Quantum transport simulations made easy
=======================================

Kwant is a Python package for numerical calculations on tight-binding models
with a strong focus on quantum transport.  It is designed to be flexible and
easy to use, while not sacrificing performance.

Tight-binding models are ubiquitous in quantum physics and they can be used to
describe a vast variety of systems and phenomena, such as semiconductors,
metals, graphene, topological insulators, quantum Hall effect,
superconductivity, spintronics, molecular electronics, any combination of the
above and many other things.  While all these systems exhibit very distinct
physical behavior, the underlying mathematical description is very similar.
Kwant has been designed so that the computer simulation of various physical
systems and phenomena is within reach of one software package.

Kwant does not use the traditional input files often found in scientific
software packages. Instead, one writes simple Python programs (using the
Python's simple and very expressive syntax) to define the system and calculate
its quantum properties (conductance, density of states, etc). This workflow is
summarized as follows:

.. image:: kwant_workflow.png
    :width: 90%

Kwant was designed to be easy to use: for example the program that generates
the right panel of the image above is only 42 lines long. Kwant is also
accessible for people without expertise in numerics. To aid that, it is
provided along with a detailed hand-on `tutorial </doc/1.0/tutorial/>`_ and the
Kwant `paper </paper>`_, which describes the guiding principles underlying its
design.

Examples of the use of Kwant
----------------------------

The following examples are mostly taken from real research projects done with
Kwant.  The tutorial_ and the `Kwant paper
<http://downloads.kwant-project.org/doc/kwant-paper.pdf>`_ each contain several
pedagogical examples with line-by-line explanations (`zipfile of all examples
<http://downloads.kwant-project.org/examples/kwant-examples.zip>`_).

3-d systems
...........

.. container:: rightside

   .. image:: quantum_wire_3d.png

.. container:: leftside

   .. image:: bilayer_graphene_3d.png

The left figure shows a piece of bilayer graphene.  On the right, one can see a
quantum wire (gray) to which a superconducting electrode (blue) is attached.
This device has been built in order to give rise to a Majorana bound state close
to the superconducting-normal interface.


Flying qubit
............

.. container:: leftside

   .. image:: flying-qubit.png

This example shows some numerical simulations (left) and experimental results
(right) for a flying Qubit sample made in a GaAs/GaAlAs heterostrucutre. See
Yamamoto et al, Nature Nanotechnology 7, 247 (2012) for details about this
experiment.  (Simulations: T. Bautze et al. to be submitted to
Phys. Rev. B). In this example, particular attention was paid to designing a
realistic model for the confining potential seen by the electrons so that
rather subtle aspects of the experiments could be reproduce. Such type of
"numerical experiments" can not only be used to interpret the experimental data
but also as an aid in designing the sample geometry or in the choice of
materials.


conductance of a Corbino disk in a quantum Hall regime
......................................................

.. container:: leftside

   .. image:: corbino.png

Transport properties of a Corbino disk across a quantum Hall transition. Left
panel: winding number of the reflection matrix, which has to change from 0 to 1
across the transition. Right panel: conductance across the transition, showing
quantized conductance peaks.