QuantumOptics.jl: simulating quantum systems in Julia

A Julia framework for quantum system simulation

The README describes QuantumOptics.jl as a numerical framework written in Julia that makes it easy to simulate various kinds of quantum systems, and the repository description is more precise: a library for the numerical simulation of closed as well as open quantum systems.

That pair of words in the description is the whole scope. A closed system evolves on its own, and an open one exchanges energy or particles with an environment, which in the formalism means a dissipator term alongside the Hamiltonian. Simulating the open case is where the numerics get interesting, because you are integrating a Lindblad equation rather than a Schrödinger equation, and the difference is not a detail of implementation but a different model of the system.

The lineage is stated too. The framework is inspired by the Quantum Optics Toolbox for MATLAB and by the Python framework QuTiP. That tells you two things: the vocabulary and the API shape come from a lineage people already know, and the point of a Julia version is to bring that vocabulary to the Julia ecosystem, where it can share a type system and a compiler with the rest of a numerical stack.

The homepage is qojulia.org, and the stable documentation lives at docs.qojulia.org, with a build badge and a codecov coverage badge in the README pointing at both.

Five repositories, one organisation

The Project Structure section is a map of the whole effort, and it is worth reading because it tells you where to look for the thing you want.

The main code is in this repository. The documentation source is under docs/src in the same repository, and the examples are under docs/examples, also here. The benchmarks are in a separate repository, QuantumOptics.jl-benchmarks, and the website is in another, QuantumOptics.jl-website, which is where the logo and the site pages come from.

All of them sit under the qojulia organisation on GitHub, which is the marker to use when you are looking for the framework elsewhere and you want to be sure you have found the maintained version rather than a copy.

The split of benchmarks into their own repository is the interesting choice. Benchmark results are only meaningful relative to a machine and a Julia version, and separating them keeps the main repository's history clean of large result files while giving the benchmark suite a home of its own.

The repository root also holds a Project.toml, which is the Julia package definition, a CITATION.bib alongside the paper reference, a LICENSE.md, and a CLAUDE.md. There is a `.buildkite` directory for continuous integration alongside the GitHub workflows that the README badge points at, and separate src and test directories for the code and its tests.

The README documents use and citation, not the API

It is worth being explicit about what this README does not contain, because the absence is unusual and it shapes how you approach the project.

There are no installation instructions, no code example showing a simulation, and no description of the API surface. The library's own details live in the documentation at docs.qojulia.org, the examples under docs/examples, and the project website at qojulia.org, which the README describes as the place for more information, documentation and examples.

What the README does cover is the surrounding project. There is a Discussion and Help section that says bug reports and feature requests go to the GitHub issue tracker, and that general discussion and help belongs on the Julia Discourse forum under the quantum topic, with a `#quantum-optics` channel on the Julia Slack for real-time conversations. Contributions of any kind are welcome, whether ideas, bug reports or pull requests, and there is an Office Hours section describing a scheduling link where anyone can bring questions or suggestions.

For a numerical library that is a deliberate division of labour, and it is the norm for scientific software: the README orients you, the documentation teaches you, and the examples show you. The one thing you should not expect to find anywhere in the repository is a quick tour of the API, because the type names and function signatures are the documentation's job.

Citing the paper is part of the workflow

The Citing section asks for two things, and the first one is unusual for an open source project. If you like QuantumOptics.jl, the README asks you to star the repository to help increase its visibility, and if you find the framework useful in your research it asks you to cite the publication, with the bibtex entry included.

The entry is the Computer Physics Communications paper, QuantumOptics.jl: A Julia framework for simulating open quantum systems, by Krämer, Plankensteiner, Ostermann and Ritsch, volume 227, pages 109 to 116, 2018, published by Elsevier:

@article{kramer2018quantumoptics,
  title={QuantumOptics. jl: A Julia framework for simulating open quantum systems},
  journal={Computer Physics Communications},
  volume={227},
  pages={109--116},
  year={2018},
  publisher={Elsevier}
}

The repository also carries a CITATION.bib at the root, so the citation is available to tooling as well as to copy and paste.

This is a small but meaningful signal about the project. A library that asks for a paper citation in its README is a library whose primary audience is academic, and the software is a means to a paper rather than the other way round. That also tells you what to expect from the documentation, which is written for readers who already know the physics, and why the examples under docs/examples matter more here than they would in a general purpose library.

Patch releases, a moving branch, and a custom licence file

The recent releases are v1.2.10 from 2026-09-11, v1.2.9 from 2026-09-06 and v1.2.8 from 2026-08-20, so the project is on a 1.2 line and shipping patch releases steadily. The repository is not archived and the last push was on 2026-10-13.

That last date is a warning rather than a promise. The README badge points at the stable documentation, and the version badge points at the releases, so the intended reading path is to install a tagged release and read the docs for that version. Working from the master branch of a Julia package that is shipping patch releases every few days is a choice with a cost, since the API can move under you between a tagged release and a branch you cloned last week.

The licence is where the repository is least clear. The licence is recorded as a custom one that GitHub cannot classify, and the LICENSE.md file at the root is the text to read. For a library published out of a research group and carrying a paper citation, a bespoke licence is not unusual, but it does mean you have to read it rather than assume a standard permissive grant, and in particular you should check what it says about commercial use and redistribution before you build a product on top of it.

There is no changelog file in the root, so the release history on the repository is the record of what changed between the patch releases.

Editorial conclusion

QuantumOptics.jl fits a physicist or graduate student who wants a Julia-native way to simulate open and closed quantum systems and who wants an example-driven API rather than a paper to implement from. It is the wrong choice if your code is in Python and already works with QuTiP, since the value here is the Julia ecosystem and the examples, not a different model. Verify first which Julia version the current release supports, since the README does not state it, and read the stable documentation at docs.qojulia.org rather than the master branch, because the repository is moving.

Frequently asked questions

Official sources

1. Issues
2. Project website
3. qojulia/QuantumOptics.jl on GitHub
4. README
5. Releases