index.txt

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 (including detailed
comments). 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 Kwant usage
-----------------------

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 system: Majorana states
...........................

.. container:: rightside

   .. image:: quantum-wire.png

Kwant allows systems of any dimensionality, for example three-dimensional ones.
This image shows a 3-d model of a semiconducting quantum wire (gray cylinder).
The red region is a tunnel barrier, used to measure tunneling conductance, the
blue region is a superconducting electrode.  In this simulated device, a
Majorana bound state appears close to the superconducting-normal interface.

Taken from: S. Mi, A. R. Akhmerov, M. Wimmer (to be published).


Numerical experiment: flying qubit
..................................

.. container:: rightside

   .. image:: flying-qubit.png

Numerical simulations and experimental results for a flying qubit sample made in
a GaAs/GaAlAs heterostrucutre. The Kwant simulations were performed with
particular attention to a realistic model of the confining potential seen by the
electrons.  This allows for rather subtle aspects of the experiment could be
reproduced.  Such "numerical experiments" can not only be used to interpret the
experimental data but also can help to design the sample geometry and in to
choose the right materials.

Taken from T. Bautze et al., to be submitted to Phys. Rev. B.  See Yamamoto et
al., `Nature Nanotechnology 7, 247 (2012)
<http://dx.doi.org/doi:10.1038/nnano.2012.28>`_ for details about the
experiment.


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

.. container:: leftside

   .. image:: corbino-layout.svg
      :width: 15em


.. container:: rightside

   .. image:: corbino-conductance.png

Transport properties of a Corbino disk across a quantum Hall transition. Left:
geometry of the sample consisting of a ring-shaped two-dimensional electron gas
(grey) in a perpendicular magnetic field.  Right: conductance across the
transition, showing quantized conductance peaks.

Taken from I. C. Fulga, F. Hassler, A. R. Akhmerov, C. W. J. Beenakker,
`Phys. Rev. B 84, 245447 (2011)
<http://link.aps.org/doi/10.1103/PhysRevB.84.245447>`_; `arXiv:1110.4280
<http://arxiv.org/abs/1110.4280>`_.