Update doc

doc/source/reference/tkwantoperator.rst

@@ -3,7 +3,7 @@ Documentation
 
 .. module:: tkwantoperator
 
-tkwantoperator implements additional operators for `tkwant <https://kwant-project.org/extensions/tkwant/>`__ that calculate energy related expectation values. They can be used with kwant's operators in the module `~kwant.operator`.
+tkwantoperator implements additional operators for `tkwant <https://kwant-project.org/extensions/tkwant/>`__ that calculate energy related expectation values. They can be used in the same way as kwant’s operators in the module `~kwant.operator`.
 
 Helper Classes
 ----------------
@@ -24,3 +24,11 @@ Energy observables
    EnergyDensity
    LeadHeatCurrent
    EnergyCurrentDivergence
+
+.. _article-ref:
+
+Reference article
+"""""""""""""""""
+
+A. Kara Slimane, P. Reck, G. Fleury,
+`Simulating time-dependent thermoelectric transport in quantum systems <https://arxiv.org/abs/1912.09386>`__
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