Measuring Entangled Qubits

The CNOT (Controlled NOT) gate uses a control input (the solid black circle) to determine the target input (the XOR symbol). If the control input is , then the target output is the same as the target input. If the control input is , then the target output is inverted. In this circuit, the control input is in an entangled state, being both and at the same time (something that can occur only in quantum, not classical, computing). The Hadamard gate (the H box) is represented by the 2×2 unitary matrix , which takes a single qubit from the upper input and produces a single qubit output.

The intermediate state after the Hadamard operation is also displayed. Quantum wire coloring, unique for each qubit state, lets you see the qubit state as time moves from left to right. The dashed line indicates an entangled state, and common coloring indicates a common qubit state.

Reference:

P. Kaye, R. Laflamme, M. Mosca, An Introduction to Quantum Computing, New York: Oxford University Press, 2007.