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.
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.
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
Ca' Foscari University of Venice, as part of its institutional aims and in fulfillment of the obligations set forth in article 13 of the EU Regulation 2016/679 ("Regulation"), gives you information regarding the processing of personal data collected by cookies and/or similar technologies when you visit the www.unive.it website (“Website”). Cookies are small pieces of information that a website sends to the user’s device where it is automatically stored and then sent back to the same website or to a third party every time that the website is visited using the same device.
In this notice, the term “cookie” will mean cookies as defined above as well as any other similar tracking technology used in the Website.
1. Data Controller
The data controller is Ca' Foscari University of Venice, with headquarters in Dorsoduro n. 3246, 30123 Venice (VE), legally represented by the Rector pro tempore.
2. Data Protection Officer
The University has appointed a "Data Protection Officer" ("DPO"), who can be contacted by writing to the email address: firstname.lastname@example.org or to the following address: Ca' Foscari University, Venice, Data Protection Officer, Dorsoduro n. 3246, 30123 Venice (VE).
3. Types of cookies, data collect, purposes and legal basis
The cookies used on the website are (please refer to the cookie list for specific information on each cookie):
essential cookies: cookies that are necessary to support the login system and the access to the “Personal Area” of the Website as well as to remember users’ cookie preferences. Personal data associated with these cookie is: user’s IP address, date and time of the visit, means of data transfer, requested resource, response, response size in byte, link connected to the user requesting the resource, user’s operating system, browser and device used. The legal basis for this processing activity is represented by art. 6.1.b) (“performance of a service asked by the data subject”) and art. 6.1.e) of the Regulation (“execution of a task of public interest or connected to the exercise of public powers''). Being the University is a public body and it is required to have a website where the users can find useful information on its activities (artt. 53 e 54 D.lgs. n. 82/2005). Is not possible to disable these cookies through the banner, as this action will have a negative effect on your navigation of the Website.
analytics cookies (firs party): cookies that collect aggregated and statistical data on the navigation of the Website aimed at measuring the number of visits for each page of the Websites as well as the performance and the usability of the Website. The analytics cookies used on the Website are set by the University. Personal data associated with these cookies is: User IP, browser language, average visit time, page actions (downloads, outlinks), first visit, last visit, device used, ecc. The legal basis for this processing activity is represented by art. 6.1.e) of the Regulation (“execution of a task of public interest or connected to the exercise of public powers''), as the University aims at collecting statistical data on the visits of its Website pages especially with regards to the pages of “Amministrazione Trasparente”. Is not possible to disable these cookies through the banner, as this action will have a negative effect on your navigation of the Website.
third party cookies (social media): these cookies are set by third parties that collect the data in order to analyze the navigation and the preferences of the users. On the website, there are cookies set by Google namely Google-Youtube that allow users to watch Youtube videos on the Website and the University to measure the performance of its communication campaigns. These cookies are managed by Google and the University only processes anonymous and statistical information collected by them, who act as joint controllers on the basis of the CJEU ruling n. C-210/16 (5 June 2018). The legal basis for this processing activity is represented by art. 6.1.a) of the Regulation (“consent of the data subject''). The first time you land on the Website, a banner pops up asking whether you consent to enable these cookies. If you do not consent or withdraw consent at a later time through the “click here to change your cookie preferences” button, this will have no effect on your navigation.
You can always manage your cookie preferences through the browser setting. If you disable all cookies, please note that some activities on the website will not be possible anymore (i.e. the ones on the pages accessible in the Personal Area). To find out more on how to manage cookies through browser settings, please visit:
The processing of personal data will be carried out by authorized employees (in compliance with Article 29 of the Regulation and art. 2-quaterdecies of D. lgs. 196/2003), with the use of computerized procedures, adopting appropriate technical and organizational measures to protect them from unauthorized or illegal access, destruction, loss of integrity and confidentiality, even if accidental in nature.
5. Data retention
For information on the retention period of each cookie, please refer to the list below.
6. Recipients and categories of recipients of personal data and data transfer outside the European Economic Area
For the purposes set out above, in addition to specifically authorized employees and collaborators of the University, personal data may also be processed by those who execute outsourced activities on behalf of the University in their capacity as data processors (the updated list is available at: https://www.unive.it/pag/36643/).
Personal data collected by third party cookies may be transferred outside the European Economic Area (EEA) in compliance with the provisions of the Regulation.
7. Data subjects rights and how to exercise them
As a data subject, you have the right to obtain from the University, in the cases provided for by the Regulation, the access, the rectification and the erasure of your personal data as well as ask for the limitation of the processing of your personal data or to object to the data processing itself (articles 15 and following of the Regulation). The request can be submitted, without any particular formal procedures, by contacting the Data Protection Officer directly at email@example.com or by sending a communication to the following address: Ca' Foscari University of Venice - Data Protection Officer, Dorsoduro 3246, 30123 Venice. Alternatively, you can contact the Data Controller, by writing a PEC (certified email) to firstname.lastname@example.org.
Data subjects, who believe that the processing of their personal data is in violation of the provisions of the Regulation, have the right to file a complaint with the Data Protection Authority, as provided for by art. 77 of the Regulation, or to take legal action (art. 79 of the Regulation).
Appendix on social media:
In addition to the privacy notice published at: www.unive.it/pag/29567, the University provides you with information on the processing of your personal data collected through its social media pages (Facebook, Instagram). In particular, the University only processes anonymous and statistical information collected by the social media providers, who act as joint controllers on the basis of the CJEU ruling n. C-210/16 (5 June 2018). Moreover, the University informs you that when you leave a comment or a post on its social media pages, it will process your data only to administer your request/comment.
Last updated: 27/07/2022
List of cookies
Last update of the list of cookies: 22/12/2022
They maintain the session data of the SingleSignOn.