
Learn Quantum Computing with Python and IBM Quantum
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Another visual representation of a qubit and its states is the Bloch sphere, named after the physicist Felix Bloch. The Bloch sphere is an ordinary three-dimensional sphere that’s generally used as a geometrical representation of the qubit. By this, we mean the sphere can represent a qubit’s state as a point anywhere on the surface of the Bloch sphere. As described in the previous chapter, the basis states are represented by the north and south poles.
Conventionally, the north pole of the Bloch sphere represents the state, while the south pole represents the
state. Any point on the surface of the Bloch sphere can represent the linear combination of states as a unit vector from the center (origin) to the surface of the Bloch sphere.
Since we have the quantum mechanical constraint that the total probability of the vector must equal 1, we get the following formula:
The vector can then only rotate around the and...