If you know anything about quantum mechanics, you’ve probably heard of quantum entanglement. This feature of quantum mechanics is one of the most extraordinary discoveries of the 20th century and one of the most promising research avenues for advanced technologies in communications, computers and more.

But what is quantum entanglement and why is it so important? Why did it upset Albert Einstein? And why does it seem to violate one of the most important laws of physics?

## What is quantum entanglement in simple terms?

Every time you discuss quantum mechanics, things get complicated, and quantum entanglement is no different.

The first thing to understand is that particles exist in a state of “superposition” until observed. In a very common demonstration, the quantum particles are used as qubits in a quantum computer *both* 0 and 1 at the same time until observed, causing them to become a 0 or 1 seemingly at random.

Well, in simple terms, quantum entanglement is when two particles are created or interact in such a way that the key properties of those particles cannot be described independently.

For example, if two photons are generated and entangled, a particle is formed *can* have a clockwise rotation on one axis, so the other will necessarily have a counterclockwise rotation on the same axis.

In and of itself it’s not that radical. But because particles can also be described as wave functions in quantum mechanics, the act of measuring a particle’s spin is said to “collapse” its wave function to produce this measurable property (like going from 0 and 1 to just 0 or just 1).

However, when you do this with entangled particles, we get to the really incredible part of quantum entanglement. If you measure an entangled particle to determine its spin along one axis and collapse its wave function, the other particle will also collapse to produce the measurable property of spin, even though you haven’t observed the other particle.

If it is a pair of entangled particles *both* 0 and 1, and you measure one particle as 0, the other entangled particle automatically collapses to produce a 1, all by itself and without observer interaction.

This seems to be happening *immediately* and independent of their distance from each other, which originally led to the paradoxical conclusion that information about the spin of the measured particle is somehow transmitted to its entangled partner faster than the speed of light.

## Is quantum entanglement real?

Quantum entanglement is not only real, but also an important part of new technologies such as quantum computing and quantum communication.

In quantum computing, how can you work with qubits in a quantum processor without observing them and therefore breaking them down into plain old digital bits? How do you spot bugs without looking at the qubits and destroying the entire mechanism that makes quantum computing so powerful?

The quantum entanglement of multiple particles in a row is crucial to create enough distance between qubits and the outside world so that the vital qubits stay in superposition long enough for them to perform calculations.

Quantum communication is another area of research that hopes to use quantum entanglement to facilitate communication, although that doesn’t mean faster-than-light communication is on the horizon (in fact, such technology is likely impossible).

## Are all particles entangled?

To a certain extent, yes.

When most people talk about quantum entanglement, they use an example of two entangled particles behaving in a certain way to demonstrate the phenomenon, but this is largely an oversimplification of an incredibly complex quantum system.

The reality is that a given particle can be entangled with many different particles to varying degrees, not just in the “maximally entangled” state where two particles are one-to-one correlated with each other and only with each other.

For this reason, measuring one member of an entangled pair does not automatically guarantee that you know the state of the other particle in real-world applications, since that other particle also has other entanglements that it maintains. However, it gives you a better than chance chance of knowing the state of the other particle.

## Who discovered quantum entanglement?

Quantum entanglement, or at least the principles that describe the phenomenon, was first proposed by Einstein and his colleagues Boris Podolsky and Nathan Rosen in a 1935 article in the journal *physical examination* entitled “Can the Quantum Mechanical Description of Physical Reality Be Considered Complete?” In it, Einstein, Podolsky, and Rosen discussed that a particularly strong correlation of quantum states between particles can result in them having a single uniform quantum state.

They also found that this uniform state can mean that the measurement of one strongly correlated particle has a direct impact on the other strongly correlated particle, regardless of the distance between the two particles.

The purpose of the Einstein-Podolsky-Rosen paper was not to herald the “discovery” of quantum entanglement per se, but rather to describe this observed and discussed phenomenon and to argue that there must be a missing component of quantum mechanics that has yet to be discovered not discovered.

Because the strong correlation phenomenon they described violated the laws established in Einstein’s theory of relativity and seemed paradoxical, the paper argued that there must be something else that physicists were lacking in order to properly place the quantum realm under the umbrella of relativity. That “something different” has still not been found almost a century later.

The first use of the word “entanglement” to describe this phenomenon came from Erwin Schrödinger, who recognized it as one of the most fundamental features of quantum mechanics and argued that it was not a puzzle that would soon be solved under the theory of relativity, but rather a stark departure from classical physics.

## What did Einstein say about quantum entanglement?

Einstein famously described quantum entanglement as “spooky action at a distance,” but he actually described it as more than just a weird quirk of spooky particles with instantaneous knowledge of one another.

Einstein actually saw quantum entanglement as a mathematical paradox, an inherent contradiction in mathematical logic that shows there must be something wrong with the arguments put forward.

In the case of the so-called Einstein-Podolsky-Rosen paradox, it is argued that the basic rules of quantum mechanics are fully known and general relativity applies. If general relativity is valid, nothing in the universe can travel faster than the speed of light, which travels at 186,000 miles per second.

When quantum mechanics is fully understood, then the rules for the strong correlation between particles will be complete, and our observations will tell us all we need to know.

Since quantum particles are “of the universe” they should be governed by the speed of light like everything else, but quantum entanglement doesn’t just seem to instantaneously exchange information between particles that could theoretically be on opposite ends of the universe. What is even stranger is that this information can even travel *back and forth through time*.

Quantum entanglement through time would have all sorts of implications for the nature of causality, which is as fundamental a law of physics as it gets. It doesn’t work the other way around, effects can’t precede their cause, but some scientists believe these rules may apply to the quantum world no more than the speed of light.

This last point is still largely speculative, but it has some experimental basis and only complicates the paradox proposed by Einstein, Podolsky, and Rosen in their 1935 paper.

## Why is quantum entanglement important?

Quantum entanglement is important for two main reasons.

First, quantum entanglement is such a fundamental mechanism of the quantum world, while at the same time being one that we can directly interact with and influence. It could be a key way to harness some of the most fundamental properties of the universe to push our technology to new heights.

We know how to entangle particles and we do it regularly both in laboratories and in real-world applications such as quantum computers. Quantum computers in particular demonstrate the potential of quantum mechanics in modern technology, and quantum entanglement is the best tool we have to actually use quantum mechanics in this way.

The other main reason quantum entanglement is important is that it is a signpost pointing to something really fundamental in our universe. It’s as clear a demonstration as you can get that the quantum world is an almost purer form of the universe than we can see, and obeys laws we can explain.

If the whole universe is a stage and matter is the actors, then quantum entanglement – and quantum mechanics more broadly – can be the hanging cords that raise the curtains, the switches that turn the lights on and off, or even the costumes of the wear actors.

When we look at a play, there are two ways to appreciate it. You can drop by the theater and set pieces to appreciate the story the play conveys, or you can appreciate the quality of performance, staging and execution.

You could see two very different things if you watch the exact same performance, and quantum mechanics seems to be giving us a different way of seeing the same universe we’ve always seen, and quantum entanglement could be the key that takes us backstage brings.