between communicating parties in a quantum communication protocol. Unlike a classical communication channel, which carries information that can be copied, amplified, or intercepted without fundamental disturbance, a quantum channel is constrained by the no-cloning theorem: any attempt to measure or copy the quantum states necessarily alters them.

Quantum channels are implemented using various physical carriers: photons transmitted through optical fiber or free space, ions in trap arrays, or superconducting circuits. The dominant implementation for quantum key distribution uses single photons or weak coherent laser pulses, typically at telecom wavelengths (1550 nm) for fiber compatibility.

The defining property of a quantum channel is its capacity to preserve quantum coherence — the superposition and entanglement properties that distinguish quantum information from classical. Decoherence from environmental noise, absorption, scattering, and detector inefficiency are the principal obstacles to practical quantum communication. The engineering of quantum channels is thus not merely a problem of signal transmission but of maintaining fragile quantum states across macroscopic distances.