The Bloch Sphere
The Bloch sphere plots one pure qubit as a point on a globe: latitude sets the 0/1 probability balance, longitude records relative phase, and gates are rotations. The picture is exact for a single qubit and does not extend to entangled multi-qubit states.
What is the Bloch sphere?
The Bloch sphere is a picture: every possible state of one qubit — one pure state, meaning a state described exactly by two amplitudes with no classical uncertainty mixed in — corresponds to a point on the surface of a globe.
The north pole is |0⟩. The south pole is |1⟩. Points on the equator are the balanced 50/50 states, including |+⟩ and |−⟩ from earlier lessons.
Think of it as a debugger view for a single qubit: gates become rotations of the globe, and two hard-to-picture complex amplitudes become one point you can track.
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