The year 2025 marks the 100th anniversary of the birth of quantum mechanics. In the century since the field’s inception, scientists and engineers have used quantum mechanics to create technologies such as lasers, MRI scanners, and computer chips.
Today, researchers are working on building quantum computers and finding ways to securely transfer information using an entirely new field called quantum information science.
But despite creating all these groundbreaking technologies, physicists and philosophers who study quantum mechanics still haven’t answered some of the big questions posed by the field’s founders. Given recent advances in quantum information science, researchers like me are using quantum information theory to explore new ways to think about these fundamental unanswered questions. And one direction we’re looking at is connecting Albert Einstein’s principle of relativity to the qubit.
Quantum computers
Quantum information science focuses on building quantum computers based on quantum information “bits” or qubits. The qubit is historically based on the discoveries of physicists Max Planck and Einstein. They initiated the development of quantum mechanics in 1900 and 1905, respectively, when they discovered that light exists in discrete or “quantum” bundles of energy.
These quanta of energy are also found in tiny forms of matter, such as atoms and electrons, which make up everything in the universe. It is the peculiar properties of these tiny packets of matter and energy that are responsible for the qubit’s computing advantages.
A computer based on a quantum bit rather than a classical bit would have a significant computational advantage because a classical bit can only produce a binary response to a query – either a 1 or a 0.
In turn, a 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 in a state called a quantum entanglement, where the entangled qubits act collectively in a way that classical bit strings cannot.
This means that a quantum computer can perform some calculations much faster than a regular computer. For example, one device reportedly used 76 entangled qubits to solve a sampling problem 100 trillion times faster than a classical computer.
But the exact force or principle of nature that is responsible for this quantum entanglement that underlies quantum computing is a big open question. One solution that my colleagues in quantum information theory and I have proposed involves Einstein’s principle of relativity.
Quantum information theory
The principle of relativity states that the laws of physics are the same for all observers, regardless of where they are in space, how they are oriented, or how they move relative to each other. My team has shown how to use the principle of relativity, along with the principles of quantum information theory, to account for quantum entangled 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 quantum physics approach, where force and energy are important concepts for doing calculations. In contrast, quantum information theorists do not need to know what kind of physical force might be causing the mysterious behavior of entangled quantum particles.
This gives us an advantage in explaining quantum entanglement because, as physicist John Bell demonstrated in 1964, explaining quantum entanglement in terms of forces requires what Einstein called “spooky action at a distance.”
This is because the results of measurements of two entangled quantum particles are related—even if those measurements are made simultaneously and the particles are physically separated by a very large distance. So, if a force causes quantum entanglement, it has to be moving faster than the speed of light. And a 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 spooky actions at a distance, such as the solution my team proposes.
Classical and quantum entanglement
In entanglement, you can know something about the collective state of two particles – let’s call them particle 1 and particle 2 – so when you measure particle 1, you can immediately know something about particle 2.
Imagine sending two friends, physicists usually call Alice and Bob, one glove from each pair of gloves. When Alice opens her box and sees the left glove, she immediately knows that when Bob opens the other box, he will see the right glove. Each combination of box and glove produces one of two outcomes, either the right glove or the left glove. There is only one possible measurement—opening the box—so Alice and Bob have mixed up the classical pieces of information.
But in quantum entanglement, the situation involves entangled qubits that behave very differently from classical bits.
Qubit behavior
Consider a property of electrons called spin. When you measure the spin of an electron using vertically oriented magnets, you will always get a spin either up or down, nothing in between. This is a binary measurement result, so this is a bit of information.
If you turn the magnets sideways to measure the spin of an electron horizontally, you will always get a spin to the left or the right, nothing in between. The vertical and horizontal orientations of the magnets create two different measurements of this same bit. So, the electron spin is a qubit – it produces a binary response to multiple measurements.
Quantum superposition
Now, suppose you first measure the spin of an electron vertically and find that it’s up, and then you measure its spin horizontally. When you’re standing upright, you don’t move to the right or left at all. So if I measure how much you move from side to side while standing upright, I get zero.
This is exactly what you would expect for vertical spin up electrons. Since they have spin up oriented vertically, similar to standing upright, they should not have spins horizontally to the left or right, similar to moving from side to side.
Surprisingly, physicists found that half of them were horizontally to the right and half were horizontally to the left. Now, it doesn’t seem logical that a vertical spin-up electron would have spin-left (-1) and spin-right (+1) results when measured horizontally, just as we wouldn’t expect side-to-side motion when standing upright.
But when you add up all the left (-1) and right (+1) spin results, you get zero, as we would expect in the horizontal direction when our spin state is vertical spin up. So, on average, it’s like standing upright with no side-to-side or horizontal movement.
This 50-50 ratio in binary (+1 and -1) outcomes is what physicists are talking about when they say that an electron with vertical spin upward is in a quantum superposition of horizontal spins left and right.
Entanglement from the principle of relativity
According to quantum information theory, all of quantum mechanics, including quantum entangled states, is based on the qubit, which is a quantum superposition.
What my colleagues and I propose is that this quantum superposition arises from the principle of relativity, which states (again) that the laws of physics are the same for all observers with different orientations in space.
If the electron, which has a vertical spin up, were to pass straight through the horizontal magnets as you would expect, it would have no spin horizontally. This would violate the principle of relativity, which says that the particle must have a spin regardless of whether it is measured horizontally or vertically.
Because an electron with a vertical spin upward has a spin when measured horizontally, quantum information theorists can say that the principle of relativity is (ultimately) responsible for quantum entanglement.
And since no forces are used in this explanation of the principle, there are none of the “spooky effects at a distance” that Einstein disparaged.
As the technological implications of quantum entanglement for quantum computing are firmly established, it’s nice to know that one of the big questions about its origin can be answered by a well-respected physics principle.
This article is republished from The Conversation, a nonprofit, independent news organization that brings you facts and trusted analysis to help you understand our complex world. By William Mark Stuckey Elizabethtown College
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William Mark Stuckey does not work for, consult, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no affiliations beyond his academic appointment.