Experiment verifies link between quantum theory and information theory

Researchers from Linköping University along with colleagues from Poland and Chile have confirmed a theory proposing a link between the complementarity principle and entropic uncertainty. Their study is published in the journal Scientific advances.

“Our results have no clear or direct application at this time. This is fundamental research that lays the foundation for future technologies in quantum information and quantum computers. There is enormous potential for completely new discoveries. “news in many different research areas,” says Guilherme B Xavier, a quantum communications researcher at Linköping University, Sweden.

But to understand what the researchers showed, you have to start at the beginning.

The fact that light can consist of both particles and waves is one of the most illogical, but at the same time fundamental, features of quantum mechanics. This is called wave-particle duality.

The theory dates back to the 17th century, when Isaac Newton suggested that light was made up of particles. Other contemporary scholars believed that light was made of waves. Newton eventually suggested it could be both, without being able to prove it. In the 19th century, several physicists, in various experiments, showed that light is actually made up of waves.

But in the early 1900s, Max Planck and Albert Einstein challenged the theory that light is just waves. However, it was not until the 1920s that physicist Arthur Compton was able to demonstrate that light also possessed kinetic energy, a classic property of particles.

The particles were called photons. Thus, it was concluded that light can be both particles and waves, just as Newton suggested. Electrons and other elementary particles also exhibit this wave-particle duality.

But it is not possible to measure the same photon in wave and particle form. Depending on how the photon measurement is carried out, either waves or particles are visible. This is called the principle of complementarity and was developed by Niels Bohr in the mid-1920s. It states that no matter what one decides to measure, the combination of wave and particle characteristics must be constant.

In 2014, a research team from Singapore mathematically demonstrated a direct link between the principle of complementarity and the degree of unknown information in a quantum system, called entropic uncertainty.

This connection means that whatever combination of waves or particles is characteristic of a quantum system, the amount of unknown information represents at least one bit of information, i.e. the wave or particle not measurable.

In this new study, the researchers managed to confirm the theory of the Singapore researchers in reality using a new type of experiment.

“From our point of view, this is a very direct way of showing the basic behavior of quantum mechanics. It is a typical example of quantum physics where we can see the results, but we cannot visualize what is happening. passes inside the experience And yet it can be used for practical applications It is very exciting and almost borders on philosophy”, says Guilherme B Xavier.

In their new experimental setup, the Linköping researchers used photons moving forward in a circular motion, called orbital angular momentum, as opposed to the more common oscillating motion, which goes up and down. The choice of orbital angular momentum allows for future practical applications of the experiment, because it can contain more information.

The measurements are made in an instrument commonly used in research, called an interferometer, where photons are projected onto a crystal (beam splitter) which splits the photon path into two new paths, which are then reflected back to cross each other. on a second beam splitter, then measured in the form of particles or waves depending on the state of this second device.

One of the special features of this experimental setup is that the second beam splitter can be partially inserted by the researchers into the light path. This allows light to be measured as waves or particles, or a combination of these in the same configuration.

According to the researchers, the results could have many future applications in the fields of quantum communication, metrology and cryptography. But there’s also a lot more to explore at the base level.

“In our next experiment, we want to observe how the photon behaves if we change the setting of the second crystal just before the photon hits it. This would show that we can use this experimental setup in communication to securely distribute keys to encryption, which is very exciting,” says Daniel Spegel-Lexne, a Ph.D. student in the Department of Electrical Engineering.