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Overhaul top-level README and convert to Markdown
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This is a major overhaul of the main Cirq README file. Much of the
content is rewritten, and there are numerous additions and
enhancements all over. The changes include but are not limited to:

- Updates and corrections
- Addition of new sections
- Centering and resizing the logo for a more professional look
- Addition of GitHub badges for extra "pizzazz"
- Addition of pointers to related Quantumlib software
- Addition of a table of contents
- Improvements to info about how to cite Cirq
- Conversion to Markdown format to allow formatting that is
  compatible with both GitHub and PyPI

This is a follow-on to now-closed PRs quantumlib#6903 and quantumlib#6901, which concerned
making roughly the same changes while keeping with the
reStructuredText format of the previous README file. Switching to
Markdown allows use of some simple raw HTML to achieving formatting
that is not possible in pure Markdown, and still do so in a way that
works on both GitHub and PyPI.
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# <!-- H1 title omitted because our logo acts as the title. -->

<div align="center">
<img width="280px" height="135px" alt="Cirq logo"
src="https://raw.githubusercontent.com/quantumlib/Cirq/refs/heads/main/docs/images/Cirq_logo_color.svg">

Python package for writing, manipulating, and running [quantum
circuits](https://en.wikipedia.org/wiki/Quantum_circuit) on quantum computers
and simulators.

[![Licensed under the Apache 2.0
license](https://img.shields.io/badge/License-Apache%202.0-3c60b1.svg?logo=opensourceinitiative&logoColor=white&style=flat-square)](https://github.com/quantumlib/Cirq/blob/main/LICENSE)
[![Compatible with Python versions 3.10 and
higher](https://img.shields.io/badge/Python-3.10+-fcbc2c.svg?style=flat-square&logo=python&logoColor=white)](https://www.python.org/downloads/)
[![Cirq project on
PyPI](https://img.shields.io/pypi/v/cirq.svg?logo=python&logoColor=white&label=PyPI&style=flat-square&color=fcbc2c)](https://pypi.org/project/cirq)
[![Archived in
Zenodo](https://img.shields.io/badge/10.5281%2Fzenodo.4062499-gray.svg?label=DOI&style=flat-square&colorA=gray&colorB=3c60b1)](https://doi.org/10.5281/zenodo.4062499)

[Features](#features) &ndash;
[Installation](#installation) &ndash;
[Quick Start](#quick-start--hello-qubit-example) &ndash;
[Documentation](#cirq-documentation) &ndash;
[Integrations](#integrations) &ndash;
[Community](#community) &ndash;
[Citing Cirq](#citing-cirq) &ndash;
[Contact](#contact)

</div>

## Features

Cirq provides useful abstractions for dealing with today’s [noisy
intermediate-scale quantum](https://arxiv.org/abs/1801.00862) (NISQ) computers,
where the details of quantum hardware are vital to achieving state-of-the-art
results. Some of its features include:

* Flexible gate definitions and custom gates
* Parameterized circuits with symbolic variables
* Circuit transformation, compilation and optimization
* Hardware device modeling
* Noise modeling
* Multiple built-in quantum circuit simulators
* Integration with [qsim](https://github.com/quantumlib/qsim) for
high-performance simulation
* Interoperability with [NumPy](https://numpy.org) and
[SciPy](https://scipy.org)
* Cross-platform compatibility

## Installation

Cirq supports Python version 3.10 and later, and can be used on Linux, MacOS,
and Windows, as well as [Google Colab](https://colab.google). For complete
installation instructions, please refer to the
[Install](https://quantumai.google/cirq/start/install) section of the online
Cirq documentation.

## Quick Start – “Hello Qubit” Example

Here is a simple example to get you up and running with Cirq after you have
installed it. Start a Python interpreter, and then type the following:

```python
import cirq

# Pick a qubit.
qubit = cirq.GridQubit(0, 0)

# Create a circuit.
circuit = cirq.Circuit(
cirq.X(qubit)**0.5, # Square root of NOT.
cirq.measure(qubit, key='m') # Measurement.
)
print("Circuit:")
print(circuit)

# Simulate the circuit several times.
simulator = cirq.Simulator()
result = simulator.run(circuit, repetitions=20)
print("Results:")
print(result)
```

Python should then print output similar to this:

```text
Circuit:
(0, 0): ───X^0.5───M('m')───
Results:
m=11000111111011001000
```

Congratulations! You have run your first quantum simulation in Cirq. You can
continue to learn more by exploring the [many Cirq tutorials](#tutorials)
described below.

## Cirq Documentation

The primary documentation site for Cirq is the [Cirq home page on the Quantum
AI website](https://quantumai.google/cirq). There and elsewhere, a variety of
documentation for Cirq is available.

### Tutorials

* [Video
tutorials](https://www.youtube.com/playlist?list=PLpO2pyKisOjLVt_tDJ2K6ZTapZtHXPLB4)
on YouTube are an engaging way to learn Cirq.
* [Jupyter notebook-based
tutorials](https://colab.research.google.com/github/quantumlib/Cirq) let you
learn Cirq from your browser – no installation needed.
* [Text-based tutorials](https://quantumai.google/cirq) on the Cirq home page
are great when combined with a local
[installation](https://quantumai.google/cirq/start/install) of Cirq on your
computer. After starting with the
[basics](https://quantumai.google/cirq/start/basics), you'll be ready to dive
into tutorials on circuit building and circuit simulation under the
[Build](https://quantumai.google/cirq/build) and
[Simulate](https://quantumai.google/cirq/simula) tabs, respectively. Check
out the other tabs for more!

### Reference Documentation

* Docs for the [current stable
release](https://quantumai.google/reference/python/cirq/all_symbols)
correspond to what you get with `pip install cirq`.
* Docs for the
[pre-release](https://quantumai.google/reference/python/cirq/all_symbols?version=nightly)
correspond to what you get with `pip install cirq~=1.0.dev`.

### Examples

* The [examples subdirectory](./examples/) of the Cirq GitHub repo has many
programs illustrating the application of Cirq to everything from common
textbook algorithms to more advanced methods.
* The [Experiments page](https://quantumai.google/cirq/experiments/) on the
Cirq documentation site has yet more examples, from simple to advanced.

### Change log

* The [Cirq releases](https://github.com/quantumlib/cirq/releases) page on
GitHub lists the changes in each release.

## Integrations

Google Quantum AI has a suite of open-source software that lets you do more
with Cirq. From high-performance simulators, to novel tools for expressing and
analyzing fault-tolerant quantum algorithms, our software stack lets you
develop quantum programs for a variety of applications.

<div align="center">

| Your interests | Software to explore |
|-------------------------------------------------|----------------------|
| Quantum algorithms?<br>Fault-tolerant quantum computing (FTQC)? | [Qualtran](https://github.com/quantumlib/qualtran) |
| Large circuits and/or a lot of simulations? | [qsim](https://github.com/quantumlib/qsim) |
| Circuits with thousands of qubits and millions of Clifford operations? | [Stim](https://github.com/quantumlib/ssim) |
| Quantum error correction (QEC)? | [Stim](https://github.com/quantumlib/ssim) |
| Chemistry and/or material science? | [OpenFermion](https://github.com/quantumlib/openfermion)<br>[OpenFermion-FQE](https://github.com/quantumlib/OpenFermion-FQE)<br>[OpenFermion-PySCF](https://github.com/quantumlib/OpenFermion-PySCF)<br>[OpenFermion-Psi4](https://github.com/quantumlib/OpenFermion-Psi4) |
| Quantum machine learning (QML)? | [TensorFlow Quantum](https://github.com/tensorflow/quantum) |
| Real experiments using Cirq? | [ReCirq](https://github.com/quantumlib/ReCirq) |

</div>

## Community

<a href="https://github.com/quantumlib/Cirq/graphs/contributors"><img
align="right" width="160em" alt="Total number of contributors to Cirq"
src="https://img.shields.io/github/contributors/quantumlib/cirq?label=Contributors&logoColor=white&logo=github&color=cccccc&style=flat-square"/></a>

Cirq has benefited from [open-source
contributions](https://github.com/quantumlib/Cirq/graphs/contributors) by over
200 people and counting. We are dedicated to cultivating an open and inclusive
community to build software for quantum computers, and have a [code of
conduct](https://github.com/quantumlib/cirq/blob/main/CODE_OF_CONDUCT.md) for
our community.

### Announcements

Stay on top of Cirq developments using the approach that best suits your needs:

* For releases and major announcements: sign up to the low-volume mailing list
[`cirq-announce`](https://groups.google.com/forum/#!forum/cirq-announce).
* For releases only:
* Via GitHub notifications: configure [GitHub repository
notifications](https://docs.github.com/github/managing-subscriptions-and-notifications-on-github/configuring-notifications)
for the Cirq repository.
* Via Atom/RSS from GitHub: subscribe to the GitHub [Cirq releases Atom
feed](https://github.com/quantumlib/Cirq/releases.atom).
* Via RSS from PyPI: subscribe to the [PyPI releases RSS
feed](https://pypi.org/rss/project/cirq/releases.xml) for Cirq.

Cirq releases take place approximately every quarter.

### Questions and Discussions

* Do you have questions about using Cirq? Post them to the [Quantum Computing
Stack Exchange](https://quantumcomputing.stackexchange.com/) and tag them
with the
[`cirq`](https://quantumcomputing.stackexchange.com/questions/tagged/cirq)
tag. You can also search past questions using that tag – it's a great way to
learn!
* Would you like to get more involved in Cirq development? _Cirq Cynq_ is our
biweekly virtual meeting of contributors to discuss everything from issues to
ongoing efforts, as well as to ask questions. Become a member of
[_cirq-dev_](https://groups.google.com/forum/#!forum/cirq-dev) to get an
automatic meeting invitation!

### Issues and Pull Requests

* Do you have a feature request or want to report a bug? [Open an issue on
GitHub](https://github.com/quantumlib/Cirq/issues/new/choose) to report it!
* Do you have a code contribution? Read our [contribution
guidelines](https://github.com/quantumlib/cirq/blob/main/CONTRIBUTING.md),
then open a [pull
request](https://help.github.com/articles/about-pull-requests)!

## Citing Cirq

When publishing articles or otherwise writing about Cirq, please cite the Cirq
version you use – it will help others reproduce your results. We use Zenodo to
preserve releases. The following links let you download the bibliographic
record for the latest stable release of Cirq in various popular formats:

<div align="center">

[![Download BibTeX bibliography record for latest Cirq
release](https://img.shields.io/badge/Download%20record-e0e0e0.svg?style=flat-square&logo=LaTeX&label=BibTeX&labelColor=106f6e)](https://zenodo.org/records/8161252/export/bibtex)&nbsp;&nbsp;
[![Download MARCXML bibliography record for latest Cirq
release](https://img.shields.io/badge/Download%20record-e0e0e0.svg?style=flat-square&label=MARCXML&labelColor=2f00cd&logo=xml)](https://zenodo.org/records/8161252/export/marcxml)&nbsp;&nbsp;
[![Download CSL JSON bibliography record for latest Cirq
release](https://img.shields.io/badge/Download%20record-e0e0e0.svg?style=flat-square&label=CSL&labelColor=2d98e0&logo=json)](https://zenodo.org/records/8161252/export/csl)

</div>

For formatted citations and records in other formats, as well as records for
all releases of Cirq past and present, visit the [Cirq page on
Zenodo](https://doi.org/10.5281/zenodo.4062499).

## Contact

For any questions or concerns not addressed here, please email
<[email protected]>.

## Disclaimer

Cirq is not an official Google product. Copyright 2019 The Cirq Developers.

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