New Phase of Matter Discovered

The first two-dimensional Bose glass—a unique phase of matter that defies statistical mechanics—has been developed by physicists at Cambridge’s Cavendish Laboratory. The study's specifics are published in Nature.

All of the particles are localized within the Bose glass, which has certain glassy characteristics as its name implies. This indicates that no particle in the system mixes with its neighbors—rather, it stays to itself. If coffee was a local product, the complex design of the black and white stripes would not fade to an average; rather, it would remain intact when milk was added.

The team overlapped several laser beams to produce a quasiperiodic pattern, which is long-range organized like a typical crystal but not periodic—that is, it never repeats like a Penrose tiling. This new phase of matter was thus created. Bose glass was created when ultracold atoms that had been cooled to nanokelvin temperatures, or almost zero, were poured into the resultant structure.

Localisation is not only one of the toughest nuts to crack in statistical mechanics, it can also help to advance quantum computing.

Ulrich Schneider, Study Lead and Professor, Many-Body Physics, Cavendish Laboratory, University of Cambridge

Quantum information stored in a localized system would remain intact for a much longer period of time since it would not blend in with its surroundings.

“A big limitation of large quantum systems is that we can’t model them on a computer. To accurately describe the system, we have to consider all its particles and all their possible configurations, a number that grows very quickly. However, we now have a real-life 2D example which we can directly study and observe its dynamics and statistics,” added Schneider.

Schneider and his colleagues research quantum simulation and quantum many-body dynamics. They employ ultracold atoms to investigate many-body processes that, without a massive complete quantum computer, cannot be numerically replicated.

This problem frequently simplifies greatly since the system will always relax into a thermal state in which just the system's temperature is crucial, and most other information is lost. This is known as being ergodic, and it serves as the foundation for statistical mechanics, which is one of the pillars of the comprehension of matter.

“For instance, simply knowing the amount of milk poured in is enough to predict the final color of our coffee after a long stirring. If we want to predict the full structure of white and dark swirls during the stirring, however, it’s important to know where the milk was poured in and how the stirring is done precisely,” stated Schneider.

Interestingly, the Bose glass does not appear to be ergodic. This means that it does not 'forget its details'; therefore, modeling it will require all of them. This makes it an excellent choice for multi-body localization.

It’s a long-term aspiration to find a system or material that has many-body localization. Such a material would offer many new possibilities, not only for fundamental studies, but also for building quantum computers, as quantum information stored in such a system should remain more local and not leak out into its environment – a process called ‘decoherence’ that plagues many current quantum computing platforms.

Dr. Jr-Chiun Yu, Study First Author, University of Cambridge

During the experiment, the researchers saw an unexpectedly quick phase shift from a Bose glass to a superfluid, similar to how ice melts as the temperature increases.

A superfluid is a fluid that flows without any resistance. Imagine particles swimming through a superfluid; there would be no friction, and the fluid would not slow them down. This property, called superfluidity, is closely related to superconductivity. Along with another quantum phase, the Mott insulator, the newly observed Bose glass and the superfluid make up the ground states of the Bose-Hubbard model that describes the physics of bosons in interacting and disordered systems.

Dr. Bo Song, Assistant Professor, Peking University

Bose glasses and superfluids are separate phases of matter, much like ice and liquid water. However, just like ice cubes in a cup of water, the atoms in their system can transition between phases within the same experiment. The experimental observations, which validate previous theoretical predictions, show how Bose glass grows and evolves, allowing scientists to consider uses for it.

However, Schneider feels that while there is great potential for the future, everyone should proceed with prudence.

“There are many things we still don’t understand about the Bose glass and its potential connection to many-body localization, both regarding their thermodynamics as well as dynamical properties. We should first focus on answering more of these questions before we try to find uses for it,” concluded Schneider.

Journal Reference:

Schneider, U., et al. (2024) Observing the two-dimensional Bose glass in an optical quasicrystal. Nature. doi.org/10.1038/s41586-024-07875-2