Physicists at UC Santa Barbara, the University of Maryland, and the University of Washington have come up with an answer to a longstanding physics question.
David Weld is an experimental physicist with specialties in ultracold atomic physics and quantum simulation. The question is in the category of "many-body" physics, which examines the physical properties of a quantum system with multiple interacting parts Many-body problems have been a matter of research and debate for decades, but the complexity of these systems makes it impossible to solve them alone. The problem is beyond the reach of computers.
The problem was not beyond the reach of the experiment. What happens when you interact in a chaotic quantum system? Weld said it was a weird quantum state. It's a state with properties which are between the classical prediction and the non-Interacting quantum prediction.
The results of the physicists are published.
Something weird is happening.
The quantum world is not afraid of strange behavior. We would expect a regular pendulum to behave in the same way when it is subjected to a pulse of energy.
A classical pendulum will start to move all over the place if you kick it and shake it up and down frequently.
Chaos in quantum systems is not the same. Particles can be brought to a halt by disorder. While a kicked quantum pendulum or "rotor" might first absorb energy from the kicks, the system stops absorbing energy and the distribution of the energy stops. This is similar to the behavior of a "dirty" electronic solid, in which disorder results in immobile electrons, causing the solid to transition from being a metal to being an insulator.
In the context of single particles, what happens in a system with multiple electrons? Weld and his co-author, University of Maryland theorist Victor Galitski, had a discussion about questions like this and related aspects of quantum chaos when they were in Santa Barbara.
Weld said that Victor wanted to know what would happen if you had a bunch of these rotors and they could all interact with each other. Is the localization retained or destroyed by the interactions?
"Indeed, it is a very difficult question that relates to the foundations of statistical mechanics and the idea that most interacting systems eventually thermalize into a universal state," he said.
Imagine pouring a cold drink into a cup of coffee. The particles in your cup will form an equilibrium state that is neither hot coffee or cold milk. Thermalization was expected of all systems. It was 16 years ago that it was argued that disorder in a quantum system could lead to MBL.
The phenomenon that was recognized by the Onsager Prize is difficult to prove.
The technology and expertise of Weld's group allowed them to give a clear picture of the situation. There is a gas of 100,000 ultracold lithium atoms in their lab. The atoms represent a quantum rotor that can be kicked by lasers.
Weld said that they could use a tool called a Feshbach resonance to keep the atoms from each other or they could make them bounce off each other. The researchers could make the atoms go from dancing to mosh pit by turning a knob.
When the atoms were invisible to each other, the laser kicked up to a point after which they stopped moving. The system appeared to absorb energy from the repeated kicks when the researchers dialed up the interaction.
While a classical system absorbs energy at a faster rate, the interacting disordered quantum system absorbs energy at a slower rate.
He said that something that absorbs energy but not as much as a classical system can be seen. The square root of time seems to be growing the same amount of energy as the time. It's still a weird quantum state, even though the interactions aren't classical.
Testing for a lot of different things.
The way in which interactions destroy time reversibility was measured by Weld's team using a technique called an echo. The destruction of time reversibility is a sign of quantum chaos.
One way to think about this is to ask how much memory the system has after a while. The system should be able to return to its initial state if the physics is reversed. He said that the experiment would reverse time by reversing the phase of the kicks. We were fascinated by the fact that no one had ever done the experiment in which different theories predicted different behaviors.
Even though the laws of motion are time-reversible, a many-particle system can be so complicated that it is almost impossible to return to its original state. Even though the system lost its ability to be time-reversed, the interactions broke the localized state.
"You would expect interactions to ruin time-reversal, but we saw something more interesting: A little bit of interaction actually helps," Sajjad said. One of the more surprising results was this.
There were other people who witnessed this fuzzy quantum state. The University of Washington physicist Subhadeep Gupta and his team ran an experiment at the same time that produced similar results using heavier atoms. The result is published by UC Santa Barbara and the University of Maryland.
The experiments at the University of Washington were conducted in a very difficult physical regime with 25-times-heavier atoms restricted to move in one direction only, yet also measured weaker-than-linear energy growth from periodic kicking, which sheds light on an area where theoretical results have been in conflict.
Many important physics results open up more questions and pave the way for more quantum chaos experiments, where the coveted link between classical and quantum physics may be discovered.
David's experiment is the first attempt to investigate a more controlled version of MBL. The data shows something strange is going on, even though it hasn't resolved the fundamental question.
We don't know how to understand these results in the context of the large body of work on many-body Localization in Condensed Matter Systems. Weld wanted to know. What can we say about this state of matter? The system is delocalizing but not with the expected linear time dependence. Future experiments will explore these and other questions.
More information: Alec Cao et al, Interaction-driven breakdown of dynamical localization in a kicked quantum gas, Nature Physics (2022). DOI: 10.1038/s41567-022-01724-7There is a kicked one-dimensional ultracold gas.
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