In the world of atoms, the laws of nature as we know them do not apply. Instead, the laws of quantum physics prevail. These may be simple in their mathematical structure, but they lead to phenomena that are difficult to understand, to say the least. On the one hand, they explain how a single particle can be in several places at once and how it can have properties similar to a wave. On the other hand, they explain that light is not only waves but can also be seen as small individual bits of energy (called photons).
Being able to look into and understand the microcosm is fascinating because of this constant dualism between particles and waves. The quantum phenomena studied can then be incorporated into new technologies for future applications. This could involve making already small electrical components even smaller (to increase their efficiency) or creating entirely new components for quantum computers.
Studying how short pulses of light interact with atoms is a well-established route into the world of atoms. In a new study, Axel Stenquist, a PhD student in mathematical physics, has looked at how intense laser pulses are absorbed by atoms. He conducted the research together with his supervisor, Associate Professor Marcus Dahlström, and a number of colleagues in the same department, as well as researchers at Madrid Autonomous University and Huazhong University in China.
Short pulses of light can be used not only to observe the motion of electrons, as has been done with attosecond pulses (by, among others, Anne L'Huillier, Nobel Laureate in Physics 2023 from Lund University), but also to control the motion of electrons within atoms and molecules.
Axel Stenquist's theoretical work is based on a new type of light pulse generated by a kilometre-long free electron laser. This type of laser allows for higher intensity and shorter wavelength than traditional laser sources, providing a unique insight into the world of quantum mechanics.
"Absorption of light from an atom usually occurs only at the wavelength that makes the atom resonate. According to Bohr's atomic model, only those photons that have the ‘right’ energy to move the atom from one quantum state to another are absorbed".
So what have they discovered? Laser pulses can be soft or more angular in shape. The study shows that the shape of the laser pulse means that the energy absorbed from the light can change significantly. The researchers found that the atom absorbs light not only at the ‘right’ energy (the transition between its atomic states), but also at both higher and lower energies. The effect depends on both the shape of the laser pulse and its strength.
"It is usually thought that the atom goes from one state to another and that the energy absorbed is identical to the transition. But it's not that simple when the intensity of the light is high. When the atom and the laser pulses interact, you could say that the atom becomes coated with light, which changes their quantum mechanical state and a new picture emerges," says Axel Stenquist.
An atom is already quantum mechanical, but when strong laser pulses couple to the atom, they merge and create a new kind of quantum mechanical state (consisting of entangled particles and photons). This process is the reason why the absorption of light cannot be explained by the individual atom, but instead by the atom being simultaneously changed by the light itself.
According to Axel Stenquist, the energy states are split and separated by an energy that depends on the strength of the light. This phenomenon of clad atoms was introduced by French physicist Claude Cohen-Tannoudji (Nobel Prize in Physics 1997), but its role in short laser pulses is a topic that still engages scientists around the world today. According to the calculations in Axel Stenquist's work, three different transitions between energy states can be created, instead of one, as expected from Bohr's atomic model.
This discovery has parallels with the previously discovered so-called ‘Mollow triplet’ in the emission of light. There are both remarkable similarities and stark differences between these phenomena.
The physicists hope that their discovery of how atoms and light interact will lead to future experiments with free-electron lasers, with a particular focus on the role of photons in atomic dynamics.
The work is part of Marcus Dahlström's ongoing Wallenberg Academy Fellow (WAF) programme.