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.

Imagine your friend has two cars. When he isn’t using one of them, the car is in his driveway. When you see your friend in town driving his car, you automatically are able to tell where his other car is: in his driveway. However you can only tell where the other car is when you observe either one of the cars. This is a fundamental property of entanglement. Entangled particles can only be described as a system and not independently of each other because observing them individually changes their properties.

Quantum entanglement is vaguely similar. When you have two particles with a correlated state, you can know what state the particle is by observing only one of the particles. Say particle 1 has a positive spin, you know by observing particle 1 that particle 2 has a negative spin. This is also applicable in computing, where if you know the state of one quantum gate, you can tell the state of another quantum gate when you observe it.

It’s important to note that particles are only in a correlated state as long as you don’t actively manipulate them to change their state. If you were to manually give particle 1 a negative spin then you wouldn’t be able to tell what state particle 2 is in.

Quantum entanglement has a lot of different possible applications in the real word. Things like quantum cryptography and quantum key distribution for example. The overarching concept is that by observing a quantum system you change its properties.

Quantum entanglement is closely related with another quantum phenomenon that you might already know: the superposition principle. Let’s say that I have a particle. My particle spins on itself. If I measure its spin, it can either spin to the left or to the right. I cannot know in advance whether it will spin to the left or to the right. And it’s not just a lack of information because we can create an experiment in which the different spins of a single particle can interfere. We say that the particle’s spin in a superposed state of both left and right, until we measure it.

Now there’s already a good thought experiment that explains quantum entanglement: Schrödinger’s cat. I have trapped a cat in a box, and I have installed a cruel setup inside. There’s a detector in which I can enter my particle. If it spins to the right, the detector breaks a bottle of poison that kills the cat. If it spins to the left, nothing happens. Importantly, the detector does not communicate the measurement to me.

Now, I insert my superposed particle inside the detector and don’t look at the result. The particle exits the detector and I can keep it. Because my particle was in a superposed state, I don’t know wether the detector has measured a right or left spin. I don’t know whether the cat is dead. Once again, it’s not just a lack of knowledge because I could imagine an experiment in which its dead and alive state interfere.

Now imagine that I make a measurement of the particle’s spin and it measures right. Then, I am 100% certain that the cat will be dead once I open the box. Even though both the particle and the cat were still in superposition, once I measure one, I will know the state of the other. That’s quantum entanglement.

It’s important because we can use this interaction between quantum objects to copy and paste information in a quantum computer without making a measurement. A measurement would damage the quantum information because it would collapse the superposed state.

Scientists are able to get pairs of particles into a state where when we

• learn what state one of the two is in
• the other will always be in that same state when we learn it.

This is useful because it seems like that “information” travels instantly between the two.

TLDR of the rest of the post, the unknown state of the particles is not the same as flipping a coin and refusing to check the result. There is not an actual state the particles are in until we learn it. Which is why quantum mechanics are so insanely counter intuitive.

You’ll not be able to get a satisfying ELI5 answer to this. Quantum mechanics are extremely counter-intuitive. You may think, like I did, that the unknown state of something is actually sort of already set in stone but we just don’t know it yet. I thought this because scientists are very clear about not saying things they haven’t verified so of course they couldn’t know the state of something until they measure it. You may think it is like flipping a coin and covering the result. You don’t know the result but it is already one way or the other. That’s a typical ELI5 answer to this but it is incredibly misleading because quantum states are not so-called “classical hidden states” which is like a fancy way of saying “I flipped a coin but haven’t looked at the result yet.”

I find this section on Wikipedia useful for seeing how Scientists know that quantum states are not the “classical hidden states” I naively suspected. https://en.wikipedia.org/wiki/Double-slit_experiment#Mach-Zehnder_interferometer

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).

I’m not sure an ELI5 answer to this set of questions is possible. Even just “what is it” is challenging with something so counter-intuitive. While I’m interested in seeing someone try, it’s well outside my abilities to even attempt.