A method created at Rice University closes the gap between light and matter and may help advance technologies like quantum computers and communications. The lab designed and built a high-quality cavity to contain an ultrathin layer of gallium arsenide. By tuning the material with a magnetic field to resonate with a certain state of light in the cavity, they prompted the formation of polaritons that act in a collective manner. Credit: Qi Zhang/Rice University
Physicists probe photon-electron interactions in vacuum cavity experiments. Rice physicists are closing in on a way to create a new condensed matter state in which all the electrons in a material act as one by manipulating them with light and a magnetic field. The effect made possible by a custom-built, finely tuned cavity for terahertz radiation shows one of the strongest light-matter coupling phenomena ever observed. It could help advance technologies like quantum computers and communications by revealing new phenomena to those who study cavity quantum electrodynamics and condensed matter physics.
Condensed matter in the general sense is anything solid or liquid, but condensed matter physicists study forms that are much more esoteric, like Bose-Einstein condensates. The Kono team is working toward something similar, but with electrons that are strongly coupled, or “dressed,” with light. Qi Zhang designed and constructed an extremely high-quality cavity to contain an ultrathin layer of gallium arsenide, a material they’ve used to study superfluorescence. By tuning the material with a magnetic field to resonate with a certain state of light in the cavity, they prompted the formation of polaritons that act in a collective manner.
“This is a nonlinear optical study of a 2D electronic material,” said Zhang, PhD. “When you use light to probe a material’s electronic structure, you’re usually looking for light absorption or reflection or scattering to see what’s happening in the material. That light is just a weak probe and the process is called linear optics. “Nonlinear optics means light does something to the material,” he said. “Light…couples strongly with the material. As you change the coupling strength, things change in the material. What we’re doing is the extreme case of nonlinear optics, where the light and matter are coupled so strongly that we don’t have light and matter anymore. We have something in between, called a polariton.”
The researchers employed a parameter known as vacuum Rabi splitting to measure the strength of the light-matter coupling. “In more than 99%of previous studies of light-matter coupling in cavities, this value is a negligibly small fraction of the photon energy of the light used,” said Xinwei Li. “In our study, vacuum Rabi splitting is as large as 10% of the photon energy. That puts us in the so-called ultrastrong coupling regime.
“This is an important regime because, eventually, if the vacuum Rabi splitting becomes larger than the photon energy, the matter goes into a new ground state. That means we can induce a phase transition, which is an important element in condensed matter physics,” he said. Phase transitions are transitions between states of matter, like ice to water to vapor. The specific transition Kono’s team is looking for is the superradiant phase transition in which the polaritons go into an ordered state with macroscopic coherence.
Kono said the amount of terahertz light put into the cavity is very weak. “What we depend on is the vacuum fluctuation. Vacuum, in a classical sense, is an empty space. There’s nothing. But in a quantum sense, a vacuum is full of fluctuating photons, having so-called 0-point energy. These vacuum photons are actually what we are using to resonantly excite electrons in our cavity.
“This general subject is what’s known as cavity quantum electrodynamics (QED),” Kono said. “In cavity QED, the cavity enhances the light so that matter in the cavity resonantly interacts with the vacuum field. What is unique about solid-state cavity QED is that the light typically interacts with this huge number of electrons, which behave like a single gigantic atom.” Solid-state cavity QED is also key for applications that involve quantum information processing, like quantum computers. “The light-matter interface is important because that’s where so-called light-matter entanglement occurs. That way, quantum information of matter can be transferred to light and light can be sent somewhere.
“For improving the utility of cavity QED in quantum information, the stronger the light-matter coupling, the better, and it has to use a scalable, solid-state system instead of atomic or molecular systems,” he said. “That’s what we’ve achieved here.”
http://news.rice.edu/2016/08/22/light-and-matter-merge-in-quantum-coupling-2/
Recent Comments