Quantum Entanglement Illustration

What is quantum entanglement? Physicist explains Einstein’s ‘frightening action at a distance’

Illustration of quantum entanglement

When two particles are entangled, the state of one is linked to the state of the other.

Quantum entanglement is a phenomenon in which the quantum states of two or more objects become correlated, meaning that the state of one object can affect the state of others even though the objects are separated by large distances. This happens because, according to quantum theory, particles can exist in multiple states at the same time (a concept known as superposition) and can be inextricably linked or “entangled” even if they are physically separate.

Three researchers have been awarded the 2022 Nobel Prize in Physics for their groundbreaking work in understanding quantum entanglement, one of nature’s most puzzling phenomena.

Quantum entanglement, in the simplest terms, means that the aspects of one particle of an entangled pair depend on the aspects of the other particle, no matter how far apart they are or what separates them. These particles could be, for example, electrons or photons, and one aspect could be the state they are in, such as whether they “spin” in one direction or another.

The weird part about quantum entanglement is that when you measure something about one particle in an entangled pair, you immediately know something about the other particle, even if they are millions of light years apart. This strange connection between the two particles is instantaneous, apparently breaking a fundamental law of the universe. This is why Albert Einstein called the phenomenon “frightening action at a distance”.

After spending the better part of two decades conducting experiments rooted in quantum mechanics, I’ve come to terms with its strangeness. Thanks to ever more precise and reliable instruments and the work of this year’s Nobel laureates, Alain Aspect, John Clauser and Anton Zeilinger, physicists are now integrating quantum phenomena into their knowledge of the world with an exceptional degree of certainty.

However, even well into the 1970s, researchers were still divided on whether quantum entanglement was a real phenomenon. And for good reason – who would dare to contradict the great Einstein, who himself doubted it? It took the development of new experimental technologies and daring researchers to finally dispel this mystery.

cat in box

According to quantum mechanics, particles are simultaneously in two or more states until they are observed – an effect clearly captured by Schrödinger’s famous thought experiment of a cat that is both dead and alive simultaneously.

Existing in multiple states at once

To truly understand the frightening nature of quantum entanglement, it is important to first understand quantum superposition. Quantum superposition is the idea that particles exist in multiple states at once. When a measurement is made, everything happens as if the particle selects one of the states of the superposition.

For example, many particles have an attribute called spin that is measured either “up” or “down” for a given analyzer orientation. But until you measure a particle’s spin, it exists simultaneously in a superposition of spin up and spin down.

There is a probability attached to each state, and it is possible to predict the average outcome from many measurements. The probability of a single measurement going up or down depends on these probabilities, but is itself unpredictable.

Although very strange, mathematics and a large number of experiments have shown that quantum mechanics correctly describes physical reality.

Two entangled particles

The frightening nature of quantum entanglement emerges from the reality of quantum superposition and was clear to the founding fathers of quantum mechanics who developed the theory in the 1920s and 1930s.

To create entangled particles, you essentially divide a system into two, where the sum of the parts is known. For example, you can split a spin-zero particle into two particles that will necessarily have opposite spins for their sum to be zero.

In 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen published a paper describing a thought experiment designed to illustrate an apparent quantum entanglement absurdity that challenged a fundamental law of the universe.

A simplified version of this thought experiment, attributed to David Bohm, considers the decay of a particle called the pi meson. When this particle decays, it produces an electron and a positron which have opposite spins and move away from each other. Therefore, if the spin of the electron is measured up, then the measured spin of the positron can only be down, and vice versa. This is true even if the particles are billions of miles apart.

It would be nice if the measured spin of the electron was always high and the measured spin of the positron was always low. But because of quantum mechanics, the spin of every particle is both higher and lower until it is measured. It is only when the measurement occurs that the spin quantum state “collapses” up or down – instantly collapsing the other particle into the opposite spin. This seems to suggest that the particles communicate with each other by means that move faster than the speed of light. But according to the laws of physics, nothing can travel faster than the speed of light. Surely the measured state of one particle cannot instantly determine the state of another particle at the other end of the universe?

Physicists, including Einstein, came up with a number of alternative interpretations of quantum entanglement in the 1930s. They hypothesized that there was an unknown property – called hidden variables – that determined the state of a particle before the measurement. But at the time, physicists didn’t have the technology or a definition of a clear measurement that could test whether quantum theory needed to be modified to include hidden variables.

disprove a theory

It took until the 1960s before there were any clues to an answer. John Bell, a brilliant Irish physicist who did not live to receive the Nobel Prize, designed a scheme to test whether the notion of hidden variables made sense.

Bell produced an equation now known as Bell’s inequality that is always correct – and only correct – for hidden variable theories, and not always for quantum mechanics. Thus, if Bell’s equation was found to be unsatisfied in a real-world experiment, local hidden variable theories can be ruled out as an explanation for quantum entanglement.

The experiments of the 2022 Nobel laureates, notably those of Alain Aspect, were the first tests of Bell’s inequality. The experiments used entangled photons, rather than pairs of an electron and a positron, as in many thought experiments. The results definitively ruled out the existence of hidden variables, a mysterious attribute that would predetermine the states of entangled particles. Collectively, these experiments and many subsequent experiments confirmed quantum mechanics. Objects can be correlated over large distances in ways that physics before quantum mechanics cannot explain.

Importantly, there is also no conflict with special relativity, which forbids faster-than-light communication. The fact that measurements over large distances are correlated does not imply that information is transmitted between the particles. Two distant parties making measurements on entangled particles cannot use the phenomenon to transmit information faster than the speed of light.

Today, physicists continue to research quantum entanglement and study potential practical applications. Although quantum mechanics can predict the probability of a measurement with incredible

precision
How well the measured value conforms to the correct value.

” data-gt-translate-attributes=”[{” attribute=””>accuracy, many researchers remain skeptical that it provides a complete description of reality. One thing is certain, though. Much remains to be said about the mysterious world of quantum mechanics.

Written by Andreas Muller, Associate Professor of Physics, University of South Florida.

This article was first published in The Conversation.The Conversation


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