Where light and matter intersect, the world illuminates. Where light and matter interact so strongly that they become one, they illuminate a world of new physics, according to Rice University scientists.
Rice physicists are closing in on a way to create a newÂ condensed matter stateÂ 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.
The work by Rice physicist Junichiro Kono and his colleagues is described inÂ Nature Physics. It could help advance technologies like quantum computers and communications by revealing new phenomena to those who studyÂ cavity quantum electrodynamicsÂ andÂ condensed matter physics, Kono said.
Condensed matter in the general sense is anything solid or liquid, but condensed matter physicists study forms that are much more esoteric, likeÂ Bose-Einstein condensates. A Rice team was one of the first to make a Bose-Einstein condensate in 1995 when it prompted atoms to form a gas at ultracold temperatures in whichÂ all the atoms lose their individual identities and behave as a single unit.
The Kono team is working toward something similar, but with electrons that are strongly coupled, or â€śdressed,â€ť with light. Qi Zhang, a former graduate student in Konoâ€™s group and lead author of the paper, designed and constructed an extremely high-quality cavity to contain an ultrathin layer ofÂ gallium arsenide, a material theyâ€™ve used to studyÂ superfluorescence. By tuning the material with a magnetic field to resonate with a certain state of light in the cavity, they prompted the formation ofÂ polaritonsÂ that act in a collective manner.
â€śThis is a nonlinear optical study of a two-dimensional electronic material,â€ť said Zhang, who based his Ph.D. thesis on the work. â€śWhen you use light to probe a materialâ€™s electronic structure, youâ€™re usually looking for light absorption or reflection or scattering to see whatâ€™s happening in the material. That light is just a weak probe and the process is called linear optics.
â€śNonlinear optics means light does something to the material,â€ť he said. â€śLight is not a small perturbation anymore; it couples strongly with the material. As you change the coupling strength, things change in the material. What weâ€™re doing is the extreme case of nonlinear optics, where the light and matter are coupled so strongly that we donâ€™t have light and matter anymore. We have something in between, called a polariton.â€ť
The researchers employed a parameter known asÂ vacuum Rabi splittingÂ to measure the strength of the light-matter coupling. â€śIn more than 99 percent of previous studies of light-matter coupling in cavities, this value is a negligibly small fraction of the photon energy of the light used,â€ť said Xinwei Li, a co-author and graduate student in Konoâ€™s group. â€śIn our study, vacuum Rabi splitting is as large as 10 percent of the photon energy. That puts us in the so-called ultrastrong coupling regime.
â€śThis 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 aÂ phase transition, which is an important element in condensed matter physics,â€ť he said.
Phase transitions are transitions between states of matter, like ice to water to vapor. The specific transition Konoâ€™s team is looking for is theÂ superradiant phase transitionÂ 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. â€śWhat we depend on is the vacuum fluctuation. Vacuum, in a classical sense, is an empty space. Thereâ€™s nothing. But in a quantum sense, a vacuum is full of fluctuating photons, having so-calledÂ zero-point energy. These vacuum photons are actually what we are using to resonantly excite electrons in our cavity.
â€śThis general subject is whatâ€™s known as cavity quantum electrodynamics (QED),â€ť Kono said. â€śIn 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.â€ť
He said solid-state cavity QED is also key for applications that involve quantum information processing, likeÂ quantum computers. â€śThe light-matter interface is important because thatâ€™s where so-called light-matter entanglement occurs. That way, the quantum information of matter can be transferred to light and light can be sent somewhere.
â€śFor 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,â€ť he said. â€śThatâ€™s what weâ€™ve achieved here.â€ť
The high-quality gallium arsenide materials used in the study were synthesized via molecular beam epitaxy by John Reno of Sandia National Laboratories and John Watson and Michael Manfra of Purdue University, all co-authors of the paper. Weil Pan of Sandia National Laboratories and Rice graduate student Minhan Lou, who participated in sample preparation and transport and terahertz measurements, are also co-authors.
Zhang is now the Alexei AbrikosovÂ PostdoctoralÂ Fellow atÂ Argonne National Laboratory. Kono is a Rice professor of electrical and computer engineering, of physics and astronomy and of materials science and nanoengineering. Li received a â€śBest First-Year Research Awardâ€ť from Riceâ€™s Department of Electrical and Computer Engineering for his work on the project.
The research was supported by the National Science Foundation, U.S. Department of Energy, Lockheed Martin Corp. and the W.M. Keck Foundation.