# Tutorial: Explore entanglement with Q#

In this tutorial, we show you how to write a Q# program that manipulates and measures qubits and demonstrates the effects of superposition and entanglement. This guides you on installing the QDK, building the program and executing that program on a quantum simulator.

You will write an application called Bell to demonstrate quantum entanglement. The name Bell is in reference to Bell states, which are specific quantum states of two qubits that are used to represent the simplest examples of superposition and quantum entanglement.

## Pre-requisites

If you are ready to start coding, follow these steps before proceeding:

• Install the Quantum Development Kit for Python or .NET.
• If you already have the QDK installed, make sure you have updated to the latest version

You can also follow along with the narrative without installing the QDK, reviewing the overviews of the Q# programming language and the first concepts of quantum computing.

## Demonstrating qubit behavior with Q#

Recall our simple definition of a qubit. Where classical bits hold a single binary value such as a 0 or 1, the state of a qubit can be in a superposition of 0 and 1. Conceptually, a qubit can be thought of as a direction in space (also known as a vector). A qubit can be in any of the possible directions. The two classical states are the two directions; representing 100% chance of measuring 0 and 100% chance of measuring 1. This representation is also more formally visualized by the bloch sphere.

The act of measurement produces a binary result and changes a qubit state. Measurement produces a binary value, either 0 or 1. The qubit goes from being in superposition (any direction) to one of the classical states. Thereafter, repeating the same measurement without any intervening operations produces the same binary result.

Multiple qubits can be entangled. When we make a measurement of one entangled qubit, our knowledge of the state of the other(s) is updated as well.

Now, we're ready to demonstrate how Q# expresses this behavior. You start with the simplest program possible and build it up to demonstrate quantum superposition and quantum entanglement.

## Setup

This tutorial uses a host programs and consists of two parts:

1. A series of quantum algorithms, implemented using the Q# quantum programming language.
2. A host program, implemented in either Python or C#, that serves as the main entry point and invokes Q# operations to execute the quantum algorithms.
1. Choose a location for your application

2. Create a file called `Bell.qs`. This file will contain your Q# code.

3. Create a file called `host.py`. This file will contain your Python host code.

## Write a Q# operation

Our goal is to prepare two qubits in a specific quantum state, demonstrating how to operate on qubits with Q# to change their state and demonstrate the effects of superposition and entanglement. We will build this up piece by piece to demonstrate qubit states, operations, and measurement.

Overview: In the first code below, we show you how to work with qubits in Q#. We’ll introduce two operations, `M` and `X` that transform the state of a qubit.

In this code snippet, an operation `Set` is defined that takes as a parameter a qubit and another parameter, `desired`, representing the state that we would like the qubit to be in. The operation `Set` performs a measurement on the qubit using the operation `M`. In Q#, a qubit measurement always returns either `Zero` or `One`. If the measurement returns a value not equal to a desired value, Set “flips” the qubit; that is, it executes an `X` operation, which changes the qubit state to a new state in which the probabilities of a measurement returning `Zero` and `One` are reversed. To demonstrate the effect of the `Set` operation, a `TestBellState` operation is then added. This operation takes as input a `Zero` or `One`, and calls the `Set` operation some number of times with that input, and counts the number of times that `Zero` was returned from the measurement of the qubit and the number of times that `One` was returned. Of course, in this first simulation of the `TestBellState` operation, we expect that the output will show that all measurements of the qubit set with `Zero` as the parameter input will return `Zero`, and all measurements of a qubit set with `One` as the parameter input will return `One`. Further on, we’ll add code to `TestBellState` to demonstrating superposition and entanglement.

### Q# operation code

1. Replace the contents of the Bell.qs file with the following code:

``````namespace Quantum.Bell {
open Microsoft.Quantum.Intrinsic;
open Microsoft.Quantum.Canon;

operation Set(desired : Result, q1 : Qubit) : Unit {
if (desired != M(q1)) {
X(q1);
}
}
}
``````

This operation may now be called to set a qubit to a classical state, either returning `Zero` 100% of the time or returning `One` 100% of the time. `Zero` and `One` are constants that represent the only two possible results of a measurement of a qubit.

The operation `Set` measures the qubit. If the qubit is in the state we want, `Set` leaves it alone; otherwise, by executing the `X` operation, we change the qubit state to the desired state.

A Q# operation is a quantum subroutine. That is, it is a callable routine that contains quantum operations.

The arguments to an operation are specified as a tuple, within parentheses.

The return type of the operation is specified after a colon. In this case, the `Set` operation has no return, so it is marked as returning `Unit`. This is the Q# equivalent of `unit` in F#, which is roughly analogous to `void` in C#, and an empty tuple (`Tuple[()]`) in Python.

You have used two quantum operations in your first Q# operation:

• The M operation, which measures the state of the qubit
• The X operation, which flips the state of a qubit

A quantum operation transforms the state of a qubit. Sometime people talk about quantum gates instead of operations, in analogy to classical logic gates. This is rooted in the early days of quantum computing when algorithms were merely a theoretical construct and visualized as diagrams similarly to circuit diagrams in classical computing.

1. Add the following operation to the `Bell.qs` file, inside the namespace, after the end of the `Set` operation:

``````operation TestBellState(count : Int, initial : Result) : (Int, Int) {

mutable numOnes = 0;
using (qubit = Qubit()) {

for (test in 1..count) {
Set(initial, qubit);
let res = M(qubit);

// Count the number of ones we saw:
if (res == One) {
set numOnes += 1;
}
}
Set(Zero, qubit);
}

// Return number of times we saw a |0> and number of times we saw a |1>
return (count-numOnes, numOnes);
}
``````

This operation (`TestBellState`) will loop for `count` iterations, set a specified `initial` value on a qubit and then measure (`M`) the result. It will gather statistics on how many zeros and ones we've measured and return them to the caller. It performs one other necessary operation. It resets the qubit to a known state (`Zero`) before returning it allowing others to allocate this qubit in a known state. This is required by the `using` statement.

By default, variables in Q# are immutable; their value may not be changed after they are bound. The `let` keyword is used to indicate the binding of an immutable variable. Operation arguments are always immutable.

If you need a variable whose value can change, such as `numOnes` in the example, you can declare the variable with the `mutable` keyword. A mutable variable's value may be changed using a `set` statement.

In both cases, the type of a variable is inferred by the compiler. Q# doesn't require any type annotations for variables.

### About `using` statements in Q#

The `using` statement is also special to Q#. It is used to allocate qubits for use in a block of code. In Q#, all qubits are dynamically allocated and released, rather than being fixed resources that are there for the entire lifetime of a complex algorithm. A `using` statement allocates a set of qubits at the start, and releases those qubits at the end of the block.

## Create the host application code

1. Open the `host.py` file and add the following code:

``````import qsharp

from qsharp import Result
from Quantum.Bell import TestBellState

initials = (Result.Zero, Result.One)

for i in initials:
res = TestBellState.simulate(count=1000, initial=i)
(num_zeros, num_ones) = res
print(f'Init:{i: <4} 0s={num_zeros: <4} 1s={num_ones: <4}')
``````

### About the host application code

The Python host application has three parts:

• Compute any arguments required for the quantum algorithm. In the example, `count` is fixed at a 1000 and `initial` is the initial value of the qubit.
• Run the quantum algorithm by calling the `simulate()` method of the imported Q# operation.
• Process the result of the operation. In the example, `res` receives the result of the operation. Here the result is a tuple of the number of zeros (`num_zeros`) and number of ones (`num_ones`) measured by the simulator. We deconstruct the tuple to get the two fields, and print the results.

## Build and run

1. Run the following command at your terminal:

``````python host.py
``````

This command runs the host application, which simulates the Q# operation.

The results should be:

``````Init:0    0s=1000 1s=0
Init:1    0s=0    1s=1000
``````

## Prepare superposition

Overview Now let’s look at how Q# expresses ways to put qubits in superposition. Recall that the state of a qubit can be in a superposition of 0 and 1. We’ll use the `Hadamard` operation to accomplish this. If the qubit is in either of the classical states (where a measurement returns `Zero` always or `One` always), then the `Hadamard` or `H` operation will put the qubit in a state where a measurement of the qubit will return `Zero` 50% of the time and return `One` 50% of the time. Conceputually, the qubit can be thought of as halfway between the `Zero` and `One`. Now, when we simulate the `TestBellState` operation, we will see the results will return roughly an equal number of `Zero` and `One` after measurement.

First we'll just try to flip the qubit (if the qubit is in `Zero` state will flip to `One` and vice versa). This is accomplished by performing an `X` operation before we measure it in `TestBellState`:

``````X(qubit);
let res = M(qubit);
``````

Now the results (after pressing `F5`) are reversed:

``````Init:Zero 0s=0    1s=1000
Init:One  0s=1000 1s=0
``````

However, everything we've seen so far is classical. Let's get a quantum result. All we need to do is replace the `X` operation in the previous run with an `H` or Hadamard operation. Instead of flipping the qubit all the way from 0 to 1, we will only flip it halfway. The replaced lines in `TestBellState` now look like:

``````H(qubit);
let res = M(qubit);
``````

Now the results get more interesting:

``````Init:Zero 0s=484  1s=516
Init:One  0s=522  1s=478
``````

Every time we measure, we ask for a classical value, but the qubit is halfway between 0 and 1, so we get (statistically) 0 half the time and 1 half the time. This is known as superposition and gives us our first real view into a quantum state.

## Prepare entanglement

Overview: Now let’s look at how Q# expresses ways to entangle qubits. First, we set the first qubit to the initial state and then use the `H` operation to put it into superposition. Then, before we measure the first qubit, we use a new operation (`CNOT`), which stands for Controlled-Not. The result of executing this operation on two qubits is to flip the second qubit if the first qubit is `One`. Now, the two qubits are entangled. Our statistics for the first qubit haven't changed (50-50 chance of a `Zero` or a `One` after measurement), but now when we measure the second qubit, it is always the same as what we measured for the first qubit. Our `CNOT` has entangled the two qubits, so that whatever happens to one of them, happens to the other. If you reversed the measurements (did the second qubit before the first), the same thing would happen. The first measurement would be random and the second would be in lock step with whatever was discovered for the first.

The first thing we'll need to do is allocate 2 qubits instead of one in `TestBellState`:

``````using ((q0, q1) = (Qubit(), Qubit())) {
``````

This will allow us to add a new operation (`CNOT`) before we measure (`M`) in `TestBellState`:

``````Set(initial, q0);
Set(Zero, q1);

H(q0);
CNOT(q0, q1);
let res = M(q0);
``````

We've added another `Set` operation to initialize the first qubit to make sure that it's always in the `Zero` state when we start.

We also need to reset the second qubit before releasing it.

``````Set(Zero, q0);
Set(Zero, q1);
``````

The full routine now looks like this:

``````    operation TestBellState(count : Int, initial : Result) : (Int, Int) {

mutable numOnes = 0;
using ((q0, q1) = (Qubit(), Qubit())) {
for (test in 1..count) {
Set (initial, q0);
Set (Zero, q1);

H(q0);
CNOT(q0,q1);
let res = M(q0);

// Count the number of ones we saw:
if (res == One) {
set numOnes += 1;
}
}

Set(Zero, q0);
Set(Zero, q1);
}

// Return number of times we saw a |0> and number of times we saw a |1>
return (count-numOnes, numOnes);
}
``````

If we run this, we'll get exactly the same 50-50 result we got before. However, what we're interested in is how the second qubit reacts to the first being measured. We'll add this statistic with a new version of the `TestBellState` operation:

``````    operation TestBellState(count : Int, initial : Result) : (Int, Int, Int) {
mutable numOnes = 0;
mutable agree = 0;
using ((q0, q1) = (Qubit(), Qubit())) {
for (test in 1..count) {
Set(initial, q0);
Set(Zero, q1);

H(q0);
CNOT(q0, q1);
let res = M(q0);

if (M(q1) == res) {
set agree += 1;
}

// Count the number of ones we saw:
if (res == One) {
set numOnes += 1;
}
}

Set(Zero, q0);
Set(Zero, q1);
}

// Return number of times we saw a |0> and number of times we saw a |1>
return (count-numOnes, numOnes, agree);
}
``````

The new return value (`agree`) keeps track of every time the measurement from the first qubit matches the measurement of the second qubit. We also have to update the host application accordingly:

``````import qsharp

from qsharp import Result
from Quantum.Bell import TestBellState

initials = {Result.Zero, Result.One}

for i in initials:
res = TestBellState.simulate(count=1000, initial=i)
(num_zeros, num_ones, agree) = res
print(f'Init:{i: <4} 0s={num_zeros: <4} 1s={num_ones: <4} agree={agree: <4}')
``````

Now when we run, we get something pretty amazing:

``````Init:Zero 0s=499  1s=501  agree=1000
Init:One  0s=490  1s=510  agree=1000
``````

As stated in the overview, our statistics for the first qubit haven't changed (50-50 chance of a 0 or a 1), but now when we measure the second qubit, it is always the same as what we measured for the first qubit, because they are entangled!

Congratulations, you've written your first quantum program!

## Next steps

The tutorial Grover’s search shows you how to build and run Grover search, one of the most popular quantum computing algorithms and offers a nice example of a Q# program that can be used to solve real problems with quantum computing.

Get Started with the Quantum Development Kit recommends more ways to learn Q# and quantum programming.