In quantum information theory, a quantum channel is a communication channel which can transmit quantum information, as well as classical information. An example of quantum information is the state of a qubit. An example of classical information is a text document transmitted over the internet.
More formally, quantum channels are completely positive, trace preserving maps between spaces of operators. In other words, a quantum channel is just a quantum operation viewed not merely as the reduced dynamics of a system but as a pipeline intended to carry quantum information.
Read more about Quantum Channel: Memoryless Quantum Channel, In The Experimental Setting, Channel Fidelity, Quantum Channel With Memory
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