qutip / qutip-qip / 16297706141

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Bug fix in the VQA module (#280)

* Simplify matplotlib importation error

Just throw the error if package is not installed.

* Fix a bug in vqa test

The overlap computes the inner product, which is unfortunately imaginary in this case, so 1-overlap has always real part 1.

This should never work. However, since the test was initialized in the right answer, pi/2, the optimizer always stopped pretty close to the right answer. So it seldom reports an error. But the changes in SciPy 1.16.0 somehow make the optimizer bolder and explores large values in the beginning, causing the test failing.

* Adapt the test example

This is a trivial example to test the plotting function. Before, it was a VQA with no parameters, but since Scipy 1.16 the minimize function raise an error if there is no value to optimize. So I adapt the test.

* Remove the VQA bitstring cost function

I do not understand how cost function can be based on one bit string sampling. Sampling over quantum state is a statistical process, information is needed for the whole distribution. Sampling one bit string does not tell anything.

The test was passing just because it was initialized in state 1.

* black style fixing

* Fix a importation bug to be compatible with qutip==4.7

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