Accidentally posted an unfinished comment earlier and lost it, but oh well.

That’s a very loaded set of questions for an ELI5 explanation.

The concept of quantum entanglement is that we are able to measure particles in a system based on one observation of parts of the system. So for example, say you have two particles that can either spin positively or negatively that form a system and you know that these particles are entangled and that the system produces a spin of 0. You observe one particle and it’s spin is negative. Automatically, you are able to infer that the other particle in the system is spinning positively.

This concept can be applied to a lot of “non quantum” things. For example, say your friend owns only two pair of shoes and that when he isn’t wearing one, the other pair is on his shoe rack. When you see your friend wearing his red shoes, you know where his blue shoes are. That’s because you know the rules of the your friends shoe system.

One misconception that people have about quantum entanglement is that changing the state of a particle/part of the system automatically changes the state of the whole system. That isn’t true. If you were to steal your friends shoes and wear them on your feet, someone seeing you wearing the shoes wouldn’t be able to tell where the blue shoes are because they are no longer entangled.

One other important concept in quantum entanglement is that the act of observing a system inherently changes the state of that system. If you don’t see your friend wearing his red or blue shoes, there’s no way for you to tell which pair is out and which pair is in. The moment you observe your friend and his shoes, the knowledge you have about the location of the other pair of shoes changes.

Applications of quantum entanglement are hard to explain. It’s present in concepts such as quantum cryptography and key distribution. You can create dice with quantum entanglement properties.

Quantum entanglement has all sorts of applications that make it very valuable in the real world. Especially in quantum computing but also that helps us observe systems and explain things in physics, biology, chemistry and many other fields.

Okay, I am up for a challenge…

I am going to skip over all of the complicated Quantum Theory stuff and give you some general rules via a metaphor.

Let us say I have a box, in the box are two balls. Due to the laws of nature, the only way to be able to have two balls in this box is of one of them is red and the other is blue. If they are both red or both blue, they cannot go into the box.

Now, if I remove one of the balls without looking at it, carry it far away (as far away as you like…light years…other galaxies…etc) if you look at that ball and it is blue, the one you left behind has to be red because they were together in the box.

This cannot be used for instantaneous communication because you cannot influence which one is red or blue.

Part of the bizarre nature of this phenomenon is that the balls are neither red nor blue before one is observed. This is why it is called “spooky action at a distance”. For the same reason Schrodinger’s cat is both dead and alive in the box.

As for applications, it is debatable. Because the information is destroyed if observed at the wrong time (ie. wiretapped), there are discussions about using it to transmit encryption keys. Tests are ongoing.

For a true ELI5, this will require leaving out a lot. This is simply an analogy. Analogies don’t hold up perfectly with very complicated things, so be careful about using them in place of complete understanding or learning more.

The basic principle is having two special particles. These particles are kind of like twins. Created together, they become “entangled” and share a special bond. It’s not magic but more like a connection that is hard for others to understand and see.

Now, let’s say we take Particle A and put it in a box, and Particle B in another box. We take the boxes far away from each other, even on different sides of the world.

Whatever happens to Particle A will instantly affect Particle B, no matter how far apart they are! It’s like they can still talk to each other and know what the other is doing.

Another way to think of it is like having two magic coins. If you flip one coin and it lands on heads, the other coin will always land on tails, no matter how far apart they are.

Scientists are still trying to understand exactly how this happens, but it’s a very special and strange thing in the world of of very small things. Some think it shows that there is a way that very small parts of the stuff in the universe are connected that we cannot measure yet, or that many different possibilities exist and we only see one of them when we look for it.

If we could create this connection reliably and stably, we could potentially use it send information across distances nearly instantaneously! After all, a lot of the information we send right now is just 1s and 0s which we put together to make more complicated messages. This has uses in protecting information to keep it secret, making very fast computers, and maybe even “teleportation” by creating a duplicate at the other end of the connection, to name a few.

Like many fields of science, we are learning more about quantum stuff all the time, so this could change really fast. If you’re interested in learning more about quantum theory and research you’ll need a strong background in math and science. Algebra, trigonometry and classical physics would be a good first step (of many).