With the aid of a new experiment, researchers have confirmed a decade-old theoretical study that links a crucial component of quantum mechanics—the complementarity principle—with information theory. This research, published in the journal Science Advances, contributes to our understanding of future advancements in quantum communication, metrology, and cryptography.
Using a recent experiment, scientists from Linköping University and other institutions have validated a theoretical study from ten years ago, which relates one of the key principles of quantum mechanics—the complementarity principle—to information theory. Their findings are featured in Science Advances and help unravel important aspects of future quantum communication, metrology, and cryptography.
“At this point, our findings don’t have direct applications. They are part of fundamental research that sets the stage for emerging technologies in quantum information and quantum computing. There is significant potential for groundbreaking discoveries across various fields,” explains Guilherme B Xavier, a quantum communication researcher at Linköping University, Sweden.
To fully grasp what the researchers have demonstrated, it’s important to start from the basics.
The concept that light can behave like both particles and waves is one of the most counterintuitive yet fundamental aspects of quantum mechanics, commonly referred to as wave-particle duality.
This theory originated in the 17th century when Isaac Newton proposed that light is made up of particles. Meanwhile, other scholars of his time argued that light is wave-based. Newton later suggested that it could be both, although he lacked the evidence to support this claim. In the 19th century, various experiments conducted by physicists confirmed that light behaves like waves.
However, in the early 1900s, both Max Planck and Albert Einstein disputed the notion that light was only wave-like. It wasn’t until the 1920s that physicist Arthur Compton demonstrated that light also possesses kinetic energy, a trait of classical particles. These particles were termed photons, confirming Newton’s original suggestion that light can indeed act as both particles and waves. This duality extends to electrons and other elementary particles as well.
It is important to note that one cannot simultaneously measure a photon as both a wave and a particle. Depending on the measurement technique used, a photon will display either its wave or particle characteristics. This limitation is encapsulated in the complementarity principle, formulated by Niels Bohr in the mid-1920s, which asserts that the wave and particle features must remain consistent, regardless of the measurement approach.
In 2014, a research team from Singapore mathematically linked the complementarity principle to the concept of unknown information in quantum systems, referred to as entropic uncertainty. This relationship implies that regardless of which characteristics of a quantum system are measured—wave or particle—there is a minimum of one bit of unknown information.
Now, researchers from Linköping University, in collaboration with colleagues from Poland and Chile, have successfully validated the Singapore team’s theoretical findings through a novel experimental approach.
“From our viewpoint, this presents a straightforward demonstration of fundamental quantum mechanical principles. It exemplifies quantum physics, where outcomes can be observed, yet the internal processes of the experiment remain elusive. Despite that, these findings can lead to practical uses, which is quite fascinating and nearly philosophical,” remarks Guilherme B Xavier.
In their latest experimental arrangement, the Linköping researchers employed photons that move in a circular pattern, known as orbital angular momentum, as opposed to the typical up-and-down oscillating motion. This orbital angular momentum not only aids in future experimental applications but also has the capacity to store more information.
Measurements were conducted using a widely utilized instrument called an interferometer, where photons are directed at a crystal (beam splitter) that divides their paths into two distinct routes. These routes intersect and are then directed onto a second beam splitter, where they are measured as either particles or waves, depending on the configuration of this second component.
A unique feature of this experiment is that the second beam splitter can be adjusted by the researchers, allowing them to measure light as waves, particles, or a mix of both within the same setup.
Researchers believe that their results could pave the way for various applications in quantum communication, metrology, and cryptography, but they also see vast potential for further foundational exploration.
“In our upcoming experiment, we aim to observe how a photon behaves if we alter the setting of the second crystal just before the photon arrives. This could allow us to utilize this setup in communication for the secure distribution of encryption keys, which is incredibly exciting,” says Daniel Spegel-Lexne, a PhD student in the Department of Electrical Engineering.
