If you know anything about quantum mechanics, there's a good chance you've heard of quantum entanglement. This feature of quantum mechanics is one of the most extraordinary discoveries of the 20th century and is one of the most promising avenues of research for advanced technologies in communications, computing, and more.
But what is quantum entanglement and why is it so important? Why did it freak Albert Einstein out? And why does it appear to violate one of the most important laws of physics?
What Is Quantum Entanglement in Simple Terms?
Any time you discuss quantum mechanics, things are going to get complicated, and quantum entanglement is no different.
The first thing to understand is that particles exist in a state of "superposition" until they are observed. In a very common demonstration, the quantum particles used as qubits in a quantum computer are both 0 and 1 at the same time until they are observed, whereby they appear to randomly become a 0 or 1.
Now, in simple terms, quantum entanglement is when two particles are produced or interact in such a way that the key properties of those particles cannot be described independently of each other.
For example, if two photons are generated and are entangled, one particle may have a clockwise spin on one axis so that the other will necessarily have a counterclockwise spin on that same axis.
In and of itself, this is not that radical. But because particles in quantum mechanics can also be described as wave functions, the act of measuring the spin of a particle is said to "collapse" its wave function to produce that measurable property (like going from both 0 and 1 to only 0 or only 1).
When you do this to entangled particles, however, we get to the really incredible part of quantum entanglement. When you measure an entangled particle to determine its spin along some axis and collapse its wave function, the other particle also collapses to produce the measurable property of spin, even though you did not observe the other particle.
If a pair of entangled particles are both 0 and 1, and you measure one particle as 0, the other entangled particle automatically collapses to produce a 1, entirely on its own and without any interaction from the observer.
This appears to happen instantaneously and regardless of their distance from each other, which originally led to the paradoxical conclusion that the information about the measured particle's spin is somehow being transmitted to its entangled partner faster than even the speed of light.
Is Quantum Entanglement Real?
Not only is quantum entanglement real, but it's also an important component of emerging technologies like quantum computing and quantum communications.
In quantum computing, how can you operate on qubits in a quantum processor without observing them and therefore collapsing them into plain old digital bits? How do you detect errors without looking at the qubits and destroying the whole mechanism that makes quantum computing so powerful?
The quantum entanglement of several particles in a row is vital to putting enough distance between qubits and the outside world to keep the vital qubits in superposition long enough for them to perform computations.
Quantum communications is another area of research that hopes to take advantage of quantum entanglement to facilitate communication, though it doesn't mean that faster than light communication is on the horizon (in fact, such a technology is likely impossible).
Are All Particles Entangled?
To some degree, yes.
When most people discuss quantum entanglement, they use an example of two entangled particles behaving in a certain way to demonstrate the phenomenon, but this is very much a simplification 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 the "maximally entangled" state where two particles are one to one correlated to one another and only to each other.
This is why measuring one part of an entangled pair doesn't automatically guarantee that you will know the state of the other particle in real-world applications, since that other particle has other entanglements it is maintaining as well. It does give you a better than random chance of knowing the other particle's state though.
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 paper in the journal Physical Review titled "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete." In it, Einstein, Podolsky, and Rosen discussed that an especially strong correlation of quantum states between particles can lead to them having a single unified quantum state.
They also determined that this unified state can result in the measurement of one strongly correlated particle having a direct effect on the other strongly correlated particle without regard to the distance between the two particles.
The purpose of the Einstein-Podolsky-Rosen paper wasn't to announce the "discovery" of quantum entanglement, per se, but rather to describe this phenomenon that had been observed and discussed and argue that there must be a missing component of quantum mechanics that hasn't been discovered yet.
Since the strong correlation phenomenon they described violated laws laid down in Einstein's relativity and appeared to be paradoxical, the paper argued there must be something else that physicists were missing that would properly place the quantum realm under the umbrella of relativity. That "something else" still hasn't been found almost a century later.
The first use of the word "entanglement" to describe this phenomenon belongs to Erwin Schrödinger, who recognized it as one of quantum mechanics' most fundamental features and argued that it wasn't a mystery that would soon be resolved under relativity, but rather a strong break from classical physics entirely.
What Did Einstein Say about Quantum Entanglement?
Famously, Einstein described quantum entanglement as "spooky action at a distance," but he actually described it as more than just a weird quirk of ghostly particles with instantaneous knowledge of each other.
Einstein actually saw quantum entanglement as a mathematical paradox, an inherent contradiction in mathematical logic that shows that something about the arguments being made must be wrong.
In the case of the Einstein-Podolsky-Rosen paradox, as it came to be called, the arguments are that the fundamental rules of quantum mechanics are completely known and that general relativity is valid. If general relativity is valid, then nothing in the universe can travel faster than the speed of light, which moves at 186,000 miles per second.
If quantum mechanics were fully understood, then the rules governing the strong correlation between particles are complete and our observations tell us everything we need to know.
Since quantum particles are "of the universe" they ought to be governed by the speed of light just like everything else, but quantum entanglement not only appears to instantaneously share information between particles that could theoretically be on opposite ends of the universe. Even weirder, this information might even travel back and forth through time.
Quantum entanglement through time would have all kinds of implications for the nature of causality, which is about 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 think that those rules might not apply to the quantum realm any more than the speed of light would.
This last point is still mostly speculative, but it has some experimental basis, and it just further complicates the paradox that Einstein, Podolsky, and Rosen proposed in their 1935 paper.
Why Is Quantum Entanglement Important?
Quantum entanglement is important for two major reasons.
First, quantum entanglement is such a fundamental mechanism of the quantum world while also being one that we can directly interact with and influence. It may provide a key way to harness some of the most fundamental properties of the universe to advance our technology to new heights.
We know how to entangle particles and do so regularly both in laboratories and in real-world applications like quantum computers. Quantum computers in particular demonstrate the potential of quantum mechanics in modern technology, and quantum entanglement is the best tool we have for actually leveraging quantum mechanics in this way.
The other major reason why quantum entanglement is important is that it is a signpost that points towards something truly fundamental about our universe. It is as clear a demonstration as you can get that the quantum world is almost a purer form of the universe than the one we can see and that obeys laws that we can explain.
If all the universe is a stage and matter is the actors, then quantum entanglement—and quantum mechanics more broadly—may be the line riggings that lift the curtains, the switches that turn the lights on and off, or even the costumes that the actors wear.
If we watch a play, there are two ways to appreciate it. You can see past the theater and the set pieces to appreciate the story that the play conveys, or you can appreciate the quality of the performance, the staging, and the execution.
You might see two very different things by watching the exact same performance, and quantum mechanics appear to give us a different way of seeing the same universe we've always seen, and quantum entanglement may be the key that gets us backstage.