# Quantum information theorists are shedding light on entanglement, one of the daunting mysteries of quantum mechanics

A computer based on a quantum bit rather than a classical bit could have a significant computational advantage. And that’s because a classic bit gives a binary answer – 1 or 0 – to a single question.

In contrast, the qubit produces a binary response to an infinite number of queries using the property of quantum superposition. This property allows researchers to link multiple qubits into a state known as an entangled quantum state. Here, the qubits act together in a way that classical bit arrays cannot.

This means that a quantum computer can perform some calculations much faster than a normal computer. For example, one device reportedly used 76 empty qubits to solve a sampling problem 100 trillion times faster than a classical computer.

But the exact force or principle of nature responsible for this quantum state that underlies quantum computing is a big unanswered question. A solution proposed by my colleagues and I in quantum information theory is related to Einstein’s principle of relativity.

## Quantum information theory

The principle of relativity states that the laws of physics are the same for all observers, no matter where they are in space, what direction they are facing or how they are moving relative to each other. My team showed how to use the principle of relativity in conjunction with the principles of quantum information theory to account for entangled quantum particles.

Quantum information theorists like me think of quantum mechanics as a theory of information principles rather than a theory of forces. This is very different from the typical approach to quantum physics, where force and energy are important concepts for making the calculations. In contrast, quantum information theorists do not need to know what kind of physical force might cause the mysterious behavior of entangled quantum particles.

This gives us an advantage in explaining quantum entanglement because, as the physicist John Bell proved in 1964, any explanation of quantum entanglement in terms of forces requires what Einstein called “scary actions at great distances.” distant”.

That is because the results of the measurements of the two quantum particles are implicitly correlated – even if those measurements are made at the same time and the particles are physically separated by a large distance. Therefore, if a force causes quantum entanglement, it would have to act faster than the speed of light. And faster-than-light force violates Einstein’s theory of special relativity.

Many researchers are trying to find an explanation for quantum entanglement that does not require alarming actions from afar, like the team’s proposed solution.

## Classical and quantum shelter

In entanglement, you can know two particles together – call them particle 1 and particle 2 – so that when you measure particle 1, you immediately know something about particle 2.

Imagine you are sending mail to two friends, whom physicists usually call Alice and Bob, all mittens from the same pair of gloves. When Alice opens her box and sees a glove on the left, she will immediately know that when Bob opens the other box he will see the glove on the right. Each combination of box and glove produces one of two outcomes, a right-handed glove or a left-handed glove. There is only one possible guess – to open the box – so Alice and Bob have captured classic pieces of information.

But in quantum entanglement the case concerns entangled qubits, which behave very differently than classical bits.

## Qubit behavior

Consider a property of electrons called spin. When you measure the spin of an electron using vertically oriented magnets, you get a spin that is always up or down, with nothing in between. That’s the result of a binary measurement, so this is a bit of information.

If you turn the magnets on their sides to measure electron spin horizontally, you will always get a left or right spin, anything in between. The vertical and horizontal orientation of the magnets are two different measurements of this same bit. Thus, electron spin is a qubit – it creates a binary response in multiple dimensions.

## Quantum superposition

Now, suppose you first measure the spin of an electron vertically and find that it is up, then you measure its spin horizontally. When you stand straight up, you don’t move to your right or left at all. So if I measure how much you move from side to side while standing straight up, I get zero.

That’s exactly what you’d expect for the vertical electron spin. Since they have a vertical up turn, like standing straight up, they should not have any left or right turn horizontally, in line with side to side movement.

Surprisingly, physicists have found that half of them are horizontally on the right and half of them are horizontally on the left. Now it doesn’t seem to make sense that a spin (-1) and a left spin (+1) results in an electron’s vertical spin when measured horizontally, just as we would expect any side-to-side movement when stand straight up.

But when you add up all the left (-1) and right (+1) spin results you get zero, as we expected in the horizontal direction when our spin state has a vertical spin. So, on average, it’s like no side-to-side or horizontal movement when we stand up straight.

This 50-50 ratio over the binary outcomes (+1 and -1) is what the physicists are talking about when they say that the vertical spin of an electron is a quantum superposition of left and right horizontal spins.

## Addressing the principle of relativity

According to quantum information theory, all of quantum mechanics, including its trapped quantum states, is based on the qubit with its quantum superposition.

What my colleagues and I have suggested is that this quantum superposition is a result of the principle of relativity, which states (again) that the laws of physics are the same for all observers with a different orientation in space.

If the electron with a vertical spin in the upward direction was going straight through the horizontal magnets as you would expect, it would not have a horizontal spin. This would violate the principle of relativity, which states that the particle should have spin regardless of whether it is being measured in the horizontal or vertical direction.

Because an electron has a vertical spin in the upward direction when measured horizontally, quantum information theorists can say that the principle of relativity is (ultimately) responsible for quantum entanglement.

And since no force is involved in this principled explanation, there are none of the “distant, frightening actions” that Einstein joked about.

With the technological implications of quantum entanglement for quantum computing firmly established, it’s nice to know that one big question about its origins could be answered with a well-respected principle of physics.

*This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.*

It was written by William Mark Stukey, *Elizabethtown College*.

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*William Mark Stuckey does not work for, consult with, or own shares in, or receive funding from, any company or organization that would benefit from this article. this article, and did not disclose any relevant connections beyond their academic appointment.*