We design two simple quantum devices applicable as an adjustable quantum
spectral filter and as a flux controller. Their function is based upon the
threshold resonance in a F\"ul\"op-Tsutsui type star graph with an external
potential added on one of the lines. Adjustment of the potential strength
directly controls the quantum flow from the input to the output line. This is
the first example to date in which the quantum flow control is achieved by
addition of an external field not on the channel itself, but on other lines
connected to the channel at a vertex.Comment: ReVTeX format, 4 pages, 6 figure
