A hallmark of so-called topological quantum states is that they are protected against local perturbations. ETH physicists now demonstrate that in the paradigmatic case of the integer quantum Hall effect, vacuum fluctuations can cause a breakdown of topological protection.
"Up to 1980 nobody expected that there exists an effect like the Quantized Hall Effect, which depends exclusively on fundamental constants and is not affected by irregularities in the semiconductor like impurities or interface effects." So spoke the German physicist Klaus von Klitzing on receiving the 1985 Nobel Prize in Physics. He was recognized for his discovery, in 1980, of a quantised version of the Hall effect in two-dimensional electron gasses. The unexpected robustness of the 'integer quantum Hall effect', as it has become known, in fact enabled von Klitzing's discovery in the first place. He was working with semiconductors—materials famously plagued by imperfections—yet observed an astonishingly 'clean' quantization of the Hall conductivity. The fact that such quantum systems can be so well protected against local perturbations was later explained in the framework of topological properties of electronic many-body states. But that protection can break down in unexpected ways, as the group of Prof. Jérôme Faist at the Institute of Quantum Electronics now reports. Writing in Science, they present experiments in which they established that exposing a quantum Hall system to the strongly enhanced quantum vacuum fluctuations of a tight cavity provides a novel and potentially general route to substantially modify quantum states. Such 'vacuum-field engineering' might lead to new experimental capability—but could also cause unwanted interference in experiments combining two-dimensional materials and resonators.
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