What if we could store and control light?

May 28, 2021

The ability to harness light itself would be a huge step forward for quantum communication. (Unsplash/Denny Müller)

Scientists are developing a way to easily manipulate and store light such that photons can be used on demand, a technique that may one day become integral in quantum-information transfer, fiber optics and theories that remain untested due to light-based restrictions.

Normally, humans aren't able to manipulate several features of light, such as direction and timing. Rather, light moves freely in space in an unrestricted manner, only allowing adjustments to brightness. But a study published May 10 in Physical Review X describes a method for accessing some of light's untouched properties by looking to a phenomenon called subradiance.

"If you're able to describe what happens there, then it's a very good academic exercise for other problems," corresponding author Igor Ferrier-Barbut told The Academic Times. 

Future applications include coupling light into thin optical fibers — a term usually used when referring to very fast fiber-optic TV — or more recently, quantum systems. Quantum communication relies on light particles, photons, because only these quantum particles can transmit information without interrupting entanglement

Entanglement is a crucial concept in quantum mechanics that involves multiple systems that each exist in superposition. That means a system is in more than one state simultaneously, sort of like a spinning coin being both heads and tails. When entangled, the delicate systems, which can hold data called quantum information, depend on each other so heavily that the interruption of one leads to the interruption of the other. 

The principle remains true even if the systems are found at other ends of the universe. As light can preserve quantum entanglement, and thus the precious information, harnessing it would be a huge step forward for humans who are interested in transmitting such data across devices.

"You can use subradiance to prepare very interesting [light] states in your quantum computer that you want to use to run quantum computations," Ferrier-Barbut said. "I think it might have potential there — but this remains to be proven."

In a normal scenario, an atom in its ground energy state is pulsed such that it moves to an excited state; as the atom falls back to the ground state, a photon is emitted. Humans can generally control activation of the atom, but not direct emission of the photon.

Subradiance, on the other hand, occurs when light is decaying slower than its natural lifetime. That means the atom doesn't reach the ground state in the expected time frame, offering an opportunity to manipulate the system before that happens — so one might be able to control the emission of a photon, after all.

"If you use subradiance … and if you were able to control it," Ferrier-Barbut said, "you modify the time that the atoms spend in the excited state; you can control how long the atom stays in the excited state and when it decays."

Along with his team, the physicist from Université Paris-Saclay's Institut d'Optique discovered a way to detect when atoms are experiencing the emission lag. Not only that, the researchers figured out how to almost switch subradiance off and on, directly controlling patterns of light.

"We realized that we have all the tools we need in our setup, and so we tried to observe subradiance," he said. "Then we decided that, actually, it should be interesting to show that we can actually control subradiance decay."

Although Ferrier-Barbut acknowledged that his team isn't the only one interested in finding ways of using subradiance to benefit technology — some researchers have even found a way to couple light into optical fibers in one go — he notes that his method's advantage is it requires materials that are relatively simple to attain.

As of now, the rough technique works with a dense "cloud" of atoms that's smaller than a light-emission wavelength. Depending on how the cloud is pulsed with a laser, the atoms behave and interact with one another in different ways.

By planning the laser pulses' features, such as their timing, the researchers say they're able to stop photons from being emitted — which would be light storage, or quantum systems' memory — and control direction of eventual emission, which can translate to quantum data transfer.

"Normally, in order to enhance the interaction between light and atoms, you put many of them — but they all add up — you just stack them together," Ferrier-Barbut said. "In our system, the light interaction is modified by the fact that the atoms interact with each other."

However, the researcher noted that with the current model of a dense atom cloud, only about 10% of the subradiant atoms can be accessed. That's why his team says the next step is to use an ordered array of atoms, instead, which is like a two-dimensional plane that's one atom thick.

"If you are able to shape your atomic sample to create an ordered array — in any kind of geometry you want — you can make a regular matrix or you can make any pattern," he said. "Then you might be able to actually shape how the light interacts with your sample and select particular properties that you want."

Another team at Ferrier-Barbut's institute has already begun a startup to build quantum simulators and sell them as atomic arrays akin to his idea, meaning subradiance can easily be integrated into the system. He noted that there are several other similar platforms in competition with each other.

"There is a team in Harvard that also created a startup on this [and] there is a startup in Berkeley as well that is starting to really try to sell quantum computers out of atomic arrays," he said. "It's fairly simple conceptually, it's just a bunch of atoms that you control well enough to be able to control, fully, all the aspects of light-matter interactions."

The paper, "Storage and release of subradiant excitations in a dense atomic cloud," published May 10 in Physical Review X, was authored by Giovanni Ferioli, Antoine Glicenstein, Igor Ferrier-Barbut and Antoine Browaeys, Université Paris-Saclay; and Loic Henriet, Pasqal.

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