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The general definition of a qubit as the quantum state of a two-level quantum system.In quantum computing, a qubit (/ ˈ k juː b ɪ t /) or quantum bit is a basic unit of quantum information—the quantum version of the classic binary bit physically realized with a two-state device.
A logical qubit specifies how a single qubit should behave in a quantum algorithm, subject to quantum logic operations which can be built out of quantum logic gates. However, issues in current technologies preclude single two-state quantum systems , which can be used as physical qubits, from reliably encoding and retaining this information for ...
An example is a qubit used in quantum information processing. A qubit state is most generally a superposition of the basis states | 0 {\displaystyle |0\rangle } and | 1 {\displaystyle |1\rangle } :
Just as the bit is the basic concept of classical information theory, the qubit is the fundamental unit of quantum information.The same term qubit is used to refer to an abstract mathematical model and to any physical system that is represented by that model.
It is usually assumed that the register consists of qubits. It is also generally assumed that registers are not density matrices, but that they are pure, although the definition of "register" can be extended to density matrices. An size quantum register is a quantum system comprising pure qubits.
The purpose of quantum computing focuses on building an information theory with the features of quantum mechanics: instead of encoding a binary unit of information (), which can be switched to 1 or 0, a quantum binary unit of information (qubit) can simultaneously turn to be 0 and 1 at the same time, thanks to the phenomenon called superposition.
Qubit is somewhat analogous to the bit in classical computation. Qubits can be in a 1 or 0 quantum state , or they can be in a superposition of the 1 and 0 states. However, when qubits are measured, the result of the measurement is always either a 0 or a 1; the probabilities of these two outcomes depend on the quantum state that the qubits were ...
Example: The qubit is measured, and the result of this measurement is a Boolean value, which is consumed by the classical computer. If ϕ {\displaystyle \phi } measures to 1, then the classical computer tells the quantum computer to apply the U gate on ψ {\displaystyle \psi } .