Current and Quantum Hall Effect

[Benevito]

Why does the presence of an energy gap in QHE guarantee dissipationless current flow?

 

[radium]

Because of the gap the system is an insulator in the bulk but has conducting surface states on the edge. These states are chiral with the chirality depending on the direction of the magnetic field: they cannot backscatter. If there is are impurities in the sample they will just go around them.