The Nobel Prize in Physics 2022 was awarded to three scientists who made revolutionary advances to understanding one of nature’s most perplexing phenomena: quantum entanglement. In its most basic form, quantum entanglement means that features of one particle in an entangled pair are dependent on aspects of the other particle, regardless of how far apart they are or what lies between them. These particles could be electrons or photons, and one aspect could be their state, such as whether they are “spinning” in one way or the other.
The remarkable thing about quantum entanglement is that when one particle in an entangled pair is measured, you immediately know something about the other particle, even if they are millions of light years distant. This strange connection between the two particles is instantaneous, breaching what appears to be a fundamental law of the cosmos. Albert Einstein memorably described the phenomena as “spooky action from afar.”
After spending the better part of two decades conducting quantum physics experiments, I’ve learned to accept its strangeness. Physicists now integrate quantum phenomena into their knowledge of the world with an exceptional degree of assurance, thanks to ever more precise and trustworthy instruments and the work of this year’s Nobel laureates, Alain Aspect, John Clauser, and Anton Zeilinger.
However, until the 1970s, scientists were divided about whether quantum entanglement was a real phenomena. And for good reason: who would dare to oppose the great Einstein, who doubted it himself? It needed the development of new experimental techniques and the bravery of researchers to eventually solve this puzzle.
Existing in multiple states at once
To really comprehend the strangeness of quantum entanglement, it is necessary to first comprehend quantum superposition. The concept of quantum superposition asserts that particles can exist in several states at the same time. When a measurement is taken, it is as if the particle chooses one of the superposition states.
Many particles, for example, have a property termed spin, which is assessed as “up” or “down” for a given analyzer orientation. However, until the spin of a particle is measured, it exists in a superposition of spin up and spin down.
Each condition has a probability, and the average outcome may be predicted using a large number of data. The possibility of a single measurement being up or down is determined by these probabilities, yet it is unpredictable in and of itself. Despite its strangeness, mathematics and a large number of tests have demonstrated that quantum physics accurately represents physical reality.
Two entangled particles
The eerie nature of quantum entanglement stems from the fact of quantum superposition, which was obvious to quantum mechanics’ founding fathers in the 1920s and 1930s. To generate entangled particles, you divide a system into two components, the sum of which is known. For example, you can split a particle with zero spin into two particles with opposite spins so that their sum is zero.
Albert Einstein, Boris Podolsky, and Nathan Rosen released a paper in 1935 that included a thought experiment intended to demonstrate the apparent absurdity of quantum entanglement, which contradicted a fundamental law of the universe. This thought experiment, credited to David Bohm, concerns the decay of a particle known as the pi meson. When this particle decays, it leaves behind an electron and a positron with opposing spins that are travelling away from each other. As a result, if the electron spin is observed to be up, the positron spin can only be measured to be down, and vice versa. Even if the particles are billions of miles distant, this is true.
This would be OK if the electron spin measurement was always up and the positron spin measurement was always down. However, according to quantum mechanics, each particle’s spin is both up and down until it is detected. Only after the measurement is performed does the quantum state of the spin “collapse” into either up or down, instantly collapsing the other particle into the opposing spin. This appears to imply that the particles communicate with one another by a way that moves faster than the speed of light. However, nothing can travel faster than the speed of light, according to physical rules. Surely the observed condition of one particle cannot indicate the state of another particle at the opposite end of the cosmos in an instant?
In the 1930s, physicists, including Einstein, suggested a number of alternate explanations of quantum entanglement. They hypothesized that there was some unknown attribute – nicknamed hidden variables – that dictated a particle’s condition prior to observation. However, physicists did not have the technology or a clear definition of a clear measurement to verify whether quantum theory needed to be changed to incorporate hidden variables at the time.
Disproving a theory
It took until the 1960s for any hints of an explanation to emerge. John Bell, a great Irish physicist who died before receiving the Nobel Prize, proposed a plan to see if the concept of hidden variables made sense. Bell devised what is now known as Bell’s inequality, an equation that is always – and only – accurate for hidden variable theories, but not always for quantum mechanics. Thus, if Bell’s equation is found to be unsatisfactory in a real-world experiment, local hidden variable theories as an explanation for quantum entanglement can be ruled out.
The experiments of the Nobel laureates of 2022, particularly those of Alain Aspect, were the first to put the Bell inequality to the test. The experiments used entangled photons rather than electron-positron couples, as in many thought experiments. The findings definitively ruled out the existence of hidden variables, an enigmatic property that would predetermine the states of entangled particles. These and many subsequent experiments have together vindicated quantum mechanics. Objects can be connected over long distances in ways that previous physics cannot explain.
Importantly, there is no problem with special relativity, which prohibits communication faster than the speed of light. The correlation of measurements over enormous distances does not indicate that information is exchanged between the particles. Two people taking measurements on entangled particles who are far apart cannot use the phenomenon to send information faster than the speed of light.
Physicists are still researching quantum entanglement and looking for potential practical applications. Despite the fact that quantum mechanics can predict the probability of a measurement with great precision, many academics remain doubtful that it provides a complete description of reality. But one thing is certain. There is still much to be said about the fascinating world of quantum mechanics.