Quantum entanglement is a complex phenomenon in physics that is usually poorly described as an invisible link between distant quantum objects that allows one to instantly affect the other. Albert Einstein famously dismissed this idea of entanglement as “spooky action at a distance.” In reality, entanglement is better understood as information, but that’s admittedly bland. So nowadays, every news article, explainer, opinion piece and artistic interpretation of quantum entanglement equates the phenomenon with Einstein’s spookiness. The situation has only worsened with the 2022 Nobel Prize in Physics going to Alain Aspect, John F. Clauser and Anton Zeilinger for quantum entanglement experiments. But it’s time to cut this adjective loose. Calling entanglement spooky completely misrepresents how it actually works and hinders our ability to make sense of it.
In 1935, physicist Erwin Schrödinger coined the term entanglement, emphasizing that it was “not one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.” He was writing in response to a famous paper (known simply to physicists as the EPR argument) by Einstein, Boris Podolsky and Nathan Rosen, that claimed quantum physics was incomplete. The New York Times headline read, “Einstein attacks quantum theory,” which solidified the widespread perception that Einstein hated quantum physics.
The EPR argument concerns the everyday notion of reality as a collection of things in the world with physical properties waiting to be revealed through measurement. This is how most of us intuitively understand reality. Einstein’s theory of relativity fits into this understanding, and says reality must be local, meaning nothing can influence anything else faster than the speed of light. But EPR showed that quantum physics isn’t compatible with these ideas—that it can’t account for a theory of local reality. In other words, quantum physics was missing something. To complete quantum physics, Einstein suggested scientists should look for a “deeper” theory of local reality. Many physicists responded in defense of quantum theory, but the matter remained unresolved until 1964 when physicist John S. Bell proposed an experiment that could rule out the existence of a local reality. Clauser was the first to perform the test, which was later improved and perfected by Aspect and Zeilinger.
A typical article about entanglement tells us it arises when particles interact to create a “link,” which persists no matter how far apart those particles are. Moreover, actions taken on one particle instantly affect the other, or so we are told. But—and here’s the thing even many experts get wrong—quantum physics doesn’t say that. Quantum physics says nothing about how the world is. Instead, quantum physics only describes the experiments we do to test our theories of how the world works—it gives us probabilities for the outcomes that may happen in an experiment. The compulsion to interpret quantum physics concepts as prescriptions for physical reality derives from the unfortunate way we traditionally teach physics.
I teach quantum physics to second-year computer science students at the University of Technology Sydney. Every autumn, I give teenagers a working knowledge of quantum entanglement without telling them it is spooky by guiding them through the process of engineering quantum phenomena for themselves. A former student said they understood the 2022 Nobel Prize in Physics reporting because I have students program quantum computers to produce entanglement. Another former student told me they were having trouble figuring out where the mysterious spookiness was supposed to be. I suggested that perhaps they needn’t look for something they’re not going to find.
Typically, a physics teacher starts a lecture on entanglement with Einstein, introducing concepts like local realism and ending with necessarily invoking the free will of the experimenter. But it doesn’t need to be this way. It’s much easier to understand how quantum physics works, and how it departs from the classical world, from the perspective of information, not physics. Let’s consider an example.
Imagine two people, Alice and Bob, are implicated in a crime and are being questioned in separate rooms with no way to communicate. They are each asked one of two possible questions. They must corroborate each other’s story to be set free. But there’s a catch: the questions contain a trap such that if they are both asked the second question, they must give opposite answers. Alice and Bob know all this before heading into their rooms for questioning. So, they do the obvious thing and devise a strategy so that their answers will be correlated in just the right way. However, it quickly becomes apparent that no possible strategy can set them free since they won’t know which question the other investigator asked. The best Alice and Bob can do is answer correctly 75 percent of the time, by both giving the same answer for every question, accepting they will fail in one of the four cases.
So far, Alice and Bob have only used classical information. But by sharing quantum information, they succeed with a probability higher than 75 percent. They do this by devising a strategy using the mathematics of quantum information rather than classical information. Intuiting the solution requires some familiarity with linear algebra, so I won’t detail it here. But it is a fact that the quantum information they share requires correlations, which means it is entangled. This appears spooky to the investigators because they only reason with classical information. But it’s not spooky. In any theory of information, correlations are ubiquitous. Through the lens of quantum information, then, entanglement is not strange or rare, but rather expected. The information perspective beautifully illustrates the core problem with demanding a classical description of quantum phenomena: it’s the wrong language. The Nobel Prize–winners were the first to demonstrate this as a fact about nature. Today, you can follow in their footsteps by creating entanglement and processing the correlated quantum information on a real quantum computer.
Einstein wanted all of nature identified with a simple and compact classical description. But we now know that quantum information provides the most accurate description of nature, which is written in a language we do not speak. Accepting this liberates us from the limits of traditional physics and makes teaching it more natural by facilitating active learning. The quantum information perspective illuminates some of the most profound questions in physics. For example, quantum information is the key to understanding the mystery of black holes and perhaps the entire universe. It also leads us to new quantum technologies that quickly and automatically encode and process quantum information.
For the second half of the 20th century, computers rapidly changed every facet of society, transforming our understanding of the universe and ourselves. We thought they were the ultimate tool for this purpose, but we were wrong. Scientists now believe the ultimate machine is a quantum computer, the full potential of which we have yet to realize. Determining when quantum computers will become ubiquitous and what problems they will solve is an exercise in crystal gazing. However, we already know they can solve a small list of problems such as factoring numbers, searching databases or simulating chemical reactions. If you have such a problem, you might be in the market for a quantum computer. You'll love it; Einstein, on the other hand, would have hated it.