Zero-index metamaterials offer new insights into the foundations of quantum mechanics

In physics, as in life, it’s always good to see things from different angles.

Since the beginning of quantum physics, the way light travels and interacts with the matter around it has mainly been described and understood mathematically through the prism of its energy. In 1900, Max Planck used energy to explain how light is emitted by heated objects, a fundamental study that formed the basis of quantum mechanics. In 1905, Albert Einstein used energy when he introduced the concept of the photon.

But light has another equally important quality, known as momentum. And it turns out that when you lose momentum, the light starts behaving in really interesting ways.

An international team of physicists led by Michaël Lobet, Research Associate at Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and Eric Mazur, Balkanski Professor of Physics and Applied Physics at SEAS, are reexamining the foundations of quantum physics from a momentum perspective and explore what happens when the momentum of light is reduced to zero.

The research is published in Natural light: science and applications.

Any object with mass and speed has momentum – from atoms to bullets to asteroids – and momentum can be transferred from one object to another. A firearm recoils when a bullet is fired because the momentum of the bullet is transferred to the weapon. At the microscopic scale, an atom recoils when it emits light because of the momentum acquired from the photon. Atomic recoil, first described by Einstein when he wrote the quantum theory of radiation, is a fundamental phenomenon that governs the emission of light.

But a century after Planck and Einstein, a new class of metamaterials is raising questions about these fundamental phenomena. These metamaterials have an index of refraction close to zero, which means that when light passes through them, it does not travel as a wave in phases of peaks and valleys. Instead, the wave is stretched to infinity, creating a constant phase. When this happens, many processes typical of quantum mechanics disappear, including atomic recoil.

Why? It all comes down to momentum. In these so-called near-zero index materials, the light momentum becomes zero and when the wave momentum is zero, strange things happen.

“Fundamental radiative processes are inhibited in near-zero index three-dimensional materials,” says Lobet, who is currently a lecturer at the University of Namur in Belgium. “We realized that momentum recoil from an atom is forbidden in near-zero index materials and no momentum transfer is allowed between the electromagnetic field and the atom.”

If breaking one of Einstein’s rules wasn’t enough, the researchers also broke perhaps the most well-known experiment in quantum physics, Young’s double-slit experiment. This experiment is used in classrooms around the world to demonstrate particle-wave duality in quantum physics, showing that light can display the characteristics of waves and particles.

In a typical material, light passing through two slits produces two coherent wave sources which interfere to form a bright spot in the center of the screen with a pattern of light and dark fringes on either side, called diffraction fringes.

“When we modeled and numerically calculated Young’s double-slit experiment, it turned out that the diffraction fringes disappeared when the refractive index was lowered,” said co-author Larissa Vertchenko, of the Technical University of Denmark.

“As can be seen, this work interrogates the fundamental laws of quantum mechanics and probes the limits of wave-particle duality,” said co-author Iñigo Liberal, from the public University of Navarre in Pamplona, ​​Spain. .

While some fundamental processes are inhibited in near-zero refractive index materials, others are enhanced. Consider another famous quantum phenomenon: Heisenberg’s uncertain principle, more accurately known in physics as Heisenberg’s inequality. This principle states that you cannot know both the position and the velocity of a particle with perfect precision and the more you know about one, the less you know about the other. But, in near-zero index materials, you know with 100% certainty that a particle’s momentum is zero, which means you have absolutely no idea where the momentum is. particle in the material at any given time.

“This material would make a really poor microscope, but it hides objects perfectly,” Lobet said. “In a way, objects become invisible.”

“These new theoretical results shed new light on near-zero refractive index photonics from a momentum perspective,” Mazur said. “It provides insights into understanding light-matter interactions in low refractive index systems, which may be useful for laser and quantum optics applications.”

The research could also shed light on other applications, including quantum computing, light sources that emit a single photon at a time, lossless propagation of light through a waveguide and more.

The team then aims to revisit other fundamental quantum experiments in these materials from a momentum perspective. After all, even though Einstein did not predict near-zero refractive index materials, he emphasized the importance of momentum. In his seminal 1916 paper on fundamental radiative processes, Einstein insisted that from a theoretical point of view, energy and momentum “should be considered on an equal footing since energy and momentum are linked in the closest possible way”.

“As physicists, it’s a dream to follow in the footsteps of giants like Einstein and push their ideas further,” Lobet said. “We hope that we can provide a new tool that physicists can use and a new perspective, which could help us understand these fundamental processes and develop new applications.”