It turns out that what goes up does not have to come down again.
Physicists have obtained a phenomenon known as subradiance, where atoms lingering in an excited state in a dense cloud of atoms for the first time.
Utilization of subradiance could allow scientists to create reliable, long-lasting quantum networks from atomic clouds, physicists reported in a new study.
Atoms gain energy by absorbing photons (light particles) that cause their electrons to jump from the lowest energy “earth” state to higher energy-excited states. Once in an excited state, atoms spontaneously emit a photon and fall back to the earth state. But this is not always the case. If many atoms are packed together and separated by a shorter distance than the wavelength of the emitted photon, the light they emit will cancel itself out, and the atoms will remain in their excited state.
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This process, called irradiation, effectively averts the decay of a large group or “ensemble” of excited atoms. Subradiance has been observed before in dilute atomic ensembles and ordered atomic arrays, but never before in dense atomic clouds.
Subradiance works due to a phenomenon called destructive interference. When two light waves of the same amplitude get the same part of space, the peaks and troughs of the waves can be adjusted to add together constructively, creating a combined wave that is twice as light or destructive and cancels both waves completely.
But how can the cancellation of light from a cloud of atoms emit these atoms in excited state? According to the researchers, the key to understanding this idea is to observe subradiance quantum mechanics – the strange, probable rules that govern the subatomic world.
On the small scale of the strange quantum world, particles both have wave-like properties and can simultaneously travel all the infinite paths between one point and another. The path that the particle “chooses” to go, and the one we observe, depends on how the wave-like particles interfere with themselves. It’s not really the destructive interference between emitted photons that traps atoms in excited states, but instead – and here’s the crazy part – the possibility that it can happen, which prevents photons from being emitted in the first place.
“To understand what the probability of a physical event is, summarize all the paths leading to that event,” co-author Loïc Henriet, a quantum software engineer at French quantum processor company Pasqal, told WordsSideKick.com in an email. “In some cases, paths constructively interfere and enhance the phenomenon, while in other cases there are destructive interference effects that suppress probability. The destructive interference of photons that would have been emitted by individual atoms prevents the decay of an excited state collectively divided into the atomic ensemble. “
To induce subluxation in a dense gas for the first time, the team confined a disorderly cloud of cold rubidium atoms inside an optical tweezers trap. This technique, which scientists won the Nobel Prize in Physics in 2018, uses a highly concentrated laser beam to hold small particles in place. Another eruption of laser light then excited the rubidium atoms.
Many of the excited atoms decayed rapidly through a process called superradiance, which is related to subradiance, but instead has atoms that combine their emitted light constructively into a super-intense flash. But some atoms lingered in a radiant or “dark” state, unable to emit light that would destructively interfere. As time went on, some atoms in super-radiation states also became sub-radiating, making the atomic cloud more and more sub-radiation.
“We were simply waiting for the system itself to decay into dark states,” Henriet said. “The decay dynamics are quite complex, but we know that interactions somehow lead the system to populate radiation states for an extended period of time.”
Once they found a way to make a subradiant cloud, the scientists pushed the atoms out of their dark states by adjusting the optical tweezers so that the atoms could emit light without destructive interference. This resulted in a light burst from the cloud.
The team also made several clouds in different shapes and sizes to investigate their properties. Only the number of atoms in an excited cloud affected its lifetime – the more atoms there were, the longer it took them to decay back to their earth states.
“Interference effects are collective effects. For that to happen, you need more emitters,” Henriet said. “And it gets more pronounced when you increase the number of emitters. With only two atoms, it would be possible to have some kind of under-radiation, but it would be a very small physical effect. By increasing the number of atoms, one can suppress photon emission more efficiently. . “
Now that scientists can produce and control radiant atomic clouds, they plan to study techniques such as arranging their clouds in regular geometric patterns that, by allowing them to precisely set the interference they want, will give them even more control over their lifetime. for the excited atoms.
The researchers believe that their discovery will help the development of many new technologies, such as new quantum computers and more accurate weather forecast sensors.
The researchers published their findings on May 10 in the journal Physical Review X.
Originally published on WordsSideKick.com.