Researchers Confirm Quantum Theory Link to Information Principle

Researchers at Linköping University, along with colleagues from Poland and Chile, have successfully confirmed a ten-year-old theoretical study that connects one of the most fundamental aspects of quantum mechanics—the complementarity principle—with information theory. This was reported in a publication by the AAAS. This breakthrough was made possible through a new experimental setup that demonstrates the direct connection between the wave-particle duality of light and the degree of unknown information in a quantum system, known as entropic uncertainty.

The concept of wave-particle duality dates back to the 17th century when Isaac Newton suggested that light could be both particles and waves. This idea was later confirmed by physicist Arthur Compton in the 1920s, who showed that light has kinetic energy, a classical particle property. The complementarity principle, developed by Niels Bohr, states that no matter what one decides to measure, the combination of wave and particle characteristics must be constant.

Guilherme B Xavier, researcher in quantum communication at Linköping University, notes that this research lays the foundation for future technologies in quantum information and quantum computers. The findings could have many applications in quantum communication, metrology, and cryptography, with potential for completely new discoveries in various research fields.

Quantum Theory and Information Theory: A Fundamental Connection

Researchers have been interested in the connection between quantum theory and information theory in recent years. A new experimental setup at Linköping University, Sweden, has confirmed a ten-year-old theoretical study linking one of the most fundamental aspects of quantum mechanics—the complementarity principle—with information theory. This breakthrough has enormous potential for future technologies in quantum information and quantum computers.

The concept of wave-particle duality is a fundamental characteristic of quantum mechanics, where light can exhibit both particle-like and wave-like properties. This phenomenon was first suggested by Isaac Newton in the 17th century and later confirmed by experiments in the 19th and early 20th centuries. The complementarity principle, developed by Niels Bohr in the mid-1920s, states that it is impossible to measure the same photon as both a wave and a particle simultaneously. Depending on how the measurement is carried out, either waves or particles are visible.

In 2014, researchers from Singapore demonstrated mathematically a direct connection between the complementarity principle and the degree of unknown information in a quantum system, known as entropic uncertainty. This connection means that no matter what combination of wave or particle characteristic of a quantum system is looked at, the amount of unknown information is at least one bit of information.

Experimental Demonstration of Entropic Uncertainty

The new experiment setup at Linköping University uses photons moving in a circular motion, called orbital angular momentum, unlike the more common oscillating motion. This choice allows for future practical applications of the experiment, as it can contain more information. The measurements are made using an interferometer, where the photons are shot at a crystal (beam splitter) that splits the path of the photons into two new paths, which are then reflected to cross each other onto a second beam splitter and measured as either particles or waves depending on the state of this second device.

One of the unique features of this experiment setup is that the second beam splitter can be partially inserted by the researchers into the path of the light. This makes it possible to measure light as waves, particles, or a combination of both in the same setup. The results confirm the theoretical connection between entropic uncertainty and wave-particle duality, demonstrating the fundamental link between quantum theory and information theory.

Implications for Quantum Communication and Cryptography

The findings of this experiment have significant implications for future applications in quantum communication, metrology, and cryptography. According to the researchers, the experimental setup could be used to securely distribute encryption keys, which is a crucial aspect of secure communication. The ability to measure light as both waves and particles in the same setup opens up new possibilities for encoding and decoding information.

Furthermore, this experiment demonstrates the potential for basic research to lay the foundation for future technologies. As Guilherme B Xavier, researcher in quantum communication at Linköping University, notes, “It’s a typical example of quantum physics where we can see the results, but we cannot visualize what is going on inside the experiment. And yet it can be used for practical applications.”

Future Directions and Applications

The researchers plan to further explore the behavior of photons in this experimental setup by changing the setting of the second crystal right before the photon reaches it. This could lead to new insights into the fundamental nature of quantum mechanics and its connection to information theory.

As Daniel Spegel-Lexne, PhD student in the Department of Electrical Engineering, notes, “It would show that we can use this experimental set-up in communication to securely distribute encryption keys, which is very exciting.” The potential applications of this research are vast, and it is likely that we will see significant advancements in quantum communication and cryptography in the coming years.