Negativity error fixed

[Joan Agusti]

Dear group,

I realized the code for negativity in qutip.entropy only allows us to take partitions with respect to one subsystem. If one has multiple subsystems, one can be interested to study the negativity with respect to multiple partitions. This can be fixed in the code by the following change: instead of creating the mask as

mask = [idx == subsys for idx, n in enumerate(rho.dims[0])]

just create a mask as    

mask=[]
    for idx, n in enumerate(rho_ss.dims[0]):
        if idx in subsys:
            mask.append(True)
        else:
            mask.append(False)

Notice the change idx in subsys, instead idx==subsys.

Now, one is able to define subsys=[0,3,4, etc.] and this will perform the partial transpose with respect to those systems and find the negativity.

[Nathan Shammah]

Dear Joan,

Thanks for sharing! Would you be keen in opening a pull request on the QuTiP GitHub repository about it? If you need help with that do let us know. 

https://qutip.org/docs/latest/development/contributing.html

Bests,

Nathan 

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Dr. Nathan Shammah

www.nathanshammah.com