First of all, qutip is not a visualisation library, even though it does provide some visualisation functionalities, mostly leveraging matplotlib.
However, because qutip does provide handy functionalities to plot Bloch spheres and points on it, it does make sense to ask how one can tweak such functionalities to for example add tangent vectors to the bloch ...
Main purpose of Qutip is to explore dynamics of quantum systems and therefore density matrices are the tool to use. According to this answer on Quantum computing, we can model a measurement operator Pi on a density matrix.
In the case of the measurement of a single qubit in the computational basis, you have
$$P_0=|0\rangle\langle 0|\qquad P_1=|1\rangle\...
I found a solution to my problem here https://github.com/qutip/qutip/issues/1027
Apparently plotting points on the Bloch sphere does not work with Matplotlib v3.1.0.
So I had to downgrade to v.3.0.3 for it to work!
That is a very old paper, corresponding to version one of the software. It is now on version 4.x. It is best to see the current documentation for how to use the Bloch sphere: http://qutip.org/docs/latest/guide/guide-bloch.html
Mixed states are represented by density matrices cf. Density matrices for pure states and mixed states. This is the QuTiP documentation for density matrices; check the 4th example. The ket2dm function takes input a ket or bra vector and returns density matrix formed by the outer product. So say for a 50-50 mixture the following code should work:
from qutip ...
QuTiP is not really meant for this I think. As said on the home page :
QuTiP is open-source software for simulating the dynamics of open quantum systems.
Simulating dynamics of open quantum systems by definition means you are interested in the quantum state as a result of your algorithm.
I tried looking at the Notebook examples provided in this Github ...