Frozen Chain of Ultracold Atoms Allows Observation of Bethe Strings

Physicists have successfully frozen a chain of ultracold atoms in place, allowing them to observe a quantum phenomenon that was predicted nearly 100 years ago but had never been directly seen before.

The phenomenon in question is related to the behavior of magnetic materials and was first described by a young physicist named Hans Bethe back in 1931. At the time, Bethe was studying the spin of electrons and discovered that when a few spins aligned, they could stick together and travel as a single unit down a row.

This phenomenon has since been found to apply not just to electrons but also to larger particles such as atoms. When these particles attract each other, they form clusters called Bethe strings, which can be predicted using mathematical equations developed by Bethe.

Physicists have long been interested in studying the behavior of interacting particles, which is a fundamental aspect of physics and underlies the properties of everything from magnets to electronic chips. However, predicting their behavior is notoriously difficult due to its complex nature.

The use of ultracold atoms has provided an ideal way for physicists to study these interactions in a controlled environment. By cooling atoms to extremely low temperatures, researchers can create precise conditions that allow them to observe the formation of Bethe strings and other quantum phenomena up close.

Physicists have made significant progress in understanding the behavior of particles at the atomic level using ultracold atoms as their test subjects.

Researchers have successfully created multiple strings of atoms at once, a feat that has eluded scientists until now.

The team achieved this by manipulating cesium atoms cooled into a Bose-Einstein condensate, a state of matter in which atoms share a single quantum state when chilled to near absolute zero.

This was done using laser beams that divided the condensate into thousands of thin tubes, each containing up to a few dozen atoms lined up in a string-like formation.

The resulting strings were found to be stable and could be made with varying numbers of atoms, including some with more than six atoms in a single line.

These findings are significant because scientists have struggled to create multiple strings of atoms at once due to the difficulties in getting atoms to attract each other without collapsing into clumps.

The team's success in creating these "necklaces of atoms opens up new possibilities for studying quantum phenomena that occur when many strings interact with each other.

The researchers employed a magnetic field to manipulate the interaction between the atoms in the necklace-like arrangement. Initially, the atoms repelled each other due to their electromagnetic properties. By gradually adjusting the magnetic field, the team was able to reduce this repulsion until it reached zero and then turned it into attraction.

Since the strings themselves are not directly observable, the researchers employed an indirect method to study them. They allowed the atoms to expand freely in all three dimensions, measuring how quickly the cloud dispersed. When the atoms were confined to one dimension, any strings present remained intact, but when they were freed in 3D, the strings broke apart and released their binding energy as motion.

The difference in the rate at which the clouds expanded in two versus three dimensions provided valuable information about the binding between the atoms. By analyzing this data, the team was able to estimate that the strings consisted of at least six atoms. This measurement also aligned closely with predictions from a theoretical framework known as generalized hydrodynamics.

This theory views the gas of atoms as a fluid composed of various types of particles, including strings of different sizes. It accounts for how each type of particle moves over time and tracks their interactions. The team's findings supported this theoretical framework but stopped short of providing definitive proof.

The researchers observed that the strings likely do not survive for long periods due to their interaction with the surrounding environment. At close range, the atoms still experience the effects of three-dimensional space, causing the strings to eventually decay into ordinary cesium molecules. In the coldest experiments, approximately 40% of the atoms were lost due to this process.

Physicists have been experimenting with a unique phenomenon known as Bethe strings, where atoms in an ultracold gas come together to form a single, stable string-like state.

According to Nägerl, these Bethe strings are only transiently created, suggesting that they may not be long-lived. However, he has proposed several ideas for future experiments that could potentially allow researchers to study them directly.

One idea involves using microwaves to shield the ultracold molecules from colliding with each other at close range. This could enable the creation of strings made from shielded molecules that might last long enough to be studied in detail.

Another approach being considered is quantum gas microscopy, which allows scientists to image individual atoms one by one and gather direct evidence of their behavior. This method could potentially provide more insight into the nature of Bethe strings.

In the long term, Nägerl hopes to use his research on Bethe strings as a stepping stone towards developing an experiment that can control both the movement and spin of individual particles in a quantum system.

Physicists have successfully frozen a necklace" of atoms, allowing them to witness a quantum phenomenon that was predicted over 100 years ago. This achievement is significant because it demonstrates the ability to control and manipulate individual particles in a quantum system.

The experiment's success has opened up new avenues for research, particularly in the field of quantum computing, where precise control over particle movement and spin is crucial. By building on this breakthrough, scientists may be able to develop more efficient and powerful quantum computers in the future.