Kyoto and Tohoku Universities realize world's first topological atomic laser through synthetic dimensions

A research group from Kyoto University's Graduate School of Science, led by Professor Yoshiro Takahashi, Assistant Professor Shintaro Taie, Associate Professor Yosuke Takasu, and Master's Student Takuto Tsuno (at the time of the research), in collaboration with Professor Tomoki Ozawa of Tohoku University's WPI-AIMR, successfully achieved the world's first lasing of a topological atomic laser through experiments using ultracold rubidium atoms. Takasu stated, "We succeeded in creating something in synthetic dimensions that, although theoretically described, would not actually exist in real spaces. We were able to expand the possibilities of quantum simulation." Their results were published in Nature Communications.

In quantum mechanics, systems that exchange energy with the external environment are called 'non-Hermitian quantum systems.' In particular, unique phenomena that arise through the control of "gain," which amplifies signals, and "loss," which attenuates signals, have been actively researched in the field of photonics (optics). This includes lasers and optical communications.

Meanwhile, research on non-Hermitian quantum mechanics has also advanced in the field of cold atomic gases, which deals with material particles such as atoms. In atomic experiments, however, introducing "loss" by removing atoms from a system is relatively easy, whereas adding "gain (amplification)" by pumping from outside, e.g. by optical pumping, has been extremely technically challenging. As a result, experiments using atoms have been limited to those dealing with "loss only." Observing phenomena where "gain" plays an essential role, such as laser oscillation, has remained a major challenge.

To overcome this limitation, the research group employed a method known as 'synthetic dimensions', which treats the internal degrees of freedom of atomic spin as if they were spatial coordinates, rather than using actual space (three dimensions).

Specifically, by irradiating multiple spin states of rubidium atoms with microwaves to couple them and enable transitions between states, the researchers created a virtual one-dimensional lattice (chain). By precisely controlling the intensity and phase of the microwaves, they realized an artificial crystal with topological properties - properties where unique states appear at the edges- known as the SSH (Su-Schrieffer-Heeger) model.

The major breakthrough of this research lies in utilizing evaporative cooling, a technique that is a standard in cold atom experiments, with a completely new approach. In conventional evaporative cooling, high-energy atoms are removed from the system, allowing the remaining atoms to cool and condense into the lowest energy state (ground state). However, the research group cleverly designed the initial atom distribution and evaporation conditions. As a result, as high-energy atoms were removed from the thermal atomic ensemble, the proportion of atoms occupying high-energy states - known as topological edge states - actually increases in relative terms. This effect is physically equivalent to atoms being injected (having gain) for the edge state of interest, and thus function as "effective gain."

Using this approach, the group observed atoms avalanching and Bose-Einstein condensation (BEC) occurring in high-energy topological edge states, where atoms would normally be unstable and unable to persist. This phenomenon is similar to specific modes of light being amplified to become laser light in a laser oscillator. It can therefore be described as the realization of a topological atomic laser using atomic waves (waves of matter). The study also confirmed that this condensate retains topological properties (robustness against structural defects and disorder).

This work has opened the way to freely controlling "gain," a long-standing barrier in cold atom systems. This makes it possible to fully introduce the knowledge of non-Hermitian quantum mechanics cultivated in the optics field into the world of atoms, which are waves of matter. Ozawa stated, "We have created an experimental system that can investigate non-Hermitian systems from a quantum mechanics perspective. This is very interesting even from a theoretical researcher's perspective."

The topological atomic laser demonstrated in this research exhibits "robustness against disorder," a characteristic of edge states. In the future, this property is expected to enable applications of the developed laser to next-generation quantum technologies. This includes robust and highly sensitive atom interferometers and gravity sensors, as well as highly directional atomic beam sources.

Takahashi stated, "The current experimental system is one-dimensional, but if it can be expanded to two dimensions, it will be possible to move along the edge of topological edge states. I am very much looking forward to future developments."

Journal Information
Publication: Nature Communications
Title: Gain engineering and atom lasing in a topological edge state in synthetic dimensions
DOI: 10.1038/s41467-025-67106-8