In physics, as in life, it is always good to look at things from different points of view.
Since the beginning of quantum physics, how light moves and interacts with matter around it has been described and understood mathematically through the lens of its energy. In 1900, Max Planck used energy to explain how hot objects emit light, a study essential in establishing quantum mechanics. In 1905, Albert Einstein used energy when he introduced the concept of the photon.
But light has another equally important property, known as momentum. And as it turns out, when you take the momentum away, the light starts to behave in really exciting ways.
An international team of physicists led by Michael Lubbet, Research Associate at Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS) and Eric Mazur, SEAS Balkan Professor of Physics and Applied Physics, re-examine the foundations of quantum physics from a momentum perspective and explore what happens when it decreases Light momentum to zero.
The search was published in Nature’s light: science and applications.
Any object of mass and velocity has momentum – from atoms to lead to asteroids – and the momentum can be transferred from one object to another. The gun bounces when a bullet is fired because the bullet’s momentum is transmitted to the gun. At the microscopic level, an atom bounces when it emits light due to the acquired momentum of a photon. Atomic bounce, first described by Einstein when he was writing the quantum theory of radiation, is a fundamental phenomenon that controls the emission of light.
But a century after Planck and Einstein, a new class of metamaterials is raising questions regarding these fundamental phenomena. These metamaterials have a refractive index close to zero, which means that when light passes through them, it does not travel like a wave in phases of peaks and troughs. Instead, the wave extends to infinity, creating a stationary phase. When that happens, many processes typical of quantum mechanics, including atomic rebound, disappear.
why? It all comes back to momentum. In these so-called near zero index materials, the wave momentum of light becomes zero and when the wave momentum is zero, strange things happen.
“Basic radioactive processes are inhibited in materials with a three-dimensional index close to zero,” says Loupt, who is currently a lecturer at the University of Namur in Belgium. “We realized that the momentum bounce of the atom is forbidden in materials with a near-zero index and that the transfer of momentum between the electromagnetic field and the atom is not allowed.”
If breaking one of Einstein’s rules wasn’t enough, researchers have also broken perhaps the most famous experiment in quantum physics – Young’s double-slit experiment. This experiment is used in classrooms around the world to demonstrate wave-particle duality in quantum physics – demonstrating that light can exhibit properties of both waves and particles.
In a typical material, light passing through two slits produces two coherent sources of waves that interfere to form a bright spot in the center of the screen with a pattern of light and dark fringes on either side, known as diffraction fringes.
“When we modeled and numerically computed Young’s two-slit experiment, it was found that the diffraction fringe disappeared when the refractive index was lowered,” said co-author Larisa Verchenko from the Technical University of Denmark.
“As we can see, this work interrogates the fundamental laws of quantum mechanics and probes the limits of wave-particle duality,” said co-author Inigo Liberal of the Public University of Navarre in Pamplona, Spain.
While some basic processes are inhibited in near-zero refractive index materials, others are enhanced. Take another famous quantum phenomenon – Heisenberg’s Uncertainty Principle, more precisely known in physics as Heisenberg’s inequality. This principle states that you cannot know a particle’s position and velocity with complete accuracy and the more you know about one, the less you know about the other. But, in materials with a near-zero index, you know with 100% certainty that the momentum of the particle is zero, which means that you have absolutely no idea where the particle is in the material at any given moment.
“This material would make a really lousy microscope, but it manages to mask things perfectly,” Lopett said. “In a way, things become invisible.”
“These new theoretical results shed new light on near-zero refractive index photons from a momentum perspective,” Mazur said. “It provides insights into understanding light-matter interactions in systems with a low refractive index, which could be useful for laser and quantum optics applications.”
The research could also shed light on other applications, including quantum computing, light sources that emit one photon at a time, lossless propagation of light through a waveguide and more.
The team next aims to revisit other foundational quantum experiments in these materials from a momentum perspective. After all, although Einstein did not predict refractive index materials close to zero, he emphasized the importance of momentum. In his seminal 1916 paper on fundamental radiative processes, Einstein insisted that from a theoretical standpoint, energy and momentum should be considered “exactly equal because energy and momentum are related in the closest possible way”.
“As physicists, it is a dream to follow in the footsteps of giants like Einstein and to push their ideas even further,” Lopett said. “We hope that we can provide a new tool that physicists can use and a new perspective, which may help us understand these fundamental processes and develop new applications.”
Depicting the spin angular momentum in water waves
Michaël Lobet et al, Momentum considerations within near-zero index materials, Light: science and applications (2022). DOI: 10.1038 / s41377-022-00790-z
Provided by Harvard College John A. Paulson Engineering and Applied Sciences
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