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Further reading: W. Dür, S. Heusler, What we can learn about quantum physics from a single qubit, (2013)
In station (U1-06) “Heart of quantum mechanics”, we revealed the link between probabilities P■ and P□ and amplitudes in the quantum dimension capable of interference. Thus, the two probabilities are extended each to become a rotating wheel with two parameters: a radius and a phase. This generalisation defines the transition from a classic bit to a quantum bit or qubit.
As a specific example, consider a photon passing a polarising beam splitter Whether the detector “Black” or the detector “White” records the photon is decided by chance.
The rotation frequencies of the turning wheel is proportional to the energy E of the photon. Overall, four real parameters emerge. The “Black” and “White” phases are independent of one another. From conservation of probabilities P■ + P□ =1, it follows that the number of free parameters or degrees of freedom of a qubit in the quantum dimension is reduced to 4-1=3. As only phase differences are observable, all observable states can be represented on a two dimensional sphere – the so-called Bloch sphere.
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