Scientists reach the gates of quantum light

Scientists reach the gates of quantum lightBasel University.


A new study has located the exact point where quantum light originates inside an atom and has also discovered how to manipulate it. The finding will boost medical research, quantum computing and astrophysics.

For the first time, an international team of physicists has shown that it is possible to identify and manipulate small numbers of photons that interact inside an atom. An unprecedented milestone in the development of quantum technologies.

To understand the scope of this technological development, one must understand how lasers work and how light emission occurs at the atomic level.

Light is produced inside atoms. It is the part of electromagnetic radiation that can be perceived by the human eye. It emerges as a result of the dynamics of the electrons that surround the nucleus: they are like heavy flies that the atom cannot get rid of. They form a kind of steps that differ from each other by their respective energy levels.

energy steps

The point is that these electrons change their energy levels at all times and that in those going up and down the stairs that are formed around the nucleus, they absorb or emit flashes of light that are called photons.

That is, when those electrons go down steps, they emit photons. This is how the light that amazes us is formed. This quantum process is called spontaneous emission of light.

But there is more: these newborn photons also get into the game played by the electrons around the atom and, with a “kick”, they can cause an electron to go down one of the steps. And when it falls, it generates another photon that is identical to the one that kicked it. This process is called stimulated light emission and today it is a powerful technological resource.

funny atoms

The game does not end there because a photon recently arrived by the game of electrons can also be absorbed and drive the ascent of an electron to a higher energy level. That process is called stimulated absorption. When that happens, the light fades.

With all this information we can then imagine that the inside of an atom is something tremendously fun: it looks like a schoolyard with many children playing wildly, pushing each other, absorbing or emitting light in the process.

With an important detail: the emission of light prevails over the absorption (darkness) if there are more children (photons) in the upper steps pushing electrons to the lower steps so that they emit more children (twin photons than those who have pushed them) and continue to illuminate the interior of an atom.

coherent lasers

This is how many photons arise inside atoms in one of the most spectacular quantum processes: the lasers. Laser is an acronym that stands for light amplification by stimulated emission of radiation.

All the electromagnetic waves that make up the laser light beam do so coherently, that is, they are constant, both temporally and spatially, which makes it possible to generate a stable interference pattern that facilitates its technological manipulation.

Of all this knowledge, the most important thing is not that we have managed to observe the bustle of the atoms inside, despite the fact that each of those atoms measures only one ten billionth of a meter.

What is really important is that we have developed a technology to amplify light in such an insignificant space: we achieve it by concentrating many electrons in the upper steps, so that the light generation process is triggered spontaneously. That process is called population inversion and it is the one that allows the generation and manipulation of the lasers.

the fence tightens

The new research, developed by scientists from the Universities of Sydney (Australia) and Basel (Switzerland), led by Sahand Mahmoodianhas refined the technology that modulates the obtaining of lasers inside an atom.

It no longer intends to stimulate the emission of light, but to narrow the range of the dynamics of the photons once they have been emitted by the electrons. It is not trivial: we use the interactions that keep photons within light rays, for example, to send information through fiber optic cables. This is how we can enjoy the movie we are watching on TV at home.

Thanks to the new research, we have been able to capture more details of the moment of photon emission that occurs in the echelons of electrons that surround the atomic nucleus.

quantum light

These scientists have been able to see what happens when an electron falls to the lower step: the emerging photons, loose and united, are concentrated at different speeds (measured in this experiment) in a single dot that researchers have called “quantum light,” from which they amplify their light effect.

Single photons and two- and three-photon states have different delays in reaching that point of light, which shorten as the number of photons is greater. This reduced delay is a fingerprint of stimulated emission that can be used technologically, the researchers explain in the article they publish in Nature Physics.

They claim that this fingerprint opens the door for manipulate quantum light: that means that we have not only identified the exact point where the light originates (we could also call it Alpha point), but we have also achieved an unprecedented ability to bind and manipulate photons in that interaction with the Alpha point, which gives us will allow new and important technological developments to be achieved.

practical uses

“By demonstrating that we can identify and manipulate united states of photons, we have taken a critical first step in harnessing quantum light to practical use,” Mahmoodian explains in a statement.

Joint lead author, Natasha Tomadds: “This experiment is beautiful, not only because it validates a fundamental effect, stimulated emission, at its maximum limit, but also represents a great technological step towards advanced applications.”

And he adds: “we can apply the same principles to develop more efficient devices that provide us with photon binding states. This holds great promise for applications in a wide range of areas: from biology to advanced manufacturing and quantum information processing.”

In particular, this discovery could contribute to the development of more robust quantum computers and to improve interferometers that are used for medical imaging, oceanography, spectrometry or even in astrophysics.


Photon bound state dynamics from a single artificial atom. Natasha Tom et al. Nature Physics (2023). DOI:

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