Experimentally verifying the relationship between quantum theory and information theory

Researchers from Linköping University, along with colleagues from Poland and Chile, have confirmed a theory suggesting a relationship between the complementarity principle and entropic uncertainty. Their research is published in the journal Science Advances.

“The results of our research have no obvious or direct applications at this time. This is fundamental research that lays the foundation for future technologies in quantum information and quantum computing. It will lead to completely new discoveries in many different research fields. “There is great potential for this,” said researcher Guilherme B. Xavier. Quantum Communications at Linköping University, Sweden.

But to understand what the researchers have shown, we need to start from the beginning.

That light can be either a particle or a wave is one of the most illogical, yet at the same time fundamental, features of quantum mechanics. This is called wave-particle duality.

This theory dates back to the 17th century, when Isaac Newton suggested that light is made up of particles. Other modern scholars believed that light was made up of waves. Newton suggested that it might ultimately be both, although he could not prove it. In the 19th century, several physicists showed in various experiments that light is actually made up of waves.

But around the early 1900s, Max Planck and Albert Einstein challenged the theory that light was just a wave. But it wasn’t until the 1920s that physicist Arthur Compton was able to prove that light also has kinetic energy, a classical particle property.

The particle was named a photon. Therefore, it was concluded that light can be both particles and waves, as Newton suggested. Electrons and other elementary particles also exhibit this wave-particle duality.

However, it is not possible to measure the same photon in wave and particle form. Depending on how the photon measurement is performed, either waves or particles appear. This is known as the complementarity principle and was developed by Niels Bohr in the mid-1920s. It states that no matter what we decide to measure, the combination of wave and particle properties must remain constant.

In 2014, a team of researchers in Singapore mathematically demonstrated a direct relationship between the complementarity principle and the degree of unknown information in quantum systems, the so-called entropic uncertainty.

This relationship means that no matter what combination of waves and particles that characterize a quantum system are observed, the unknown amount of information is at least one bit of information, i.e., an unmeasurable wave or particle. .

In a new study, researchers have managed to confirm the Singapore researchers’ theory in practice with the help of a new type of experiment.

“From our point of view, this is a very direct way of showing fundamental quantum mechanical behavior. It allows us to see the results, but it is difficult to visually see what is happening inside the experiment. “It’s very fascinating, almost philosophical,” says Guilherme B. Xavier. I say.

In their new experimental setup, the Linköping researchers used photons traveling forward in a circular motion called orbital angular momentum, as opposed to the more common oscillating motion up and down. Choosing orbital angular momentum allows for the inclusion of more information, allowing for practical application of future experiments.

Measurements are made with an instrument called an interferometer, which is commonly used in research. In an interferometer, photons are shined onto a crystal (beam splitter), which splits the photon’s path into two new paths that are then reflected back to cross each other. It is directed into a second beam splitter and measured as a particle or a wave depending on the state of this second device.

One of the things that makes this experimental setup special is that researchers can insert a second beam splitter partially into the light path. This allows light to be measured as waves, particles, or a combination of both in the same setup.

The researchers say their discovery could have many future applications in quantum communications, metrology and cryptography. But there is much more to explore at a basic level.

“In our next experiment, we want to observe how the photons behave if we change the settings of the second crystal just before they arrive. We can use this experimental setup for communication to securely store the encryption keys. It will show that it can be distributed.”It’s very exciting,” says Dr. Daniel Spägel-Rexne. Student in the Department of Electrical Engineering.