5.4 Quantum circuits

In this section, you will make use of what you have already learnt about single-qubit and two-qubit gates to construct and interpret quantum circuits. A quantum circuit performs an algorithm in the sense that it executes a predefined sequence of quantum operations that embody an algorithm’s logic. However, unlike classical circuits, quantum circuits rely on principles like superposition and entanglement, and their output is often probabilistic rather than deterministic.

By the end of this section you will be able to predict the outcome of the application of a quantum circuit to a given input state. As well as combining a series of gates, measurements can also be made in quantum circuits when one of the eigenstates of the measurement operator will be obtained with a certain probability. Therefore, the prediction of an output from a quantum circuit may involve more than one possible outcome and the associated probabilities of these outcomes. At the end of this section there is an activity to test your understanding of quantum circuits.