What is quantum entanglement? All about this 'spooky' quirk of physics


Quantum entanglement appears to defy basic physics rules, yet it also supports a number of crucial technologies.

There's a fair probability you've heard about quantum entanglement if you know anything about quantum mechanics. This property of quantum mechanics is one of the most remarkable discoveries of the twentieth century, as well as one of the most promising study areas for new technologies in communications, computers, and other fields.

But what exactly is quantum entanglement, and why is it so crucial? What was it about it that frightened Albert Einstein? And why does it appear to defy one of physics' most fundamental laws?

What Is Quantum Entanglement in Simple Terms?

Things get confusing whenever you talk about quantum mechanics, and quantum entanglement is no exception.

The first thing to realise is that until they are noticed, particles exist in a condition of "superposition." The quantum particles employed as qubits in a quantum computer are both 0 and 1 at the same time in a common demonstration, until they are detected, at which point they appear to randomly become a 0 or 1.

To put it another way, quantum entanglement occurs when two particles are formed or interact in such a way that their key properties cannot be explained separately.

If two photons are created and entangled, for example, one particle may have a clockwise spin on one axis, whereas the other must have a counterclockwise spin on the same axis.

This isn't particularly revolutionary in and of itself. However, because quantum physics describes particles as wave functions, measuring a particle's spin is said to "collapse" its wave function to generate that observable feature (like going from both 0 and 1 to only 0 or only 1).

However, when we apply this to entangled particles, we get the most fascinating portion of quantum entanglement. Even if you did not observe the second particle, when you measure an entangled particle to determine its spin along some axis and collapse its wave function, the other particle collapses as well, producing the measurable property of spin.

If you measure one of two entangled particles as 0, the other entangled particle collapses to generate a 1 on its own, without any interaction from the observer.

This appears to happen instantly and regardless of their distance from one another, leading to the bizarre conclusion that information about the measured particle's spin is somehow communicated to its entangled partner faster than the speed of light.

Are All Particles Entangled?

Yes, to some extent.

When most people talk about quantum entanglement, they use an example of two entangled particles behaving in a certain way to illustrate the phenomena, although this is a highly simplified version of a very complex quantum system.

In actuality, a single particle can be entangled with a variety of other particles to differing degrees, rather than only the "maximally entangled" state, in which two particles are one to one correlated and only to each other.

This is why, in real-world applications, measuring one part of an entangled pair does not guarantee that you will know the state of the other particle, because the other particle has additional entanglements to maintain as well. It does, however, give you a higher probability than chance of knowing the state of the other particle.

Who Discovered Quantum Entanglement?

In a 1935 publication in the journal Physical Review titled "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete," Einstein and his colleagues Boris Podolsky and Nathan Rosen postulated quantum entanglement, or at least the principles that characterise the phenomena. Einstein, Podolsky, and Rosen explained how a very strong connection of quantum states between particles can lead to a single unified quantum state between them.

They also discovered that due to this unified state, measurements of one strongly correlated particle can have a direct effect on the measurement of the other strongly correlated particle, regardless of the distance between the two particles.

The goal of the Einstein-Podolsky-Rosen paper wasn't so much to proclaim the "finding" of quantum entanglement as it was to describe a phenomenon that had already been observed and discussed, and to argue that there must be a missing component of quantum mechanics that hasn't been discovered yet.

Because the strong correlation phenomenon they described looked to be counterintuitive and defied Einstein's relativity equations, the paper contended that physicists were missing something else that would correctly fit the quantum realm under the cover of relativity. Almost a century later, that "something else" has yet to be discovered.

Erwin Schrödinger was the first to adopt the term "entanglement" to characterise this occurrence, recognising it as one of quantum mechanics' most fundamental properties and arguing that it wasn't a riddle that would be solved shortly under relativity, but rather a complete break from classical physics.

What Did Einstein Say about Quantum Entanglement?

Quantum entanglement is famously defined as "spooky action at a distance," but Einstein described it as more than simply a strange quirk of ghostly particles with instantaneous knowledge of each other.

Quantum entanglement, according to Einstein, is a mathematical paradox, or an inherent contradiction in mathematical logic that indicates that something about the arguments being made is incorrect.

The arguments in the so-called Einstein-Podolsky-Rosen conundrum are that the fundamental rules of quantum mechanics are entirely understood and that general relativity is correct. Nothing in the cosmos can go faster than the speed of light, which is 186,000 miles per second if general relativity is correct.

If quantum mechanics is fully understood, the rules regulating strong particle correlations are complete, and our observations provide all the information we require.

Because quantum particles are "of the universe," they should, like everything else, be governed by the speed of light. However, quantum entanglement appears to transfer information instantly between particles that may conceivably be on opposite ends of the cosmos. Even worse, this data may be able to go back and forth in time.

Quantum entanglement in time would have a lot of ramifications for the nature of causality, which is about as fundamental a physical law as you can get. It doesn't work the other way around; consequences can't come before their causes, but some scientists believe that those constraints don't apply to quantum mechanics any more than they do to the speed of light.

This last argument is still somewhat speculative, but it does have some experimental support, and it only adds to the paradox provided by Einstein, Podolsky, and Rosen in their 1935 study.

Why Is Quantum Entanglement Important?

Entanglement in quantum mechanics is significant for two reasons.

To begin with, quantum entanglement is a fundamental mechanism of the quantum universe that we can directly interact with and control. It could be a route to unlocking some of the universe's most fundamental features and pushing our technology to new heights.

We understand how to entangle particles and do so on a daily basis in both laboratory and real-world applications such as quantum computers. Quantum computers, in particular, show how quantum mechanics may be used in modern technology, and quantum entanglement is the best instrument we have for utilising quantum mechanics in this way.

Another crucial argument for quantum entanglement is because it is a signpost pointing to something very basic about our reality. It's as evident as it gets that the quantum world is a purer version of the cosmos than the one we can see, and that it follows principles that we can understand.

If the entire universe is a stage, and matter is the actors, quantum entanglement—and quantum mechanics in general—could be the line riggings that raise the curtains, the switches that turn on and off the lights, or even the actors' clothing.

When we see a play, we can appreciate it in two ways. You can appreciate the tale told by the play by looking past the theatre and set pieces, or you can appreciate the quality of the performance, staging, and execution.

By seeing the same performance again, you can see two quite different things, and quantum mechanics appears to provide us a new way of experiencing the same universe we've always seen, and quantum entanglement may be the key to getting us backstage.

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