In this note, we consider a physical system given by a two-dimensional quantum walk in an external magnetic field. In this setup, we show that both the topological structure as well as its type depend sensitively on the value of the magnetic flux $Phi$: while for $Phi/(2{pi})$ rational the spectrum is known to consist of bands, we show that for $Phi/(2{pi})$ irrational the spectrum is a zero-measure Cantor set and the spectral measures have no pure point part.
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