A new hands-off probe uses light to explore electron behavior in a topological insulator: Just as pressing a guitar string produces a higher pitch, sending laser light through a material can shift it to higher energies and higher frequencies.

Topological insulators are one of the most puzzling quantum materials. Their edges are electron superhighways where electrons flow with no loss, while the bulk of the material blocks electron flow - properties that could be useful in quantum computing and information processing. Researchers used a process called high harmonic generation to separately probe electron behavior in both of those domains. The method should be applicable to a broad range of quantum materials.