3 Setting the scene in quantum physics

In this section, some quantum physics needed to study quantum computing is introduced. You will learn about spin-½ particles, which have two fundamental basis states but can also exist in a superposition of these states. This concept is central to quantum mechanics and directly relates to quantum computing, where quantum bits (qubits) similarly have two basis states and can exist in any superpositions of these two states. In the context of spin-½ particles the two states are called spin-up and spin-down; in quantum computing the two states are referred to as logical states. At the end of this section, the essential concept for quantum computing of entanglement is illustrated using spin-states.

• 1 Particles are indistinguishable when they are identical (i.e. they have the same intrinsic properties like mass, charge, and spin) and they are so close together that their wavefunctions overlap so that we cannot tell them apart, even in principle.