Scientists from Harvard University and the Massachusetts Institute of Technology see the light with new eyes
Scientists from Harvard University and the Massachusetts Institute of Technology (MIT) are challenging the concept of light as scientists know it today, and they're doing it without going all the way to very distant galaxies.
A group of scientists led by Harvard University physics professor Mikhail Lukin and MIT physics professor Vladan Vuletic managed to force photons to join together to form molecules - a mechanism that until recently existed only in theory. The research findings are described in an article published in the prestigious scientific journal Nature.
The discovery, explains the lead researcher, contradicts decades of accepted knowledge about the nature of light. Photons are described as massless particles that do not react with each other, for example - if you shine two laser beams opposite each other, they simply pass through each other. However, "photonic molecules" behave less like ordinary lasers and more like an item known from the movies of the Star Wars, especially Star Wars, a lightsaber [a fictional weapon in the world of Star Wars. This is the traditional weapon of the Jedi Knights and the Sith. A lightsaber is a sword whose blade is made of light energy produced from a crystal, and can cut through any other material].
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"Most of the properties of light, as we know them, originate from the fact that photons are massless particles that do not react with each other," explains the lead researcher. "What we did was to create a special type of medium in which the photons react with each other with such great intensity as if they had mass, and as a result they join together to form molecules. This type of photonic state has been the subject of theoretical discussions for a long time, but there has been no evidence of its existence until now. "It's not quite the same as a lightsaber," explains the lead researcher. "When these photons react with each other, they push against each other and cause their mutual deviation. The physics behind this mechanism is similar to what we see in MDB movies."
In order to make the massless photons connect with each other the researchers could not use the fictional "force" - instead they used much more extreme conditions. The researchers began their experiment by injecting rubidium atoms into a vacuum chamber and then used laser beams to cool the "cloud" of atoms to several degrees above absolute zero. Then, using extremely weak laser beams, they beamed individual photons into the cloud of atoms. As soon as the photons penetrate the cloud of cold atoms, their energy excites the atoms in their path, dramatically slowing down the speed of the photons themselves. As the photon passes through the cloud, this energy is transferred from atom to atom, eventually being emitted from the cloud with the outgoing photon.
"When the photon leaves this special medium, its identity is still preserved," explains the lead researcher. "It's like the mechanism we see when light refracts through a glass of water - the light penetrates through the water, transfers some of its energy to the medium and within the medium it exists as light and matter combined together. However, when this light is emitted, it still comes out as light. The process that takes place is the same, but it takes place under somewhat more extreme conditions - the light slows down to a considerable extent and there is a much greater transition of energy than the process of light rays in water." When the researchers beamed two photons into the cloud of atoms they were surprised to find that they came out together as a single molecule.
The researchers' explanation for this phenomenon lies in the quantum concept called "Rydberg blockade" [see also efficient production of separate photons for quantum computing], which states that when an atom is energetically excited, atoms adjacent to it cannot be excited to the same energetic level. In practical terms, this means that when two photons penetrate the cloud of atoms, the first of them excites an atom, but this photon must move forward before the second one is able to excite the atoms next to it. The result, explains the lead researcher, is that the two photons push and pull each other through the cloud of atoms while their energy is transferred from one atom to another. "These are photonic interactions mediated by atomic interactions," explains the lead researcher. "This phenomenon causes the two photons to behave similarly to a molecule, and when they are emitted from the medium it is more likely that they will exit together than each one in turn."
Despite the fact that this phenomenon is unusual, there may still be practical applications of it. "We do this for fun, and also to expand the boundaries of science," says the lead researcher. "However, this is still done within the overall study of photons which holds that photons still remain the best possible means of transmitting quantum information. The problem, until now, was that photons do not react with each other."
In order to build a quantum computer, the lead researcher points out, researchers will need to build a system that can store the quantum information and still process it using quantum logic operations. The challenge, however, lies in the fact that quantum logic requires interactions between discrete packets of quanta so that quantum systems can be branded in order to process information. "What we demonstrated in our experiment allows us to do exactly that," explains the lead researcher. “Before we can develop an efficient and practical quantum logic switch or gate, we must improve the performance of our system; The idea is still in its infancy, but it is an important step on this long road. The physical principles we demonstrated here are extremely important."
The system could be useful even in classical computing, notes the lead researcher, this in light of the challenges facing the chip industry today. Several computer companies, including IBM, are working to develop systems based on optical routers that convert light signals into electrical signals, but these systems have their limitations. The researchers believe that their system could, one day, even be used to create complex three-dimensional structures, for example crystals, from light alone.
"We still don't know what the future uses will be, but what we have discovered is a new state of matter, so we are hopeful that new applications will be discovered as we continue the research of these photonic systems," says the lead researcher. The news about the study