# Q Sharp

Q# (pronounced Q sharp) is a domain-specific programming language used for expressing quantum algorithms. It was initially released to the public by Microsoft as part of the Quantum Development Kit.
Q# works in conjunction with classical languages such as C#, Python and F#, and is designed to allow the use of traditional programming concepts in quantum computing, including functions with variables and branches as well as a syntax-highlighted development environment with a quantum debugger.

## History

Historically, Microsoft Research had two teams interested in quantum computing: the QuArC team based in Redmond, Washington, directed by Krysta Svore, that explored the construction of quantum circuitry, and Station Q initially located in Santa Barbara and directed by Michael Freedman, that explored topological quantum computing.
During a Microsoft Ignite Keynote on September 26, 2017, Microsoft announced that they were going to release a new programming language geared specifically towards quantum computers. On December 11, 2017, Microsoft released Q# as a part of the Quantum Development Kit.
At Build 2019, Microsoft announced that it would be open-sourcing the Quantum Development Kit, including its Q# compilers and simulators.
To support Q#, Microsoft developed Quantum Intermediate Representation (QIR) in 2023 as a common interface between programming languages and target quantum processors. The company also announced a compiler extension that generates QIR from Q#.
Bettina Heim currently leads the Q# language development effort.

## Usage

Q# is available as a separately downloaded extension for Visual Studio, but it can also be run as an independent tool from the command line or Visual Studio Code. Q# was introduced on Windows and is available on MacOS and Linux.
The Quantum Development Kit includes a quantum simulator capable of running Q# and simulated 30 logical qubits.
In order to invoke the quantum simulator, another .NET programming language, usually C#, is used, which provides the (classical) input data for the simulator and reads the (classical) output data from the simulator.

## Features

A primary feature of Q# is the ability to create and use qubits for algorithms. As a consequence, some of the most prominent features of Q# are the ability to entangle and introduce superpositioning to qubits via controlled NOT gates and Hadamard gates, respectively, as well as Toffoli Gates, Pauli X, Y, Z Gate, and many more which are used for a variety of operations (See quantum logic gates).
The hardware stack that will eventually come together with Q# is expected to implement Qubits as topological qubits. The quantum simulator that is shipped with the Quantum Development Kit today is capable of processing up to 32 qubits on a user machine and up to 40 qubits on Azure.

## Documentation and resources

Currently, the resources available for Q# are scarce, but the official documentation is published: Microsoft Developer Network: Q#. Microsoft Quantum Github repository is also a large collection of sample programs implementing a variety of Quantum algorithms and their tests.
Microsoft has also hosted a Quantum Coding contest on Codeforces, called Microsoft Q# Coding Contest - Codeforces, and also provided related material to help answer the questions in the blog posts, plus the detailed solutions in the tutorials.
Microsoft hosts a set of learning exercises to help learn Q# on GitHub: microsoft/QuantumKatas with links to resources, and answers to the problems.

## Syntax

Q# is syntactically related to both C# and F# yet also has some significant differences.

### Similarities with C#

* Uses namespace for code isolation
* All statements end with a ;
* Curly braces are used for statements of scope
* Single line comments are done using //
* Variable data types such as Int Double String and Bool are similar, although capitalised (and Int is 64-bit)
* Qubits are allocated and disposed inside a using block.
* Lambda functions are defined using the => operator.
* Results are returned using the return keyword.

### Similarities with F#

* Variables are declared using either let or mutable
* First-order functions
* Modules, which are imported using the open keyword
* The datatype is declared after the variable name
* The range operator ..
* for … in loops
* Every operation/function has a return value, rather than void. Instead of void, an empty Tuple is returned.
* Definition of record datatypes (using the newtype keyword, instead of type).

### Differences

* Functions are declared using the function keyword
* Operations on the quantum computer are declared using the operation keyword
* Lack of multiline comments
* Asserts instead of throwing exceptions
* Documentation is written in Markdown instead of XML-based documentation tags

## Example

The following source code is a multiplexer from the official Microsoft Q# library repository.