A connection between quantum theory and information theory

“Our results have no clear or direct applications yet. Fundamental research lays the foundation for future technologies in quantum information and quantum computers. There is huge potential for completely new discoveries in many different research fields.” from Linköping University in Sweden. Quantum communication.

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

The fact that light can be both a particle and a wave is one of the most illogical yet fundamental features of quantum mechanics. This is called wave-particle duality.

The theory dates back to the 17th century, when Isaac Newton proposed that light is made of particles. Other contemporary scholars believe that light is composed of waves. Newton eventually suggested that it might be both, but failed to prove it. In the 19th century, several physicists proved through various experiments that light is actually composed of waves.

But in the early 1900s, Max Planck and Albert Einstein both challenged the theory that light was just a wave. However, it was not until the 1920s that physicist Arthur Compton showed that light also has kinetic energy, a classical particle property. These particles are named photons. Therefore, it is concluded that light can be both a particle and a wave, as Newton suggested. Electrons and other elementary particles also exhibit this wave-particle duality.

But it’s impossible to measure the same photon in both wave and particle form. Depending on how the photons are measured, the waves or particles are visible. This is called the principle of complementarity and was proposed by Niels Bohr in the mid-1920s. It states that the combination of wave and particle characteristics must be constant no matter what you decide to measure.

In 2014, a team of researchers in Singapore mathematically demonstrated a direct link between the principle of complementarity and the degree of unknown information in quantum systems, known as entropic uncertainty. This connection means that no matter how the wave or particle properties of the quantum system are combined, the unknown information quantity is at least one bit of information, that is, an immeasurable wave or particle.

Researchers from Linköping University, together with colleagues from Poland and Chile, have succeeded in confirming the theory of the Singaporean researchers in real life with the help of a new type of experiment.

“From our perspective, it’s a very straightforward way of showing fundamental quantum mechanical behavior. It’s a classic example of quantum physics where we can see the results, but we can’t imagine what’s going on inside the experiment. But it’s The practical applications it can have are fascinating, almost philosophical.

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

Measurements are made in an instrument commonly used in research, called an interferometer, in which photons are fired at a crystal (beam splitter), which splits the photon’s path into two new paths, which are then reflected to cross each other into the second split. On the beamer, measurements are then made in the form of particles or waves depending on the state of the second device.

One of the special features of this experimental setup is that the researchers could insert a second beam splitter section into the light path. This makes it possible to measure light from waves, particles, or a combination of them in the same device.

The researchers say the findings could have many future applications in quantum communications, metrology and cryptography. But there’s 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 before the photons reach it. This will show that we can safely use this experimental setup in communications. Distributing encryption keys, that’s very exciting,” said Daniel Spegel-Lexne, a doctoral student in the Department of Electrical Engineering.