The “Back to basics” approach helps untangle a new phase of the material

A new phase of matter, considered understandable only using quantum physics, can be studied with much simpler classical methods.

Researchers at the University of Cambridge have used computer modeling to study potential new phases of matter known as pre-thermal discrete time (DTC) crystals. The properties of prethermal DTCs were thought to depend on quantum physics: the strange laws governing particles on the subatomic scale. However, researchers have found that a simpler approach, based on classical physics, can be used to understand these mysterious phenomena.

Understanding these new phases of matter is a step forward in the control of complex N-body systems, a long-standing goal with various potential applications, such as simulations of complex quantum networks. The results are reported in two joint articles in Physical examination letters and Physical examination B.

When we discover something new, whether it’s a planet, an animal, or a disease, we can learn more about it by examining it more and more carefully. Simpler theories are tried first, and if they don’t work, more complicated theories or methods are tried.

“This is what we thought to be the case with pre-thermal DTCs,” said Andrea Pizzi, a Ph.D. candidate at Cambridge’s Cavendish Laboratory, first author of the two papers. “We thought they were basically quantum phenomena, but it turns out that a simpler classical approach allows us to learn more about them.”

DTCs are very complex physical systems, and much remains to be learned about their unusual properties. Just as a standard spatial crystal breaks spatial translational symmetry because its structure is not the same everywhere in space, DTCs break distinct temporal translational symmetry because, when periodically “shaken”, their structure changes to each “push”.

“You can think of it like a parent pushing a child on a swing on a playground,” Pizzi said. “Normally the parent pushes the child, the child will swing back, and the parent pushes him again. In physics, it’s a pretty simple system. But if several swings were on the same playing field, and if the children on top of them held hands with each other, then the system would become much more complex, and much more interesting and less obvious behaviors could emerge.A pre-thermal DTP is one of those behaviors, in which atoms, acting sort of like oscillations, just ‘come back’ every second or third push, for example. “

First predicted in 2012, DTCs opened up a new field of research and have been studied in various types, including experimentally. Among these, pre-thermal DTCs are relatively simple systems to make that do not heat up quickly as one might expect, but rather exhibit crystalline behavior over time for a very long time: the faster they are shaken, the longer they survive. long time. However, it was believed that they were based on quantum phenomena.

“Developing quantum theories is complicated, and even when you manage it, your simulation capabilities are usually very limited, as the computational power required is incredibly large,” Pizzi said.

Now Pizzi and his co-authors have discovered that for pre-thermal DTCs, they can avoid using overly complicated quantum approaches and instead use much more affordable classical approaches. This way, researchers can simulate these phenomena much more fully. For example, they can now simulate many more elementary constituents, accessing the most relevant scenarios for experiments, such as two and three dimensions.

Using a computer simulation, the researchers studied many interacting rotations, like children on swings, under the action of a periodic magnetic field, like the parent pushing the swing, using Hamiltonian dynamics. classic. The resulting dynamics clearly and clearly showed the properties of pre-thermal DTCs: for a long time, the magnetization of the system oscillates with a period greater than that of the drive.

“It’s surprising how clean this method is,” said Pizzi. “Because it allows us to look at larger systems, it shows very clearly what’s going on. Unlike what happens when we use quantum methods, we don’t have to fight this system for it. We hope that this research will establish a dynamic classical Hamiltonian as an appropriate approach to large-scale simulations of complex N-body systems and open up new avenues in the study of non-equilibrium phenomena, of which pre-thermal DTCs are only ‘an example.”

Pizzi’s co-authors on the two papers, both of whom were recently based in Cambridge, are Dr Andreas Nunnenkamp, ​​now at the University of Vienna, and Dr Johannes Knolle, now at the Technical University of Munich. .

Meanwhile, at UC Berkeley, Norman Yao’s group also used classical methods to study pre-thermal DTCs. Remarkably, the Berkeley and Cambridge teams simultaneously addressed the same issue. Yao’s group will release its results shortly.