To grasp the significance of this study, it's essential to revisit a core concept of quantum mechanics: wave-particle duality. This principle reveals that light-and other elementary particles-exhibits both wave-like and particle-like properties, a notion that has puzzled scientists for centuries. While Isaac Newton speculated in the 17th century that light might have a dual nature, the evidence for its wave-like behavior emerged in the 19th century. Then, in the 20th century, Max Planck, Albert Einstein, and Arthur Compton confirmed the particle nature of light, coining the term photons to describe light particles.

This duality poses a measurement paradox: light can be observed as either a wave or a particle, but not both simultaneously. Niels Bohr's complementarity principle states that the total characteristics of wave and particle behavior in a quantum system remain constant, no matter the measurement focus.

In 2014, researchers in Singapore mathematically linked this principle to entropic uncertainty-a measure of unknown information in a quantum system. They concluded that at least one bit of information about a quantum system's wave-particle state remains unmeasurable. Now, researchers from Linkoping University, collaborating with colleagues in Poland and Chile, have experimentally validated this theory using an innovative setup.

"From our perspective, it's a very direct way to show basic quantum mechanical behaviour. It's a typical example of quantum physics where we can see the results, but we cannot visualise what is going on inside the experiment. And yet it can be used for practical applications. It's very fascinating and almost borders on philosophy," says Guilherme B Xavier.

The experiment introduced a novel approach by using photons moving in a circular motion, called orbital angular momentum, instead of the more common up-and-down oscillating motion. This choice enhances the potential for practical applications, as the circular motion allows the system to carry more information. Measurements were conducted using an interferometer, where a beam splitter divides photon paths into two, which are then recombined and measured.

What sets this setup apart is the ability to partially insert the second beam splitter into the light's path. This enables measurements of light as waves, particles, or a combination of both, all within the same experimental setup. The results open new avenues for quantum communication, metrology, and cryptography, while also advancing foundational quantum research.

"In our next experiment, we want to observe the behaviour of the photon if we change the setting of the second crystal right before the photon reaches it. It would show that we can use this experimental set-up in communication to securely distribute encryption keys, which is very exciting," says Daniel Spegel-Lexne, PhD student in the Department of Electrical Engineering.

Research Report:Experimental demonstration of the equivalence of entropic uncertainty with wave-particle duality

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Linkoping University
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