Quantum information theorists shed light on entanglement, one of the spooky mysteries of quantum mechanics
A computer based on a quantum bit instead of a classical bit could have a significant computational advantage. And that’s because a classical bit produces a binary response — a 1 or a 0 — to just one query.
In contrast, the qubit produces a binary response to infinitely many queries using the property of quantum superposition. This property allows researchers to connect multiple qubits in what is called a quantum entangled state, where the entangled qubits behave collectively in a way that arrays of classical bits cannot.
This means that a quantum computer can perform some calculations much faster than a regular computer. For example, a device using 76 entangled qubits is said to have solved a sampling problem 100 trillion times faster than a classical computer.
But the exact force or principle of nature responsible for this quantum entanglement that underlies quantum computing is a big unanswered question. One solution that my colleagues and I in quantum information theory 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 are moving relative to each other. My team showed how you can use the principle of relativity in combination with the principles of quantum information theory to explain quantum entangled particles.
Quantum information theorists like me regard 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, in which force and energy are important concepts for performing the calculations. Quantum information theorists, on the other hand, do not need to know what kind of physical force might be causing the mysterious behavior of entangled quantum particles.
That gives us a head start in explaining quantum entanglement, because, as physicist John Bell proved in 1964, any explanation of quantum entanglement in terms of forces requires what Einstein called “spooky actions at a distance.”
That’s because the measurements of the two entangled quantum particles are correlated, even when the measurements are made simultaneously and the particles are physically far apart. So if a force causes quantum entanglement, it would have to act 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 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 email two friends, whom physicists usually call Alice and Bob, each with a glove from the same pair of gloves. When Alice opens her box and sees a 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 a right glove or a left glove. There is only one possible measurement – opening the box – so Alice and Bob have classically entangled bits of information.
Quantum entanglement, however, involves entangled qubits, which behave very differently from classical bits.
Qubit behavior
Think of a property of electrons called spin. When you measure the spin of an electron with magnets that are oriented vertically, you always get a spin that goes up or down, nothing in between. That’s a binary measurement result, so this is a bit of information.
If you turn the magnets on their sides to measure the spin of an electron horizontally, you always get a spin that is either left or right, nothing in between. The vertical and horizontal orientations of the magnets produce two different measurements of the same bit. So electron spin is a qubit – it produces a binary response to multiple measurements.
Quantum superposition
Now imagine that you first measure the spin of an electron vertically and see that it is up, then you measure the spin horizontally. If you are standing upright, you are not moving to the right or left at all. So if I measure how much you move from left to right when you are standing upright, I get zero.
That is exactly what you would expect from the vertically spin-up electrons. Since they have vertically oriented spin-up, analogous to standing upright, they should have no spin left or right horizontally, analogous to moving sideways.
Surprisingly, physicists have discovered that half are horizontally right-handed and the other half are horizontally left-handed. Now, it doesn’t seem logical that a vertically spun-up electron would have left-hand spin (-1) and right-hand spin (+1) results when measured horizontally, just as we wouldn’t expect sideways motion when we’re standing upright.
But if 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 is vertical spin up. So on average, it’s like we have no side-to-side or horizontal movement when we’re standing upright.
This 50-50 ratio over the binary (+1 and -1) outcomes is what physicists are talking about when they say that an electron with vertical spin up is in a quantum superposition of horizontal spins on the 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 with its quantum superposition.
My colleagues and I proposed that this quantum superposition is the result of the principle of relativity, which (again) states that the laws of physics are the same for all observers with different orientations in space.
If the electron with a vertical spin were to pass straight through the horizontal magnets in the upward direction as you would expect, it would have no spin horizontally. This would violate the principle of relativity, which states that the particle must have a spin regardless of whether it is measured in the horizontal or vertical direction.
Because an electron with a vertical spin in the upward direction also has a spin when measured horizontally, quantum information theorists can argue that the principle of relativity is (ultimately) responsible for quantum entanglement.
And since there is no mention of force in this statement of principle, there is also no mention of the “scary actions at a distance” that Einstein laughed about.
Now that the technological implications of quantum entanglement for quantum computing have been clearly established, it is nice to know that a key question about its origins may be answered by a highly regarded physical principle.
This article is republished from The Conversation, an independent nonprofit organization that brings you facts and analysis to help you understand our complex world.
It was written by: William Mark Stuckey, Elizabethtown College.
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William Mark Stuckey is not an employee of, an advisor to, an owner of stock in, or a recipient of funding from any company or organization that would benefit from this article. Additionally, he has disclosed no relevant affiliations beyond his academic appointment.