Binding atoms via long-range repulsion
- Mateo Cardinal
- 3 hours ago
- 4 min read
A research team from the Max Planck Institute of Quantum Optics succeeded in realising tuneable long-range interactions between atoms. In their study, published in Science, the scientists were able to increase the system’s lifetime by more than a factor of 100 compared to previous experiments. This allows to study the effect of long-range interactions on a microscopic level in tunnel-coupled quantum systems. Notably, the team experimentally observed an unusual binding mechanism of two atoms and its influence on the atom arrangement in an optical lattice. The study represents a breakthrough in the control of atoms in quantum simulators and opens new perspectives for the investigation of quantum many-body systems with long-range interactions.
Long-range interactions are all around us – earth’s gravity keeps us on the ground, magnets attract or repel each other across larger distances, and electric forces hold molecules together. While most day-to-day examples can be described by classical physics, in the quantum world, similar interactions give rise to rich and complex phenomena. Understanding and controlling such interactions on a microscopic level is a major goal of modern physics.
Analogue quantum simulators provide a powerful platform for exploring such complex quantum systems. A prominent example uses ultracold atoms trapped in optical lattices, periodic potentials created by intersecting laser beams that resemble the crystalline structure of solids. These systems are described by the quantum tunnelling of atoms between neighbouring lattice sites and the repulsive interaction that occurs when two atoms occupy the same site, captured by the so-called Hubbard model.
Realizing long-range interactions
For over a decade, scientists have tried to extend the Hubbard model to include long-range interactions. In the present study, the researchers used ultracold rubidium atoms trapped in an optical lattice and induced long-range interactions through a technique called Rydberg-dressing. Here, a laser couples the atoms to a highly excited Rydberg state, which possesses a strong electric dipole moment. This dipolar character is partially “inherited” by the ground-state atoms, creating effective long-range interactions on the micrometre scale, compatible with optical lattices and quantum gas microscopy.
Rydberg-dressed interactions have been explored before, but were experimentally limited by strong collective losses, causing the quantum system to break down quickly. The team overcame this by applying the laser in a pulsed, stroboscopic manner rather than continuously. “By using a train of light pulses instead of a continuous beam, we can engineer long-range interactions while dramatically increasing the system’s lifetime,” explains Pascal Weckesser, research group leader and lead author. “Normally, collective losses are triggered by the laser, but since it is mostly off, we can extend the effective lifetime by more than a factor of 100 compared to previous studies. This is particularly exciting because the community has been trying to realize a Rydberg-dressed extended Hubbard model for over a decade.”
Capturing the invisible: long-range bound state
One fascinating prediction of the extended Hubbard model is the existence of so-called repulsively bound pairs. These unusual states appear when long-range interactions between atoms become the dominant energy scale in the system. Normally, atoms in an optical lattice can tunnel between sites. However, when strong repulsion acts over longer distances, it can actually bind two atoms together in an unexpected way – not by attraction, but by pushing their combined energy above the range of regular single-particle tunnelling. This counterintuitive effect can only occur when the atoms are confined by the external potential of the optical lattice.
“Using the single-site resolution of our quantum gas microscope, we were able to prepare these bound pairs locally and directly track their motion,” explains Kritsana Srakaew, PhD student and second author of the study. “For the first time, we could microscopically confirm this repulsive binding mechanism and observe its slower propagation through the lattice. It was exciting to see these pairs in action, as they open the pathway to study versatile phenomena.”
Correlated density-density ordering
One remarkable consequence of these repulsively bound pairs is their ability to create strongly correlated quantum systems. Without long-range interactions, atoms in an optical lattice move and arrange themselves freely, resulting in a mostly random distribution. But by gradually increasing the repulsive long-range interaction between neighbouring atoms, the formation of bound pairs can be suppressed. This restriction forces the atoms into an organized pattern, giving rise to long-range correlations in their positions. “At the beginning of our study, it was unclear whether such density-ordering could be observed in Rydberg-dressed quantum systems. It was therefore exciting to see that we could clearly demonstrate this effect in a small one-dimensional setting.”, concludes Pascal Weckesser. The finding was further supported by theoretical calculations from Annabelle Bohrt’s group at the University of Regensburg.
“In the future, these tuneable long-range interactions could be used to create adjustable quantum spin systems, like the transverse-field Ising model,” explains research group leader Johannes Zeiher. “By combining this with our ability to prepare arbitrary initial states in a spin chain, we will be able to explore fascinating phenomena such as spin breaking or the collisions of domain walls.”
Reference Realization of a Rydberg-dressed extended Bose-Hubbard model
Pascal Weckesser, Kritsana Srakaew, Tizian Blatz, David Wei, Daniel Adler, Suchita Agrawal, Annabelle Bohrdt, Immanuel Bloch, Johannes Zeiher
