Quantum information theory
In physics and computer science, quantum information is information that is held in the state of a quantum system. Quantum information is the basic entity of study in quantum information theory, and can be manipulated using engineering techniques known as quantum information processing. Much like classical information can be processed with digital computers, transmitted from place to place, manipulated with algorithms, and analyzed with the mathematics of computer science, so also analogous concepts apply to quantum information.
Quantum information differs strongly from classical information, epitomized by the bit, in many striking and unfamiliar ways. Among these are the following:
The study of all of the above topics and differences comprises quantum information theory.
The theory of quantum information is a result of the effort to generalize classical information theory to the quantum world. Quantum information theory aims to investigate the question "How is information stored in a state of a quantum system?". Under the no-teleportation theorem, a quantum state cannot be precisely converted into classical bits. The information content of a message M can nevertheless be measured in terms of the minimum number n of qubits needed to encode the message. Such a message M is encoded with n qubits and n2 classical bits that describe the relative arrangement of the n qubits. The qubit is the smallest possible unit of quantum information.
Quantum information can be transmitted through quantum channels. These have finite capacity and are analogous to the classical case, described by the noisy-channel coding theorem which defines the maximum channel capacity of a classical communications channel. An important breakthrough for the theory of quantum information occurred when quantum error correction codes and fault-tolerant quantum computation schemes were discovered.
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