Quantum entanglement is the stuff of science fiction. A quantum computer could break encryption, and quantum entanglement could be the basis for faster-than-light communication. It’s so wild, Einstein spent decades trying to disprove it.
Origins of entanglement
It was 1935. Europe was rolling towards WWII, and Einstein had just settled down in the US after fleeing Nazi Germany. Together with Boris Podolsky and Nathan Rosen, he posited a thought experiment, known as the EPR paradox. It described a strange problem resulting from quantum mechanics, where particles affected each other at a distance. Erwin Schrödinger, the one from the cat, wrote Einstein about it, and coined the term ‘entanglement’. And the rest is history.
Well, not so much history as a long period of more thinking, experimenting, and trying to figure out what the heck is actually going on. The paradox Einstein and his companions thought up was not meant to introduce some new and cool concept. It was a paradox. A problem the three saw in trying to unify quantum mechanics and the theory of relativity.
Einstein called the implications of quantum entanglement ‘spooky interaction at a distance’. And it has proven to be exactly that.
What is entanglement?
To understand quantum entanglement, you first have to understand superposition. And that leads to the Schrödinger’s cat.
For those who don’t know it, the thought experiment is about a cat in a box. There’s also a flask of poison, a radioactive source, and a Geiger counter in there. When the radioactive source decays — a random event — it sets off the Geiger counter, killing the cat. So, at some point the cat will die.
The thing is, in quantum mechanics a particle can exist in different states simultaneously until you measure that state. Translating this to the thought experiment, the cat is both alive and dead at the same time, until you open the box. This state of both-alive-and-dead is called a quantum superposition. It’s weird, but it is how particles at the quantum scale behave. They don’t have a fixed position, momentum, etcetera, until you measure those properties.
On to entanglement.
It is possible to create a group of particles that have connected quantum properties and are in superposition as a group.
Imagine if you have two boxes, with two cats, with interconnected airflow. In both of these boxes, the cat is dead-and-alive. But! If the cat is dead in one box, it follows the one in the other box must be dead as well. This is even true if you move the boxes apart before opening them. As soon as you open one box and find a live cat, you know it is alive in the other box, and vice versa.
Translate that back to particles. If you measure a property of one particle in a group in superposition, this ends the superposition and the corresponding property is known for the other particles in the group.
This result of entanglement had Einstein and his companions in a state back in 1935. Because they concluded quantum entanglement violates the theory of relativity.
If you move two entangled particles apart and end their superposition is ended, two particles are affected at the same time. But that should be impossible. Because nothing can travel faster than light. If you look at all interactions of a particle in one second, it should not be able to interact with particles beyond the distance light travels in that second. This is called ‘locality’. So how can quantum entanglement cause two particles to be affected at the same time when they are not in the same place? There should be a delay related to the speed of light!
This paradox is why Einstein called it ‘spooky interaction at a distance’. Quantum entanglement is a non-local effect. He hated it, and he thought some hidden mechanic was missing from quantum mechanics that could solve the problem.
It took until 1956 for John Stewart Bell to come up with an alternative theory. He proved that these hidden mechanics in quantum mechanics actually require non-locality. He reasoned that while quantum entanglement is non-local, it does not lead to non-local exchange of information.
Basically, quantum entanglement still doesn’t allow faster-than-light communication.
It comes down to a kind of synchronized randomness. Let’s look at the boxed cats. Imagine two people that are far apart open the boxes, they will each randomly find a dead cat or a live one. But when they compare results (which they cannot do faster than light) they find they always match. There is no meaningful communication from the entanglement, it just turns out a predictable way after the fact.
The principle above cannot be used to communicate faster than light, but the entanglement does have effects. It’s hard to wrap your head around, but using this superposition of a group of particles, you can solve certain mathematical problems. Because of the way entangled particles circumvent locality, this allows these superpositioned particles to solve certain problems very fast. So, people are putting entanglement in computers.
The kind of problems this solves have real-world applications. The most important one is encryption. Encryption is based on the fact that multiplying two large prime numbers is very easy, but separating the resulting number back into two primes is very hard. Quantum computers change that equation, because they can solve the separation much faster. So, in theory, quantum computers can break encryption. All encryption. Everywhere. And very quickly.
Luckily, actual working quantum computers are scarce, and they are not capable of solving meaningfully large prime calculations to break actual encryption. If they were, we’d have a problem. And that day might come.
Quantum Mechanics are weird, and entanglement is a weird part of those mechanics. But it also really cool. Well, until criminals use it to break every encryption known to man.