Add documentation for Q#

qsharp.html.markdown

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+---
+language: Q#
+contributors:
+    - ["Vincent van Wingerden", "https://github.com/vivanwin"]
+    - ["Mariia Mykhailova", "https://github.com/tcNickolas"]
+filename: LearnQSharp.qs
+---
+
+Q# is a high-level domain-specific language which enables developers to write quantum algorithms. Q# programs can be executed on a quantum simulator running on a classical computer and (in future) on quantum computers.
+
+This is the new outline
+```C#
+// Single-line comments start with //
+
+/////////////////////////////////////
+// 1. Quantum data types and operators
+
+// The most important part of quantum programs is qubits. 
+// In Q# type Qubit represents the qubits which can be used.
+// This will allocate an array of two new qubits as the variable qs.
+using (qs = Qubit[2]) {
+
+    // The qubits have internal state that you cannot access to read or modify directly.
+    // You can inspect the current state of your quantum program if you're running it on a classical simulator.
+    // Note that this will not work on actual quantum hardware!
+    DumpMachine();
+
+    // If you want to change the state of a qubit, you have to do this by applying quantum gates to the qubit.
+    H(q[0]);    // This changes the state of the first qubit from |0⟩ (the initial state of allocated qubits) to (|0⟩ + |1⟩) / sqrt(2).
+    // q[1] = |1⟩; - this does NOT work, you have to manipulate a qubit by using gates.
+
+    // You can apply multi-qubit gates to several qubits.
+    CNOT(qs[0], qs[1]);
+
+    // You can also apply a controlled version of a gate: a gate that is applied if all control qubits are in |1⟩ state.
+    // The first argument is an array of control qubits, the second argument is the target qubit.
+    Controlled Y([qs[0]], qs[1]); 
+
+    // If you want to apply an anti-controlled gate (a gate that is applied if all control qubits are in |0⟩ state), you can use a library function.
+    ApplyControlledOnInt(0, X, [qs[0]], qs[1]);
+
+    // To read the information from the quantum system, you use measurements.
+    // Measurements return a value of Result data type: Zero or One.
+    // You can print measurement results as a classical value.
+    Message($"Measured {M(qs[0])}, {M(qs[1])}");
+}
+
+
+/////////////////////////////////////
+// 2. Classical data types and operators
+
+// Numbers in Q# can be stored in Int, BigInt or Double.
+let i = 1;            // This defines an Int variable i equal to 1
+let bi = 1L;          // This defines a BigInt variable bi equal to 1
+let d = 1.0;          // This defines a Double variable d equal to 1
+
+// Arithmetic is done as expected, as long as the types are the same
+let n = 2 * 10;                // = 20
+// Q# does not have implicit type cast, so to perform arithmetic on values of different types, you need to cast type explicitly
+let nd = IntAsDouble(2) * 1.0; // = 20.0
+
+// Boolean type is called Bool
+let trueBool = true;
+let falseBool = false;
+
+// Logic operators work as expected
+let andBool = true and false;
+let orBool = true or false;
+let notBool = not false;
+
+// Strings
+let str = "Hello World!";
+
+// Equality is ==
+let x = 10 == 15; // is false
+
+// Range is a sequence of integers and can be defined like: start..step..stop
+let xi = 1..2..7; // Gives the sequence 1,3,5,7
+
+// Assigning new value to a variable:
+// by default all Q# variables are immutable;
+// if the variable was defined using let, you cannot reassign its value.
+
+// When you want to make a variable mutable, you have to declare it as such, 
+// and use the set word to update value
+mutable xii = true;
+set xii = false;
+
+// You can create an array for any data type like this
+let xiii = new Double[10];
+
+// Getting an element from an array 
+let xiv = xiii[8];
+
+// Assigning a new value to an array element
+mutable xv = new Double[10];
+set xv w/= 5 <- 1;
+
+
+/////////////////////////////////////
+// 3. Control flow
+
+// If structures work a little different than most languages
+if (a == 1) {
+    // ...
+} elif (a == 2) {
+    // ... 
+} else {
+    // ...
+}
+
+// Foreach loops can be used to iterate over an array
+for (qubit in qubits) {
+    X(qubit);
+}
+
+// Regular for loops can be used to iterate over a range of numbers
+for (index in 0 .. Length(qubits) - 1) {
+    X(qubits[index]);
+}
+
+// While loops are restricted for use in classical context only
+mutable index = 0;
+while (index < 10) {
+    set index += 1;
+}
+
+// Quantum equivalent of a while loop is a repeat-until-success loop.
+// Because of the probabilistic nature of quantum computing sometimes
+// you want to repeat a certain sequence of operations until a specific condition is achieved; you can use this loop to express this.
+repeat {
+    // Your operation here
+}
+until (success criteria) // This could be a measurement to check if the state is reached
+fixup {
+    // Resetting to the initial conditions, if required
+}
+
+
+/////////////////////////////////////
+// 4. Putting it all together
+
+// Q# code is written in operations and functions
+operation ApplyXGate(source : Qubit) : Unit {
+    X(source);
+}
+
+// If the operation implements a unitary transformation, you can define 
+// adjoint and controlled variants of it. 
+// The easiest way to do that is to add "is Adj + Ctl" after Unit. 
+// This will tell the compiler to generate the variants automatically.
+operation ApplyXGateCA (source : Qubit) : Unit is Adj + Ctl {
+    X(source);
+}
+
+// Now you can call Adjoint ApplyXGateCA and Controlled ApplyXGateCA.
+
+
+// To run Q# code, you can put @EntryPoint() before the operation you want to run first
+@EntryPoint()
+operation XGateDemo() : Unit {
+    using (q = Qubit()) {
+        ApplyXGate(q);
+    }
+}
+
+// Here is a simple example: a quantum random number generator. 
+// We will generate a classical array of random bits using quantum code.
+@EntryPoint()
+operation QRNGDemo() : Unit {
+    mutable bits = new Int[5];                // Array we'll use to store bits
+    using (q = Qubit()) {                     // Allocate a qubit
+        for (i in 0 .. 4) {                   // Generate each bit independently
+            H(q);                             // Apply Hadamard gate to prepare equal superposition
+            let result = M(q);                // Measure the qubit to get Zero or One with 50/50 probability
+            let bit = result == Zero ? 0 | 1; // Convert measurement result to an integer
+            set bits w/= i <- bit;            // Write generated bit to an array
+        }
+    }
+    Message($"{bits}");                       // Print the result
+}
+```
+
+
+## Further Reading
+
+The [Quantum Katas][1] offer great self-paced tutorials and programming exercises to learn quantum computing and Q#. 
+
+[Q# Documentation][2] is official Q# documentation, including language reference and user guides.
+
+[1]: https://github.com/microsoft/QuantumKatas
+[2]: https://docs.microsoft.com/quantum/