Quantum entanglement

The second phenomenon that is used in quantum computing is entanglement. This means that the quantum states of two particles are related and dependent on each other, and reading the result of one immediately causes a change in the state of the other. This property of particles makes it possible to create quantum states consisting of more particles, so that the number of simultaneously computed solutions grows exponentially.

Entanglement binds 2 or more qubits together, through any multicubit gate. The simplest example of such a gate might be a controlled X gate (often written as CNOT), in which one qubit controls whether a negation operation is performed to the other.

In the simple circuit presented below, the CNOT gate entangles 2 cubits with each other. Only 2 possible states can appear at the output: |00〉 or |11〉, because despite the probabilistic nature of the measurement, the state of the first cubit determines the state of the second.