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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.
[1] [2] A logical qubit is a physical or abstract qubit that performs as specified in a quantum algorithm or quantum circuit [3] subject to unitary transformations, has a long enough coherence time to be usable by quantum logic gates (c.f. propagation delay for classical logic gates). [1] [4] [5]
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.
Hybrid qubit memory is developed. [129] A qubit is stored for over 1 second in an atomic nucleus. [130] Faster electron spin qubit switching and reading is developed. [131] The possibility of non-entanglement quantum computing is described. [132] D-Wave Systems claims to have produced a 128-qubit computer chip, though this claim had yet to be ...
An n-qubit (reversible) quantum gate is a unitary mapping U from the space H QB(n) of n-qubit registers onto itself. Typically, we are only interested in gates for small values of n . A reversible n -bit classical logic gate gives rise to a reversible n -bit quantum gate as follows: to each reversible n -bit logic gate f corresponds a quantum ...
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 } .
Unlike classical digital states (which are discrete), a qubit is continuous-valued, describable by a direction on the Bloch sphere. Despite being continuously valued in this way, a qubit is the smallest possible unit of quantum information, and despite the qubit state being continuous-valued, it is impossible to measure the value precisely ...
The prototypical example of a finite-dimensional Hilbert space is a qubit, a quantum system whose Hilbert space is 2-dimensional. A pure state for a qubit can be written as a linear combination of two orthogonal basis states | 0 {\displaystyle |0\rangle } and | 1 {\displaystyle |1\rangle } with complex coefficients: