Processing and Transmission of Quantum Information

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Quantum information consists of qubits rather than bits. A qubit is a two-level quantum system where the levels represent 0 and 1. Unlike a classical bit, it can exist in a superposition of 0 and 1, and this property makes novel kinds of information processing possible. Proposed applications are quantum cryptography, superdense coding, and quantum computing. Qubits can be processed by devices as a quantum gates. A universal quantum device is one whose performance does not depend on the input state. An example, which has all ready been studied, is a quantum copier, which produces two imperfect copies of an input qubit. Other universal devices are a universal NOT gate, a universal entangler, a universal disentangler, and a universal transposer. All of these devices should be relatively simple and able to be realized by a small number of quantum gates. Multilevel and continuous systems can also be used to represent quantum information. Squeezed states can be used as the basis of quantum cryptographic protocol which would allow amplifiers to be used to overcome transmission losses.