Blind witnesses quench quantum interference without transfer of which-path information

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.

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