Figure 1:
Quantum computers operate on a register of quantum objects that store information (qubits). The upper panel shows the difference between classical bits and qubits. Each classical bit takes a definite value while quantum bits can be prepared in superposition of several values forming entangled states (depicted as tilted bits). The lower panel shows the basic concept of quantum computation. An input register of qubits, initialized in some state, is manipulated by a set of quantum gates (unitary transformations of the qubit states) that act on one (orange) or two (green) qubits at a time. The result of the computation is read out at the end of all these processing steps. [Adapted from [34].]