Artistic illustration of the research study published in Nature. Credit: Georgy Ermakov and Sergey Lebedyanskiy
Physicists build theories to describe nature. Let us describe it through an example with something that we can do in our daily life, like going on a walking in the mountains. To prevent getting lost, we generally utilize a map. The map is a representation of the mountain, with its homes, rivers, paths, and so on. By utilizing it, it is rather easy to find our way to the top of the mountain. However the map is not the mountain. The map makes up the theory we utilize to represent the mountains truth.
Presented in the early 20th century to represent the tiny world, the introduction of quantum theory was a game-changer. Amongst the numerous extreme modifications it brought, it was the first theory phrased in terms of intricate numbers.
Invented by mathematicians centuries ago, intricate numbers are made from a genuine and imaginary part. It was Descartes, the famous philosopher considered as the dad of reasonable sciences, who coined the term “fictional,” to strongly contrast it with what he called “genuine” numbers. Regardless of their fundamental function in mathematics, complex numbers were not expected to have a comparable role in physics due to the fact that of this fictional part. And in reality, before quantum theory, Newtons mechanics or Maxwells electromagnetism used real numbers to describe, state, how objects move, in addition to how electro-magnetic fields propagate. The theories often employ complicated numbers to streamline some calculations, but their axioms just utilize real numbers.
Schrödingers bewilderment
Quantum theory drastically challenged this state of affairs because its building postulates were phrased in regards to complicated numbers. The brand-new theory, even if really helpful for anticipating the results of experiments, and for example completely explains the hydrogen atom energy levels, went against the instinct in favor of genuine numbers. Looking for a description of electrons, Schrödinger was the first to introduce complicated numbers in quantum theory through his well-known formula. Nevertheless, he might not develop that complex numbers might really be essential in physics at that fundamental level. It was as though he had actually discovered a map to represent the mountains however this map was actually made out of abstract and non-intuitive drawings. Such was his bewilderment that he composed a letter to Lorentz on June 6, 1926, specifying “What is unpleasant here, and indeed straight to be objected to, is using complex numbers. Ψ is surely fundamentally a real function.” Several decades later on, in 1960, Prof. E.C.G. Stueckelberg, from the University of Geneva, demonstrated that all predictions of quantum theory for single-particle experiments might equally be derived utilizing only genuine numbers. Ever since, the agreement was that intricate numbers in quantum theory were just a practical tool.
Antonio Acín (right), group leader at ICFO and Marc Olivier Renou (left), first author of the research study. Credit: IFCO
In a recent research study released in Nature, ICFO researchers Marc-Olivier Renou and ICREA Prof. at ICFO Antonio Acín, in partnership with Prof. Nicolas Gisin from the University of Geneva and the Schaffhausen Institute of Technology, Armin Tavakoli from the Vienna University of Technology, and David Trillo, Mirjam Weilenmann, and Thinh P. Le, led by Prof. Miguel Navascués, from the Institute of Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences in Vienna have proven that if the quantum postulates were phrased in terms of genuine numbers, rather of complex, then some predictions about quantum networks would always differ. Undoubtedly, the group of researchers came up with a concrete experimental proposal including 3 parties connected by two sources of particles where the prediction by basic complex quantum theory can not be expressed by its genuine counterpart.
2 sources and three nodes
To do this, they thought of a specific scenario that involves two independent sources (S and R), placed in between three measurement nodes (B, c, and) in an elementary quantum network. That is, they have actually correlated polarization in a method that is allowed by (both complex and genuine) quantum theory however difficult classically. The essential point in this research study was to find the suitable method to measure these 4 photons in the nodes A, B, C in order to acquire forecasts that can not be explained when quantum theory is restricted to genuine numbers.
As ICFO researcher Marc-Olivier Renou comments “When we discovered this result, the obstacle was to see if our idea experiment might be done with current innovations. After discussing with colleagues from Shenzhen-China, we found a method to adjust our protocol to make it possible with their state-of-the-art gadgets. And, as anticipated, the experimental results match the predictions!” This amazing experiment, understood in partnership with Zheng-Da Li, Ya-Li Mao, Hu Chen, Lixin Feng, Sheng-Jun Yang, Jingyun Fan from the Southern University of Science and Technology, and Zizhu Wang from the University of Electronic Science and Technology is released at the exact same time as the Nature paper in Physical Review Letters.
The results published in Nature can be viewed as a generalization of Bells theorem, which provides a quantum experiment that can not be discussed by any local physics formalism. Bells experiment includes one quantum source S that produces 2 knotted photons, one to A, and the second to B, prepared in a knotted state. Here, on the other hand, one needs two independent sources, the assumed self-reliance is vital and was carefully created in the experiment.
When integrating the concept of a quantum network with Bells ideas, the research study likewise reveals how outstanding forecasts can be. For sure, the tools established to acquire this first outcome are such that they will allow physicists to accomplish a better understanding of quantum theory, and will one day trigger the awareness and materialization of so far unfathomable applications for the quantum web.
Reference: “Quantum theory based upon real numbers can be experimentally falsified” by Marc-Olivier Renou, David Trillo, Mirjam Weilenmann, Thinh P. Le, Armin Tavakoli, Nicolas Gisin, Antonio Acín and Miguel Navascués, 15 December 2021, Nature.DOI: 10.1038/ s41586-021-04160-4.
Quantum theory radically challenged this state of affairs due to the fact that its structure postulates were phrased in terms of complicated numbers. Looking for a description of electrons, Schrödinger was the first to introduce complex numbers in quantum theory through his famous formula. A number of years later, in 1960, Prof. E.C.G. Stueckelberg, from the University of Geneva, showed that all predictions of quantum theory for single-particle experiments could equally be obtained using just genuine numbers. Considering that then, the agreement was that intricate numbers in quantum theory were just a convenient tool.
The essential point in this research study was to discover the suitable way to measure these four photons in the nodes A, B, C in order to acquire forecasts that can not be explained when quantum theory is limited to real numbers.