Quantum entanglement is a totally mind-blowing phenomenon in quantum physics. The idea was first dreamed up in 1935 by Einstein, Podolsky and Rosen as a thought experiment whilst trying to prove that quantum mechanics wasn’t a fully baked theory yet. Ironically though, it turned out to be very real – and that was experimentally shown in 1972 using something called Bell’s Inequality.

So what actually is quantum entanglement? It’s when two or more particles get so closely linked together that their individual quantum states are tied up with each other. It’s like they become one connected system, even if the particles are far apart in space. So if you measure one entangled particle, the properties of its partner particle snap into correlation instantly, faster than the speed of light.

It’s utterly bizarre – the particles act interconnected as if they’re not separate entities anymore. Experiments have proven this “spooky action-at-a-distance” is real, and it reveals something deep and non-intuitive about our quantum reality. Quantum entanglement shows that physics at the atomic scale is full of radical phenomena that clash with our everyday assumptions about causality and connections in space and time.

## Correlation of entanglement

So when particles get entangled, they end up having this bizarre, instant connection that totally defies common sense. If you measure one entangled particle, you instantly know something about the other one – even if they’re light years apart! This is called correlation due to entanglement.

To give this an analogy, entangled particles are like synchronized dancers – if one particle spins clockwise, its entangled partner will instantly spin counterclockwise. Spookily, this happens immediately, no matter how far apart they are. Einstein dubbed the effect “spooky action at a distance” and thought it was too weird to be real – but experiments have proven that it is a real effect.

This instant correlation only happens with entangled particles – there’s no actual physical force connecting them after they split up. It’s like they share a quantum state rather than being separate individual particles. This entanglement correlation is completely unique to the quantum realm. In our everyday world, two separated objects can’t instantly affect each other without a force acting between them. But at tiny quantum scales, the world plays by different rules. Particles can remain mysteriously connected across space and time in a way that defies explanation.

## Non-locality

Non-locality is a concept in quantum physics that refers to the phenomenon where two or more entangled particles exhibit instantaneous correlations in their measurements, regardless of the distance between them. This means that when one particle’s quantum state is measured and a property is determined, the corresponding property of the other entangled particle is immediately known, even if it is far away and the two particles are spatially separated, seemingly violating the speed-of-light limit for information transfer.

## Bell’s Theorem

Bell’s Theorem is a huge deal in quantum physics. It was thought up in the 1960s by a physicist named John Bell. He wanted to figure out if the freaky quantum correlations between entangled particles could be explained by old-school classical physics, or if we needed something totally new – a non-classical, quantum explanation.

Basically, Bell came up with this mathematical inequality, now called Bell’s inequality. He showed that if quantum particles like photons operated under classical physics, they would obey this inequality when measured. But if the quantum theory was correct, certain measurements on entangled particles would violate Bell’s inequality.

This was big news because it meant scientists could design experiments to test Bell’s theorem. Over the years, physicists have run all kinds of tests on entangled particles. And you know what? The results have consistently shown that entangled particles violate Bell’s inequality, just as quantum theory predicts.

As a result of Bell’s theorem, it was proven that these inexplicable quantum correlations really can’t be explained by classical physics – we definitely need quantum mechanics to account for it. This was groundbreaking because it showed that the quantum realm is utterly different from the world we see around us every day. At the quantum level, reality is nonlocal and things are interconnected in mind-bending ways that classical physics just can’t grasp.

Philosophically, this shakes the foundations of how we think about reality, causality, and connections in space and time. Practically, it also allows new quantum technologies like unhackable cryptography and quantum computing. But at its core, Bell’s theorem definitively proved that to make sense of the tiny quantum world, we need an entirely new set of rules – the strange but powerful rules of quantum mechanics.

## Future applications of quantum entanglement

Quantum entanglement, a captivating phenomenon in the realm of quantum physics, has transcended its enigmatic nature to find practical applications across a spectrum of scientific and technological domains. Despite its initial paradoxical appearance, entanglement has been harnessed to yield substantial benefits in diverse fields.

One notable application of entanglement is in quantum cryptography, where it underpins the principles of quantum key distribution (QKD). QKD protocols, exemplified by the renowned BB84 protocol which was the first example of QKD, facilitate secure communication between parties by enabling the establishment of an impervious shared secret key. The remarkable attribute of entanglement ensures that any attempt to intercept the key would disrupt the entangled state, alerting communicators to potential eavesdropping.

Entanglement’s role extends to the tantalizing concept of quantum teleportation. Leveraging entanglement, quantum teleportation allows the quantum state of one particle to be instantaneously transferred to another, even if separated by significant distances. This groundbreaking phenomenon holds promise for quantum communication and the prospective development of quantum internet infrastructure.

In essence, quantum entanglement, with its seemingly paradoxical properties, has evolved from a thought experiment to a wellspring of practical applications. Its utilization spans quantum cryptography, computing, communication, imaging, sensing, and fundamental research, holding the potential to reshape the landscape of modern science and technology.

Quantum entanglement has been experimentally verified through various tests, including Bell inequality tests, which have consistently shown correlations that cannot be explained by classical physics. While entanglement has been widely studied and demonstrated in laboratories, it remains one of the most intriguing and counterintuitive aspects of quantum mechanics.