Q Sharp

Q# (pronounced as 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.

History
Historically, Microsoft Research had two teams interested in quantum computing: the QuArC team based in Redmond, 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.

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. The Quantum Development Kit ships with a quantum simulator which is capable of running Q#.

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 the list at the article on 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.