It turns out that what goes up should not go back down.
Physicists have achieved a phenomenon known as radiation atoms linger in an excited state, in a dense cloud of atoms for the first time.
The use of radiation could allow scientists to create reliable, long-lasting quantum networks of clouds of atoms, physicists say in a new study.
Atoms gain energy by absorbing photons (light particles) that cause their electrons to jump from the lowest energy “ground” state to excited states with higher energy. Once excited, atoms spontaneously emit a photon and fall back to the ground state. But this is not always the case. If many atoms are packed together and separated at a shorter distance than the wavelength of the emitted photon, the light they emit will be canceled and the atoms will remain in their excited state.
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This process, called subradiation, effectively prevents the disintegration of a large group or “ensemble” of excited atoms. Sub-radiation has been observed before in dilute atomic ensembles and ordered atomic massifs, but never before in dense atomic clouds.
Subradiation works because of a phenomenon called destructive interference. When two light waves of the same amplitude are occupied to occupy the same part of space, the vertices and troughs of the waves can be aligned to assemble constructively, making a combined wave that is twice as bright or destructive, canceling both waves completely.
But how can removing the light that a cloud of atoms emits keep those atoms excited? According to researchers, the key to understanding this idea is to observe the radiation quantum mechanics – the strange, probabilistic rules that govern the subatomic region.
On the small scale of the strange quantum world, both particles have wave-like properties and can travel simultaneously on all endless paths between one point and another. The path a particle “chooses” to take and the path we observe depends on how the wave-like particles interfere. In fact, it is not the destructive interference between the emitted photons that captures the atoms in excited states, but instead – and here is the crazy part – the possibility of this happening, which stops the photons from emitting in the first place.
“To understand the probability of a physical event, you need to summarize all the pathways leading to that event,” co-author Loïc Henriet, a quantum software engineer at French quantum processor company Pasqal, told Live Science in an email. “In some cases, pathways intervene constructively and amplify the phenomenon, while in other cases there are destructive interference effects that suppress probability. The destructive interference of photons that would be emitted by individual atoms prevents the collective decay of the excited state shared in the atomic ensemble.”
To cause radiant radiation in dense gas for the first time, the team limited an unsettled cold cloud. rubidium atoms inside an optical tweezers trap. This technique, for which scientists won the Nobel Prize in Physics in 2018, uses a highly concentrated beam of laser light to keep small particles in place. Then a second explosion of laser light excited the rubidium atoms.
Many excited atoms decay rapidly through a process called superradiation, which is associated with subradiation, but instead there are atoms that constructively combine their emitted light into a super-intense flash. But some atoms remained in a sub-radiation or “dark” state, unable to emit light that would destroy destructively. Over time, some atoms in superradial states also become subradial, making the atomic cloud increasingly radiant.
“We just waited for the system to disintegrate into dark states on its own,” Henriette said. “The dynamics of decay are quite complex, but we know that interactions somehow lead the system to fill in rays longer.”
Having found a way to make a subradiate cloud, the researchers pushed the atoms out of their dark states by adjusting the optical tweezers, allowing the atoms to emit light without destructive interference. This led to a burst of light from the cloud.
The team also made many clouds of different shapes and sizes to study their properties. Only the number of atoms in an excited cloud affected his life – the more atoms there were, the longer it took them to decay back to their ground states.
“Interference effects are collective effects, to make it happen, you need several emitters,” Henriette said. “And it becomes more pronounced when you increase the number of emitters. With just two atoms, it would be possible to have some radiant radiation, but that would be a very small physical effect. By increasing the number of atoms, one can suppress more efficient radiation. of photons. “
Now that researchers can create and control radiant atomic clouds, they plan to study techniques such as arranging their clouds in regular geometric patterns that, by allowing them to fine-tune the amount of interference they want, will give them even more greater control over the life of excited atoms.
The researchers believe their discovery will help develop many new technologies, such as new quantum computers and more accurate weather sensors.
The researchers published their findings on May 10 in the journal Physical Review X.
Originally published in Live Science.