Bloch sphere

North on each globe is the state certain to read 0, and south is the state certain to read 1. The third globe puts the arrow on the equator at the |+⟩ state. Its height says 0 and 1 are equally likely, and its longitude says the two amplitudes have the same sign.

The Bloch sphere is a globe used to draw the state of a single qubit, with the state certain to read 0 at the north pole and the state certain to read 1 at the south pole. Height on the globe sets the chances of reading 0 and 1, and the longitude sets the relative phase. A pure state is a point on the surface. A mixed state, which carries some ordinary randomness, is a point inside.

Two qubits do not fit on one globe of this kind, however many posters squeeze them there. The qubit page on this site says what the poles and the phase are for, and the puzzles ask for a named state from the gates alone.

Bloch sphere

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