A quantum casino helps define atoms in the big chill

The problem:
Atoms arranged in lattice structures in the deep cold can behave in mysterious ways, which may hold the key to understanding the inner workings of exotic materials. Simulating their behaviour accurately is difficult yet crucial for interpreting recent lab experiments.

The solution:
Using the Mahuika and Māui supercomputers and working with NeSI team members to apply a new variant of the Monte Carlo method that considers the quantum motion of atoms.

The outcome:
More accurate simulations will help improve the international understanding of exotic atoms and have already been applied to experiments.

Professor Joachim Brand is a theoretical physicist who works with PhD student Mingrui (Ray) Yang and postdoctoral fellows Péter Jeszenski and Ulrich Ebling. The team researches strongly interacting Fermi and Bose gases at Massey University in collaboration with Dr Elke Pahl, a senior lecturer at the University of Auckland, and international collaborators. With the help of NeSI, Joachim, Elke, Ray, Péter, and Ulrich are simulating the behaviour of ultra-cold atomic gases using a new mathematical variant of the quantum Monte Carlo method.

An ultra-cold gas is a cloud of particles with very little kinetic energy. In the air that we breathe, atoms and molecules collide with each other often and violently, causing pressure and friction. Importantly, the wave-nature of atoms is lost at our usual ambient temperatures. In an ultra-cold environment, atomic gases exist without the constant bustle. Under certain conditions their wave nature can take over to create a frictionless flow as it occurs in superconductors. Understanding and predicting the mechanisms of exotic superconductivity is an important research goal of material science.

Researchers often use a group of algorithms called Monte Carlo methods to simulate interacting particles. Monte Carlo methods combine random samples of information to make statistical predictions about the solutions of complex equations, such as the ones defining the behaviour of an ultra-cold gas.

In certain ultra-cold gases, typical Monte Carlo methods don't work well. So, Joachim and his team applied a new variant called Full Configuration Interaction Quantum Monte Carlo (FCIQMC).

“Monte Carlo methods are a class of algorithms where we use random sampling to simulate how atoms behave. The problem with classical Monte Carlo methods is they don’t really work for ultra-cold Fermi gases. Instead, we use quantum Monte Carlo methods,” Joachim said.

Applying FCIQMC to simulate the ultra-cold gases is easier said than done. The strong interactions of many quantum particles meant the researchers had to run heavy randomised simulations. To this end, they turned to NeSI’s Mahuika and Maui supercomputers.

“Access to NeSI was crucial for our work. It allowed us to perform highly accurate quantum Monte Carlo simulations in a reasonable amount of time. The results were crucial for demonstrating the efficacy of our new approach to simulating Fermi gasses. These results are the centre-piece for a publication that is currently under review,” said Joachim.

In addition to accessing NeSI's high performance computing platform, the researchers also made use of NeSI's Consultancy Service to get support with optimising their code. NeSI research software engineers Chris Scott and Alex Pletzer worked with Ray and Joachim to make their FCIQMC code run more efficiently on Mahuika, cutting hours of run-time off their project.

“We had help from NeSI for the development of our quantum Monte Carlo code through a consultancy project. Chris and Alex provided invaluable assistance to profile our parallel Julia code. Chris and Alex provided motivation and profiling tools, so we were able to rewrite our code's parallelisation strategy. This led to a 10-to-20-fold speed up,” Ray said.

Joachim and Ulrich also applied their research to another project. The pair gave theoretical knowledge to an experiment bringing three rubidium atoms together. It was part of a cross-university collaboration with a University of Otago experimental team led by Mikkel Andersen.

Forcing the three rubidium atoms together with a pair of optical tweezers, the team simulated atoms interacting in close proximity. Joachim and Ulrich's work helped the team interpret the results of this experiment and uncover the puzzling fact that the atoms did not form molecules as quickly as existing theory predicted. Joachim and Mikkel now plan to take this project further and refine the theory that describes the interactions of the atoms with the help of Monte Carlo simulations at the NeSI high-performance computers. This work could be applied to future research into the fields of superconductive materials and neutron star behaviour.

While it’s important to note that the applications of the research done by Joachim, Elke, Ray, Péter, and UIlrich are still speculative, they have been able to show a more accurate view of the behaviour of atoms in Bose and Fermi gases. It’s a look into how reality behaves in the coldest, densest and most exotic parts of our universe.

It helps contribute to the knowledge at the core of physics and chemistry and gives us a better understanding of the universe around us. To this end, NeSI has been able to help Joachim and his team explore these questions and bring their findings to the international stage.

Do you have an example of how NeSI support or platforms have supported your work? We’re always looking for projects to feature as a case study. Get in touch by emailing support@nesi.org.nz.