Add common repo files such as CoC, SECURITY.md, and .editorconfig

.editorconfig

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+# Common editor configurations for this project.
+#
+# EditorConfig is a file format for specifying some common style parameters.
+# Many IDEs & editors read .editorconfig files, either natively or via plugins.
+# We mostly follow Google's style guides (https://google.github.io/styleguide/)
+# with only a few deviations for line length and indentation in some files.
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+root = true
+
+[*]
+charset = utf-8
+indent_style = space
+insert_final_newline = true
+spelling_language = en-US
+trim_trailing_whitespace = true
+max_line_length = 120
+
+[{BUILD,*.BUILD,*.bzl,*.bazel,.bazelrc}]
+indent_size = 4
+
+[{*.cc,*.h}]
+indent_size = 4
+
+[{*.js,*.ts}]
+indent_size = 4
+
+[*.json]
+indent_size = 2
+
+[*.py]
+indent_size = 4
+
+[*.sh]
+indent_size = 4
+
+[{*.yaml,*.yml}]
+indent_size = 2

.github/SECURITY.md

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+# Reporting security issues
+
+This project's developers and community are committed to addressing security
+bugs promptly and effectively. We appreciate your efforts to disclose your
+findings responsibly, and will make every effort to acknowledge your
+contributions.
+
+Please **do not** use GitHub issues to report security vulnerabilities; GitHub
+issues are public, and doing so could allow someone to exploit the information
+before the problem can be addressed. Instead, please use the *Report a
+vulnerability* interface from the *Security* tab at the top of this GitHub
+repository page.
+
+<div align="center">
+<img width="75%" alt="Location of the report button on the repository page"
+    src="./report-vulnerability-button.png">
+</div>
+
+Please report security issues in third-party modules to the person or team
+maintaining the module rather than this project's stewards, unless you believe
+that some action needs to be taken specifically with this project in order to
+guard against the effects of a security vulnerability in third-party software.
+
+## Responses to security reports
+
+The project stewards at Google Quantum AI will send a response indicating the
+next steps in handling your report. After the initial reply to your report, the
+project stewards will keep you informed of the progress towards a fix and full
+announcement, and may ask for additional information or guidance.
+
+## Additional points of contact
+
+Please contact the project stewards at Google Quantum AI via email at
+quantum-oss-maintainers@google.com if you have questions or other concerns. If
+for any reason you are uncomfortable reaching out to the project stewards,
+please email opensource@google.com instead.

.markdownlintrc

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+{ // -*- jsonc -*-
+  // Copyright 2025 Google LLC
+  //
+  // Licensed under the Apache License, Version 2.0 (the "License");
+  // you may not use this file except in compliance with the License.
+  // You may obtain a copy of the License at
+  //
+  //     https://www.apache.org/licenses/LICENSE-2.0
+  //
+  // Unless required by applicable law or agreed to in writing, software
+  // distributed under the License is distributed on an "AS IS" BASIS,
+  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  // See the License for the specific language governing permissions and
+  // limitations under the License.
+
+  // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // Markdownlint linter configuration for this project.
+  //
+  // Note: there are multiple programs programs named "markdownlint". We use
+  // https://github.com/igorshubovych/markdownlint-cli/, which is the one you
+  // get with "brew install markdownlint" on MacOS.
+  //
+  // These settings try to stay close to the Google Markdown Style as
+  // described at https://google.github.io/styleguide/docguide/style.html
+  // with very few differences.
+  //
+  // For a list of possible configuration options, see the following page:
+  // https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md
+  // (Beware that the above looks similar but is NOT the same as the page
+  // https://github.com/markdownlint/markdownlint/blob/main/docs/RULES.md.)
+  // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+  "$schema": "https://raw.githubusercontent.com/DavidAnson/markdownlint/main/schema/markdownlint-config-schema.json",
+
+  // Require ATX-style headings.
+  // https://google.github.io/styleguide/docguide/style.html#atx-style-headings
+  "headings": {
+    "style": "atx"
+  },
+
+  // Google style does not require that the first line of a file is a heading
+  // for the title; it only states that the first heading should be a level 1.
+  // https://google.github.io/styleguide/docguide/style.html#document-layout
+  "first-line-heading": false,
+
+  // The Google style does not define what to do about trailing punctuation in
+  // headings. The markdownlint default disallows exclamation points, which
+  // seems likely to be more annoying than useful – I have definitely seen
+  // people use exclamation points in headings in README files on GitHub.
+  // This setting removes exclamation point from the banned characters.
+  "no-trailing-punctuation": {
+    "punctuation": ".,;:。，；："
+  },
+
+  // No trailing spaces.
+  // https://google.github.io/styleguide/docguide/style.html#trailing-whitespace
+  "whitespace": {
+    "br_spaces": 0
+  },
+
+  // Google style is 80 characters.
+  // Google style exempts some constructs from the line-length limit:
+  // https://google.github.io/styleguide/docguide/style.html#exceptions
+  "line-length": {
+    "line_length": 120,
+    "code_block_line_length": 120,
+    "heading_line_length": 120,
+    "code_blocks": false,
+    "headings": false,
+    "tables": false
+  },
+
+  // Google Markdown style specifies 2 spaces after item numbers, 3 spaces
+  // after bullets, so that the text itself is consistently indented 4 spaces.
+  // https://google.github.io/styleguide/docguide/style.html#nested-list-spacing
+  "list-marker-space": {
+    "ol_multi": 2,
+    "ol_single": 2,
+    "ul_multi": 3,
+    "ul_single": 3
+  },
+
+  "ul-indent": {
+    "indent": 4
+  },
+
+  // Bare URLs are allowed in GitHub-flavored Markdown and in Google’s style.
+  "no-bare-urls": false,
+
+  // Basic Markdown allows raw HTML. Both GitHub & PyPI support subsets of
+  // HTML, though it's unclear what subset PyPI supports. Google's style guide
+  // recommends against using raw HTML, but does allow it. (C.f. the bottom of
+  // https://google.github.io/styleguide/docguide/style.html) Google's in-house
+  // documentation system allows many inline and block-level tags, but strips
+  // others that can pose security risks (e.g., <object> and standalone <svg>).
+  // The list below tries to capture the intersection of what GitHub allows
+  // (c.f. https://github.com/github/markup/issues/245#issuecomment-682231577),
+  // what PyPI seems to allow, what Google allows, and what seems likely to be
+  // most useful in situations where someone needs to reach for HTML.
+  "html": {
+    "allowed_elements": [
+      "a",
+      "abbr",
+      "b",
+      "blockquote",
+      "br",
+      "caption",
+      "cite",
+      "code",
+      "dd",
+      "del",
+      "details",
+      "dfn",
+      "div",
+      "dl",
+      "dt",
+      "em",
+      "figcaption",
+      "figure",
+      "h1",
+      "h2",
+      "h3",
+      "h4",
+      "h5",
+      "h6",
+      "hr",
+      "i",
+      "img",
+      "ins",
+      "kbd",
+      "li",
+      "mark",
+      "ol",
+      "p",
+      "picture",
+      "pre",
+      "q",
+      "s",
+      "samp",
+      "small",
+      "span",
+      "strong",
+      "sub",
+      "summary",
+      "sup",
+      "table",
+      "tbody",
+      "td",
+      "tfoot",
+      "th",
+      "thead",
+      "time",
+      "tr",
+      "tt",
+      "ul",
+      "var",
+      "wbr"
+    ]
+  }
+}

.yamllint.yaml

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+# Yamllint configuration to match project settings like line length.
+# See https://yamllint.readthedocs.io/ for info about configuration options.
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+rules:
+  line-length:
+    max: 120
+    # When the last item on a line can't be broken (e.g., a URL as a value),
+    # it's rarely useful to flag it. The next 2 settings save developer time by
+    # not requiring comment directives to disable warnings at every occurrence.
+    allow-non-breakable-words: true
+    allow-non-breakable-inline-mappings: true

CODE_OF_CONDUCT.md

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+# Code of Conduct
+
+## Our Pledge
+
+In the interest of fostering an open and welcoming environment, we as
+contributors and maintainers pledge to making participation in our project and
+our community a harassment-free experience for everyone, regardless of age, body
+size, disability, ethnicity, gender identity and expression, level of
+experience, education, socio-economic status, nationality, personal appearance,
+race, religion, or sexual identity and orientation.
+
+## Our Standards
+
+Examples of behavior that contributes to creating a positive environment
+include:
+
+*   Using welcoming and inclusive language
+*   Being respectful of differing viewpoints and experiences
+*   Gracefully accepting constructive criticism
+*   Focusing on what is best for the community
+*   Showing empathy towards other community members
+
+Examples of unacceptable behavior by participants include:
+
+*   The use of sexualized language or imagery and unwelcome sexual attention or
+    advances
+*   Trolling, insulting/derogatory comments, and personal or political attacks
+*   Public or private harassment
+*   Publishing others' private information, such as a physical or electronic
+    address, without explicit permission
+*   Other conduct which could reasonably be considered inappropriate in a
+    professional setting
+
+## Our Responsibilities
+
+Project maintainers are responsible for clarifying the standards of acceptable
+behavior and are expected to take appropriate and fair corrective action in
+response to any instances of unacceptable behavior.
+
+Project maintainers have the right and responsibility to remove, edit, or reject
+comments, commits, code, wiki edits, issues, and other contributions that are
+not aligned to this Code of Conduct, or to ban temporarily or permanently any
+contributor for other behaviors that they deem inappropriate, threatening,
+offensive, or harmful.
+
+## Scope
+
+This Code of Conduct applies both within project spaces and in public spaces
+when an individual is representing the project or its community. Examples of
+representing a project or community include using an official project e-mail
+address, posting via an official social media account, or acting as an appointed
+representative at an online or offline event. Representation of a project may be
+further defined and clarified by project maintainers.
+
+This Code of Conduct also applies outside the project spaces when the Project
+Stewards have a reasonable belief that an individual's behavior may have a
+negative impact on the project or its community.
+
+## Conflict Resolution
+
+We do not believe that all conflict is bad; healthy debate and disagreement
+often yield positive results. However, it is never okay to be disrespectful or
+to engage in behavior that violates the project’s Code of Conduct.
+
+If you see someone violating the Code of Conduct, you are encouraged to address
+the behavior directly with those involved. Many issues can be resolved quickly
+and easily, and this gives people more control over the outcome of their
+dispute. If you are unable to resolve the matter for any reason, or if the
+behavior is threatening or harassing, report it. We are dedicated to providing
+an environment where participants feel welcome and safe.
+
+Reports should be directed to quantumai-oss-maintainers@googlegroups.com,
+the project stewards at Google Quantum AI. They will then work with a committee
+consisting of representatives from the Open Source Programs Office and the
+Google Open Source Strategy team. If for any reason you are uncomfortable
+reaching out to the Project Stewards, please email opensource@google.com.
+
+We will investigate every complaint, but you may not receive a direct response.
+We will use our discretion in determining when and how to follow up on reported
+incidents, which may range from not taking action to permanent expulsion from
+the project and project-sponsored spaces. We will notify the accused of the
+report and provide them an opportunity to discuss it before any action is taken.
+The identity of the reporter will be omitted from the details of the report
+supplied to the accused. In potentially harmful situations, such as ongoing
+harassment or threats to anyone's safety, we may take action without notice.
+
+## Attribution
+
+This Code of Conduct is adapted from the Contributor Covenant, version 1.4,
+available at
+https://www.contributor-covenant.org/version/1/4/code-of-conduct.html