QPlayLearn - Entanglement

(02/10/2021)

Get entangled at QuantumComputing.com!

You just took learned the basics of the resource of quantum computation, entanglement. Now, you can see how it is generated in a quantum computer.
Strangeworks and QPlayLearn have partnered together to help reach and educate the expanding audience of the future quantum workforce. This series of posts is an extension of the QPlayLearn education platform with Strangeworks developed projects that allow readers to quickly explore and apply concepts with real quantum code using the free community edition of the Strangeworks platform.
Didn't come here from QPlayLearn? No problem! QPlayLearn is a great resource that seeks to provide multilevel education on quantum science and technologies to everyone, regardless of their age or background by using interactive tools to make the learning process more effective and fun. Head over to QPlayLearn's Quest quantum dictionary and start learning what a qubit is through their three different approaches: Play, Discover, and Learn. When you're ready, come back and practice what you've learned on the Strangeworks Platform. This also serves as a great place to start looking at different quantum programing languages.
Entangling Circuit - create a Bell state

Quantum entanglement is the basic resource of quantum computation. You know, entanglement, the term introduced in that paper by none other than Schrödinger. Well, to be honest, there he called it Verschränkung, but then he himself translated into English as entanglement.

For some reason, every single popular article that mentions entanglement adds the clichéd phrase "what Einstein called spooky action at a distance" as way to make it sound unnatural. Let's settle this once and for all. Entanglement is our basic tool. We understand it, we love it, we use it and that is all. Einstein might be a bit spooky now, but nobody that programs quantum computers is spooked by it.

The cat is the best anarchist. - Ernest Hemingway

Introduction

Let's begin with a brief explanation of the controlled-not gate. The controlled-not gate is a two qubit gate that changes the state of a target qubit given the state of a control qubit. If the control qubit is in the $|0\rangle$ state, nothing happens. If it's in the $|1\rangle$​ state, an X gate is applied to the target qubit.

What happens if our qubits are not in the $|0\rangle$​ or $|1\rangle$​ state, but are in some unmeasured quantum state? The qubits are now entangles and measuring either of them affects the outcome of the other.

This is the basic circuit to create an entangled state. It starts with two qubits $\lvert 00 \rangle$, does a Hadamard gate to the first, then a controlled-not to both. The the outcome is the entangled state $\frac{\lvert 0 \rangle + \lvert 1 \rangle}{\sqrt{2}}$, also called a Bell state. And that's it! Well, just one more thing. The last step of the circuit measures each qubit separately. Each time you run it, you can see how the outcome might be random, but both bits are always correlated to the same outcome.

Go ahead and run some code. The project here will allow you to create the same basic entangling circuits in many different quantum programming languages!