How are Q# programs structured?

  • 6 minutes

Before you start writing quantum programs, it's important to understand the structure and components of a Q# program.

In this unit, you'll review the main components of a Q# program.

Namespaces

Every Q# file starts with a namespace. Here's an example:

namespace HelloQuantum {
    // Your code goes here.
}

The namespace keyword is used to define a namespace. Namespaces are used to organize Q# code into logical units. Their use becomes important when you use Q# libraries in your programs and when you write your own libraries.

Allocating qubits

In Q#, to obtain a qubit, you use the use keyword. Every qubit you allocate with the use keyword starts in the $\ket{0}$ state.

You can allocate one or many qubits at a time. Here's an example that allocates one and five qubits:

use q1 = Qubit(); // Allocate one qubit
use q5 = Qubit[5]; // Allocate five qubits

Quantum operations

Operations are the basic building blocks of a Q# program. A Q# operation is a quantum subroutine. That is, it's a callable routine that contains quantum operations that modify the state of the qubit register.

To define a Q# operation, you specify a name for the operation along with its inputs and its output. Here's a basic example:

operation SayHelloQ() : Unit {
    Message("Hello quantum world!");
}

Here, SayHelloQ is the name of the operation. It takes zero arguments as its input and returns type Unit, which means that the operation returns no information.

Q# libraries also provide operations that you can use in your programs, such as the Hadamard, H, operation, which puts a qubit into a superposition state.

Types

Q# provides many built-in types you might be already familiar with, including Int, Double, Bool, and String.

In this example, the Unit type is used to indicate that the operation doesn't return any information:

operation SayHelloQ() : Unit {
    Message("Hello quantum world!");
}

In the following units, you'll work with the Result type, which is specific type to quantum computing. A Result type is the result of measuring a qubit and can be either One and Zero.

Quantum libraries

Q# libraries contain functions and operations that you can use in quantum programs. When you call a function or operation from a library, you use the open directive and specify the library's namespace. For example, to use the Microsoft.Quantum.Intrinsic library, you do the following:

    open Microsoft.Quantum.Intrinsic;

Entry point

EntryPoint tells the Q# compiler where to begin executing the program. Every Q# program must have one entry point followed by an operation. For example, the following code defines an entry point operation, HelloQ, that prints "Hello quantum world!" to the console:

namespace HelloQuantum {

    open Microsoft.Quantum.Intrinsic;

    @EntryPoint()
    operation HelloQ() : Unit {
        Message("Hello quantum world!");
    }
}

Note

Note that the above Q# program opens theMicrosoft.Quantum.Intrinsic library to use the Message operation, which writes a message to the console.

Measuring qubits

In Q#, the Measure operation performs a joint measurement of one or more qubits in the specified Pauli bases, which can be PauliX, PauliY, or PauliZ. The Measure operation returns a Result type that is either One or Zero.

To implement a measurement in the computational basis $\lbrace\ket{0},\ket{1}\rbrace$ you can also use the M operation, which performs a measurement in the Pauli Z-basis. Thus, the M operation is equivalent to applying Measure([PauliZ], [qubit]).

Resetting qubits

In Q#, qubits must be in the $\ket{0}$ state by the time they are released. When you have finished using a qubit, you use the Reset operation to reset the qubit to the $\ket{0}$.

    // Reset the qubit so it can be safely released.
    Reset(qubit);