Underdamped terminator

[cerisola]

The current PR is a follow up of #124 which adds the terminator for underdamped spectral densities.

I have not been able to find analytical expressions for the terminator in the literature (or other packages, QuTiP lacks it as well), so I derived it myself. I attach here the calculation for reference / cross-checking:

I have tested the implemented terminator for a couple of different Hamiltonians (2, 3, and 4 level systems) and a few different parameter sets and they seem to work correctly.

Here I attach an example for the case of a spin-boson model:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.69%. Comparing base (a62be5c) to head (b5d847c).
Report is 6 commits behind head on main.

Additional details and impacted files

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##             main     #131   +/-   ##
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  Coverage   91.68%   91.69%           
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  Files          24       24           
  Lines        1480     1481    +1     
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+ Hits         1357     1358    +1     
  Misses        123      123           

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