Why is θ Limited to π/2 in Basis Choice for Distinct States?

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SUMMARY

The discussion centers on the limitation of the angle θ to π/2 when choosing a basis for distinct quantum states ## \ket{\phi_{0}}## and ##\ket{\phi_{1}}##. It is established that the states can be expressed as ## \ket{\phi_{0}}= \cos\frac{\theta}{2}\ket{0} + \sin\frac{\theta}{2}\ket{1}## and ## \ket{\phi_{1}}= \cos\frac{\theta}{2}\ket{0} - \sin\frac{\theta}{2}\ket{1}##, with θ constrained between 0 and π/2. The reasoning is that angles greater than π/2 result in equivalent states through reordering and rephasing, thus not providing new distinct states.

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Homework Statement


Have to read a paper and somewhere along the line it claims that for any distinct ## \ket{\phi_{0}}## and ##\ket{\phi_{1}}## we can choose a basis s.t. ## \ket{\phi_{0}}= \cos\frac{\theta}{2}\ket{0} + \sin\frac{\theta}{2}\ket{1}, \hspace{0.5cm} \ket{\phi_{1}}= \cos\frac{\theta}{2}\ket{0} - \sin\frac{\theta}{2}\ket{1}##

where $$\theta$$ is between 0 and pi/2. Why pi/2? doesn't the upper bound have to be pi so that the inner product of the two can be anywhere between 1 and -1 (rather than between 1 and 0)

Homework Equations

The Attempt at a Solution

 
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The values of the angle > pi/2 is just a reordering and rephasing of the values < pi/2.
 

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