  We model the fundamental behavior of a two-branch quantum interference
device. Quantum interference oscillations are visible in the output as the
magnetic flux through the paths is varied. Multiple witness systems are
field-coupled to each branch. Each witness state entangles with the device
state, but for our {\em blind} witnesses which-path information is not
transferred to the quantum state of witnesses--they cannot "see" or make a
record of which path is traversed. Yet the presence of these minimal witnesses
rapidly quenches quantum interference. Thus, it is not the imprinting of
which-path information in the witness states that is essential for decoherence,
but simply the entanglement that embeds the device degrees of freedom in the
larger Hilbert space that includes the witnesses. The loss of interference
visibility can be understood as the result of phase cancellations from
different paths through the larger state space.
