Particle physics research

UW-Madison research provides basis for Nobel Prize in Physics

For the first time in almost 10 years, quantum mechanics reappeared as the inspiration for the Nobel Prize in Physics. In recognition of their work on quantum entanglement, this year’s highly esteemed physics prize has been divided into three between physicists Alain Aspect, John Clauser and Anton Zeilinger.

Quantum entanglement – a unique subatomic phenomenon that connects the properties of two seemingly non-interacting particles – defies all known intuitions of our macroscopic world. This bizarre behavior, along with its connection to real-world applications such as quantum computing and communication, has led quantum entanglement to capture the minds of amateurs and experts alike.

This new Nobel Prize marks the next chapter in this field.

The three laureates have participated in the design and execution of various experiments that have deepened our understanding of quantum entanglement. Zeilinger, a physics professor at the University of Vienna and designer of the most recent set of experiments, wanted to understand a particular side effect of quantum entanglement called quantum teleportation. Essentially, Zeilinger’s experiments showed that by using two entangled particles, information could be shared over arbitrary distances, paving the way for a potentially highly secure commercial quantum network.

Zeilinger’s experiments were only possible thanks to the experiments carried out by Clauser, a researcher at JF Clauser & Associates, and Aspect, a professor at the Institut d’Optique Graduate School of the University of Paris-Saclay. The two physicists worked independently to design an instrument capable of analyzing the presence of quantum entanglement. Clauser was the first of the two – having started testing in the 1970s.

Clauser received promising results, but problems persisted. Ten years later, Aspect has managed to eliminate the remaining problems with an improved version of Clauser’s original experience.

While that may have been the scope of this year’s Nobel Prize in Physics, it’s only the most recent shots at a much larger picture of quantum entanglement.

To get to the beginning of this story, you have to go back in time to the mid-1930s. It was only 10 years since the field of quantum mechanics has even been theorized. The novelty and uniqueness of quantum mechanics has caused any discussion of the subject to spread through the physics community like wildfire. Naturally, given the time period and the subject, Albert Einstein worked his way up to the middle of this speech.

In the early 1930s, Einstein would publish a handful of papers dealing with the various intricacies of quantum mechanics. However, it was in 1935 that Einstein would publish one of his most influential papers on quantum mechanics. Teaming up with contemporary physicists Boris Podolsky and Nathan Rosen, the three published a paper under the provocative title, “Can the quantum mechanical description of physical reality be considered complete??

As the title may suggest, Einstein and his colleagues argued that certain “physical irregularities” seemed so wild and out of the ordinary that the only logical conclusion is that quantum mechanics must be incomplete. The most obvious of these irregularities? Quantum entanglement.

Einstein, Podolsky and Rosen (EPR) set up their thought experiment using first the notion of the uncertainty principle – a famous interpretation that shows that for a given particle, only its position or quantity could be known. of motion with absolute certainty, not both, because measuring one would change the other. Thanks to this, physicists then established the idea of ​​non-commutability between two particles.

Enjoy what you read? Get content from The Daily Cardinal delivered to your inbox

Simply put, two operators – for example, X and Y – are not commutable when XY≠YX. It can be useful to think of these operators as “actions” performed on something. In a mathematical context, these operators act on wave functions — the equations that represent atomic particles — to produce unique numerical results. However, real-world analogies, like the uncertainty principle, are also useful in highlighting this same idea.

By replacing X and Y with the act of measuring location and momentum respectively, it becomes reasonable to assert that for a single particle, these two actions are not commutable. For example, if we were measuring the momentum of a particle, doing so would send it flying in a different direction, giving a different location from its original starting point when measured. If we had instead measured the position first, it would be detected in its unchanged starting point, already showing the inequality between XY and YX.

For the proponents of quantum mechanics at the time, this idea was sound. What made the EPR paper so shocking was that they were able to show, through a bit of math, that in a quantum mechanical framework, standard physics begins to break down. It would seem that “two physical quantities, with non-commutating operators, can have simultaneous realities”, describe Einstein and his co-authors. That is to say, two quantities whose very interactions should envelop them in a cloud of uncertainty are somehow linked, or rather entangled, in such a way that the knowledge of one modifies the other in a certain way. predictable.

Theoretically, if two people at opposite ends of the universe looked at the same pair of entangled particles, and one person decided to change the quantum state of their particle, the other person would be able to instantly see their own particle react to this change, apparently far faster than the speed of light would allow. To EPR, this seemed physically impossible, and they concluded that quantum mechanics must be incomplete.

The idea of ​​quantum entanglement, later described by Einstein as “remote scary actionlaunched a new approach towards a common understanding of quantum mechanics. For several decades after the article’s publication, a prominent theory pointed to the existence of a set of “hidden variables” that explain this behavior.

This would not be seriously challenged until 1964, when Northern Irish physicist John S. Bell was working at the University of Wisconsin-Madison while on leave from the European Council for Nuclear Research (CERN). Bell stumbled across the 1935 EPR paper, and after reading his interpretation of quantum entanglement and hidden variables, Bell used relatively simple statistical principles to derive the set of inequalities that would become more later known as Bell’s inequalities.

Bell published his work in an article titled “On the paradox of Einstein Podolsky Rosen.“Its content was devoted to the careful implementation of capital inequalities. Basically, Bell’s inequalities were able to show that it was in fact possible to “detect” the presence of quantum hidden variables, all without knowing what they were.

If the hidden variables were really causing the quantum entanglement, then Bell’s inequalities would be true, which would prove that the EPR interpretation is correct. However, if it could be statistically shown that the inequalities were invalid, then this could be taken as evidence that the original understanding of quantum entanglement is in fact correct.

Bell, for his part, believed the latter – that the inequalities would be violated – thus solidifying the previous understanding of quantum mechanics. For now, however, Bell will have to wait for that confirmation.

Several years later, John Clauser, then a postdoctoral fellow at the University of California-Berkeley, discovered Bell’s work and began to think about experience who would ultimately become the first of that year’s Nobel Prize-winning trio.

To complete this experiment, Clauser built an instrument capable of measuring the variables required by Bell’s inequalities. To the surprise of many, Clauser’s experimental data violated a Bell inequality, marking the first step in proving the “frightening action” introduced by the EPR 30 years earlier.

John S. Bell died in 1990, leaving him unable to see the more recent progress his work has made possible. However, the mathematics he left cemented its place as an essential and foundational element of this year’s Nobel Prize in Physics.

The Daily Cardinal has covered the University and community of Madison since 1892. Please consider donating today.