Update qsphere plots to show state labels and phases

[aasfaw]

Summary

Theplot_state_qsphere visualization currently shows quantum states without state labels and phase angles. This makes things difficult to interpret when many basis states are being visualized, especially in the case of multiple qubits. This PR does three things:

-- Adds basis state labels to each blob by default. Can be disabled using argument show_state_labels
-- Adds a show_state_phases argument to enable showing phases next to each basis state
-- Adds a use_degrees argument which allows switching to degrees when desired

Details and comments

Previous:

This PR:

show_state_phases = True

show_state_phases = True, use_degrees = True

[maddy-tod]

This definitely makes the q sphere easier to understand - thanks! There are a couple of things I think need adjusting but looks good.

[nonhermitian]

Radians should be default. This is what is used everywhere else in qiskit.

[aasfaw]

Default is now radians

done

[mtreinish]

CI passed here: https://github.com/Qiskit/qiskit-terra/runs/668101667 the checks api looks like it just got stuck. Going to merge this

[TanveshT]

I wasn't able to use these 2 parameters on the Qiskit version that is loaded on the IBM Q Experience: show_state_labels parameter and show_state_phases.
Qiskit Version in IBM Q Experience = '0.14.1'

[HuangJunye]

Is this feature included in the latest 0.19.6 version? I am not able to use it.

[mtreinish]

@HuangJunye This change has not been included in a terra release yet. Qiskit 0.19.6 has terra 0.14.2 which does not have this PR. This will be included in the next release 0.15.0

[HuangJunye]

@mtrrinish Thanks for the reply! Can’t wait to get this feature. It’s so handy for my presentations! I’ll use the unstable version for now :)

[pifparfait]

Hi @mtreinish,

I just read that the mentioned changes are already included. Please could you confirm it?

Many thanks

[aasfaw]

Hi @pifparfait -- the meta package isn't done yet, but the subpackages are. So you probably want to do pip install --upgrade qiskit-terra if you're looking for this change specifically.

[pifparfait]

@aasfaw got it!!! many thanks!!

[pragyakatyayan]

I just updated my qiskit package to 0.21.0. I was trying to plot a q-sphere with all the additions from this PR, but the state |00> and its degree value don't show up in the plot. What am I doing wrong?

[aasfaw]

Hi @pragyakatyayan -- I wrote the PR thinking that the state labels are only shown for states that have non-zero weight. Otherwise, if you show them all the time, then you might end up in situations where the image gets very crowded for many qubits. One option to do what you want is to open a new PR with a new argument to plot_state_qsphere that shows all state labels when set to True

[pragyakatyayan]

Hi @pragyakatyayan -- I wrote the PR thinking that the state labels are only shown for states that have non-zero weight. Otherwise, if you show them all the time, then you might end up in situations where the image gets very crowded for many qubits. One option to do what you want is to open a new PR with a new argument to plot_state_qsphere that shows all state labels when set to True

Thanks @aasfaw, I am on it. The only doubt left is that in the snapshot I submitted shows the label and phase for state |11>, which had zero weight. The experiment I was doing was aimed at getting |01> and |10> with 50% probabilities each, so both |00> and |11> had zero weights. Hence, I got confused when label/phase representation was visible for |11> but not for |00>. I'll dive into it a little deeper, may be I find what led to this particular case.