Research Highlights

The arrow of time points the way

Max McGinley and Nigel Cooper.

Researchers uncover an effect that limits the potential usefulness of certain types of quantum systems, thus narrowing the search for new quantum devices.

Everyday experience tells us that the forward and backward directions of time don’t look the same. For example, a hot cup of coffee will cool down over time, but a cold cup of coffee will not spontaneously heat up. Somewhat paradoxically, the fundamental laws of physics actually look the same in both time directions. It has been suggested that this property, known as `time reversal symmetry’, can be exploited to help design a new class of devices known as `topological systems’, which could have interesting technological applications, including for quantum computation. Here, the authors show that any system based on such a strategy suffers from a fundamental flaw: Even though the individual particles from which matter is made behave the same in both time directions, when many of them interact together (as they do in your coffee cup), their collective motion is not time-reversal symmetric. This emergent breaking of time-reversal symmetry leads to detrimental effects which raise doubts about the effectiveness of these new devices. In contrast, they show that related systems based on other kinds of symmetries can in principle be shielded from the same effects, further highlighting their potential usefulness in real-world settings.

Fragility of time-reversal symmetry protected topological phases M McGinley and NR Cooper, Nature Physics (2020)

See also the highlight in the Condensed Matter Journal Club