
It turns out that what goes up shouldn’t come back.
Physicists have achieved a phenomenon known as subradiance, in which atoms linger in an excited state, for the first time in a dense cloud of atoms.
The use of sub-radiation could allow scientists to create reliable, long-lived quantum networks from clouds of atoms, physicists said in a new study.
Atoms gain energy by absorbing photons (light particles), which cause their electrons to transition from the “ground” state with the lowest energy to an excited state with the highest energy. When in an excited state, atoms spontaneously emit a photon and return to the ground state. But this is not always the case. If many atoms are packed together and separated by a distance less than the wavelength of the emitted photon, the light they emit will be extinguished and the atoms will remain in an excited state.
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This process, called subradiation, effectively prevents the decay of a large group or “ensemble” of excited atoms. Subradiation has previously been observed in rarefied atomic ensembles and ordered atomic arrays, but never before in dense atomic clouds.
Subradiation works due to a phenomenon called destructive interference. When two light waves of the same amplitude occupy the same portion of space, the peaks and troughs of the waves can align to constructively stack together, creating a combined wave that is twice as bright or destructive, neutralizing both. whole waves.
But how can suppressing the light emitted by a cloud of atoms keep those atoms excited? The key to understanding this idea, according to the researchers, is the observation of sub-radiation. quantum mechanics – strange probabilistic rules governing the subatomic realm.
On the tiny scale of the strange quantum world, particles have wave properties and can simultaneously traverse all infinite paths between one point and another. The path that the particle “chooses”, and the one that we observe, depends on how the wavy particles interfere with themselves. It’s not really destructive interference between any emitted photons that traps atoms in excited states, but instead – and here’s the weird thing – the possibility that this could happen, which stops the emission of photons in the first place.
“To understand the probability of a physical event, you need to summarize all the paths 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, the pathways constructively interfere and amplify the phenomenon, while in other cases there are destructive interference effects that suppress the probability. The destructive interference of photons that would be emitted by individual atoms prevents the collective decay of the excited state from participating in the atomic ensemble.”
To induce subradiation in a dense gas for the first time, the team created a disordered cloud of cold. rubidium atoms inside the optical trap of tweezers. This method, for which scientists won the Nobel Prize in Physics in 2018, uses a highly concentrated beam of laser light to hold tiny particles in place. Then a second flash of laser light excited the rubidium atoms.
Many of the excited atoms decayed quickly through a process called superradiance, which is associated with subradiance, but instead, the atoms constructively combine the light they emit into a super-intense burst. But some atoms remained in a subradiant or “dark” state, unable to emit light that could destructively interfere. Over time, some atoms in superradiant states also became subradiant, making the atomic cloud increasingly subradiant.
“We were just waiting for the system to go dark on its own,” Henriet said. “The dynamics of decay are quite complex, but we know that interactions somehow cause the system to populate subradiative states for a longer time.”
After finding a way to create a sub-emitting cloud, the researchers lifted the atoms out of their dark state by adjusting optical tweezers, allowing the atoms to emit light without destructive interference. This resulted in a flash of light from the cloud.
The team also created several clouds of different shapes and sizes to study 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 return to their ground state.
“Interference effects are collective effects; you need multiple emitters for that to happen, ”Henriet said. “And this becomes more pronounced as you increase the number of emitters. With just two atoms, one could have some kind of sub-radiation, but that would be a very small physical effect. By increasing the number of atoms, the emission of photons can be suppressed more efficiently. “
Now that researchers can create and manipulate sub-radiant atomic clouds, they plan to explore techniques such as arranging their clouds into regular geometric patterns that, by allowing them to fine-tune the amount of interference they need, will give them even greater control over them. lifetime of excited atoms.
The researchers believe their discovery will help develop many new technologies, such as new quantum computers and more accurate weather forecasting sensors.
The researchers published their findings on May 10 in Physical Review X.
Originally published on Live Science.