Editing Qubit (section)

==Operations on qubits==
{{Further|DiVincenzo's criteria|Physical and logical qubits}} There are various kinds of physical operations that can be performed on qubits.
* [[Quantum logic gate]]s, building blocks for a [[quantum circuit]] in a [[quantum computing|quantum computer]], operate on a set of qubits (a [[quantum register|register]]); mathematically, the qubits undergo a ([[reversible computing|reversible]]) [[unitary transformation]] described by [[matrix multiplication|multiplying]] the quantum gates [[unitary matrix]] with the [[quantum state]] vector. The result from this multiplication is a new quantum state vector.
* [[Quantum measurement]] is an irreversible operation in which information is gained about the state of a single qubit, and [[quantum coherence|coherence]] is lost. The result of the measurement of a single qubit with the state <math>|\psi\rangle = \alpha |0\rangle + \beta |1\rangle</math> will be either <math>|0\rangle</math> with probability <math>|\alpha|^2</math> or <math>|1\rangle </math> with probability <math>|\beta|^2</math>. Measurement of the state of the qubit alters the magnitudes of <var>α</var> and <var>β</var>. For instance, if the result of the measurement is <math>|1\rangle</math>, <var>α</var> is changed to 0 and <var>β</var> is changed to 1, while the phase factor <math>e^{i \phi}</math> is no longer experimentally accessible. If measurement is performed on a qubit that is [[quantum entanglement|entangled]], the measurement may [[Wave function collapse|collapse]] the state of the other entangled qubits.
* Initialization or re-initialization to a known value, often <math>|0\rangle</math>. This operation collapses the quantum state (exactly like with measurement). Initialization to <math>|0\rangle</math> may be implemented logically or physically: Logically as a measurement, followed by the application of the [[Quantum logic gate#X gate|Pauli-X gate]] if the result from the measurement was <math>|1\rangle</math>. Physically, for example if it is a [[Superconducting quantum computing|superconducting]] [[phase qubit]], by lowering the energy of the quantum system to its [[ground state]].
* Sending the qubit through a [[quantum channel]] to a remote system or machine (an [[Input/output|I/O]] operation), potentially as part of a [[quantum network]].