Credits: Niklas Elmehed. Left to right: Alain Aspect, John Clauser and Anton Zeilinger

Nobel Prize in Physics 2022: Entangled Photons and Quantum Mechanics

The Nobel Prize in Physics 2022 was awarded to Alain Aspect, John F. Clauser, and Anton Zeilinger “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”. In this article we will try to explain one of the most complicated Nobel Prizes without relying on previous knowledge of quantum mechanics.

One of the fundamental principles of quantum mechanics is Heisenberg's uncertainty principle. According to this principle, we cannot know both the position and speed of microscopic particles (such electrons and photons) with arbitrary precision. Moreover, these measurements cannot be taken without modifying the motion and position of the particle itself. When objects are bigger (such as tennis balls), we cannot perceive this limitation: we know exactly where the ball is and its speed, and this information is not modified by the subject who views and measures it. All of us can understand this intuitively. However, the strange thing that happens in the microscopic world is considered a scientific principle: we cannot explain it on a fundamental level, we cannot find a reason for it, but we know that every experiment that has been conducted so far, and the entire theory of quantum mechanics, satisfies Heisenberg's uncertainty principle. 

At the beginning of quantum mechanics, just about a century ago, between WWI and WWII, physicists were dissatisfied with this inexplicable principle. Three of them — Einstein, Podolsky and Rosen — wrote an article expressing doubt regarding the completeness of the theory of quantum mechanics: it was the so-called ​​EPR paradox. The paradox was the following: if we create two “entangled” particles (to borrow the term that Schrödinger used in replying to this article), which according to quantum mechanics are absolutely identical to each other, in principle we could measure the first particle to know the position and speed of the second one, without touching it. However, this would violate Heisenberg's uncertainty principle. The three physicists thus suggested that the theory of quantum mechanics was incomplete — that there were some “hidden variables” yet to be discovered that could resolve the paradox. Einstein ironically stated that quantum mechanics was implicitly hypothesising a “spooky action at a distance”, suggesting that there was some sort of supernatural force at play.

© Johan Jarnestad/The Royal Swedish Academy of Sciences

In the following years, the world of physics was shaken by much debate and theories that were put forward, but no empirical evidence was obtained. Richard Feynman once said, “It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong.” In this case, however, theory was not the only problem: physicists did not know which experiments could solve the issue. 

In 1964, the Irish physicist John Stewart Bell proposed a theorem that would lead to mathematical expressions later be named “ Bell inequalities”. When measurements were performed independently on the two separated particles of an entangled pair, the results would serve to assess whether quantum mechanics was correct or whether the theory of hidden variables could explain some phenomena.  

The experiments of Clauser, Aspect and Zeilinger

John Clauser, the first recipient of the Nobel Prize in Physics 2022, conducted an experiment in 1972 and measured a violation of Bell’s inequality. He measured the polarisation of two rays of light  with different wavelengths (one green and one blue), emitted by  calcium atoms. Polarisation is the direction along which a wave “fluctuates” — in this case, light. When we wear polarised lenses, we select, thanks to specifically designed materials, just one polarisation of light (for example, the one that is vertical compared to the road). Vertically polarised light does not cause the annoying glow that light can cause when it is reflected horizontally on the road. By accurately measuring the polarisation of the light emitted by calcium atoms, John Clauser proved that quantum mechanics cannot be replaced by a theory that uses hidden variables.

© Johan Jarnestad/The Royal Swedish Academy of Sciences

Alain Aspect’s contribution to the measurement of individual photons was fundamental for the even more complex experiments conducted in 1998 by Anton Zeilinger. He was able to use the light emitted by galaxies. This allowed testing the predictions of quantum mechanics to an even greater degree. Moreover, the most innovative aspect of Zeilinger’s work was that he generated entangled photons not from calcium atoms, but from special crystals developed for non-linear optical experiments. By using such crystals, he managed to set up a new experiment that was fundamental for the creation of quantum technology. He took two pairs of entangled photons and created an entanglement among the four photons. This possibility was crucial in order to extend the distance over which the entanglement can be created, and it allows for the creation of a quantum communication web, a quantum Internet that can reach every corner of the planet.

Why are these three experiments important?

These experiments have laid the theoretical and practical foundations for “quantum information”, which happens via quantum optical communication. One advantage of using quantum properties of light for communication is that it is intrinsically safe. As previously stated in this article, if we measure individual microscopic particles that behave according to quantum mechanics, they are automatically modified. In a quantum Internet, if someone wanted to spy on the private communication of two interlocutors (conventionally named Alice and Bob, placed on either end of an AB segment), the act of spying would change the content of that communication, and the two interlocutors would immediately notice. IT security is crucial in today’s world. The discoveries made by these three scientists have paved the way for this to be made possible in the near future.

Over the last few years, the exchange of encryption keys using quantum mechanics has been demonstrated using photons generated by a satellite in orbit, allowing for communication among continents. The development of devices that can measure very little light (single photons) is greatly indebted to this research. This has not only brought us closer to a quantum Internet that will be safer — it has also greatly improved the accuracy of microscopes and of other instruments that can be used in physics, chemistry, biology, medicine, and all other branches of science. The fundamental research in physics on such important topics as the properties of light and matter is rarely for its own sake. Historically it has often had an impact on technology or on scientific research fields.   

Engineering Physics at Ca’ Foscari

Ca’ Foscari has recently inaugurated a Bachelor’s Degree Programme and a Master’s Degree Programme in Engineering Physics. They both include courses in Quantum Science and Technology. The goal is to enable students to understand and use counter-intuitive laws of quantum mechanics, applying it to existing and future technology. Quantum physics has awarded and will continue to award Nobel Prizes in Physics, but more importantly, it is the basis of the technological world in which we live. Without the first quantum revolution that happened a century ago, today we would not have computers, lasers, the Internet, and therefore advanced communication, medicine and diagnostic equipment. We would not have been able to communicate with any other human being on this planet through cell phone signals. During the second quantum revolution that is under way, the objective will be even more ambitious. We will create a technology that has an increasingly small impact on the planet’s resources and an increasingly large impact on the quality of life of its inhabitants.

Stefano Bonetti, Full Professor of Condensed Matter Physics, Department of Molecular Sciences and Nanosystems