Question on Decoherence and the Delayed Choice Quantum Eraser

[dtdynd]

From what I have read online, decoherence is an irreversible process that gives the appearance of wave function collapse. For example, a macroscopic measuring device will always interact with the particle it is trying to measure, and the particle becomes entangled to this environment, and appears to "collapse" in an irreversible way.

However, can someone PLEASE explain to me why, after we erase the which-path information (in the delayed choice quantum eraser setup) the interference pattern emerges? I thought it was irreversible! As wikipedia puts it "Any measuring device or apparatus acts as an environment...[and] decoherence happens when different portions of the system's wavefunction become entangled in different ways with the measuring device. I know I'm missing something here, please help me understand this!

Lastly, I'm also confused about the Mach–Zehnder interferometer. If an object (or say a detector) is blocking one of the photon's potential paths, the photon will take ONLY ONE of the available paths and cannot interfere with itself. Now, on the delayed choice quantum eraser setup, doesn't there exist a similar path it could take (hitting one of the detectors that would provide us with which-path information)? In this case, I would imagine that the photon in this DCQE experiment should also take only one path (or slit, in the initial double slit part of this experiment) ...yet, we still find an interference pattern that indicates it had interfered with itself! Again, I know there's something very obvious I'm not understanding... Please help explain it to me!

 

[Demystifier]

Decoherence is irreversible only when caused by a LARGE number of degrees of freedom. A quantum eraser involves a small number of degrees of freedom, which is why it is reversible.

 

[San K]

Lastly, I'm also confused about the Mach–Zehnder interferometer. If an object (or say a detector) is blocking one of the photon's potential paths, the photon will take ONLY ONE of the available paths and cannot interfere with itself. Now, on the delayed choice quantum eraser setup, doesn't there exist a similar path it could take (hitting one of the detectors that would provide us with which-path information)? In this case, I would imagine that the photon in this DCQE experiment should also take only one path (or slit, in the initial double slit part of this experiment) ...yet, we still find an interference pattern that indicates it had interfered with itself! Again, I know there's something very obvious I'm not understanding... Please help explain it to me!

- The path is not blocked, (in quantum eraser experiments) only messed with, a bit...;), spoiling the ordering of phase angles

- Coherence is restored via additional equipment/manipulation i.e. ordering of phase angles is (almost) brought back to prior state i.e. whatever mess was created, is sorted out and it is brought back to prior phase relationship between the components of the system

- On a separate note: Agree with demystifier post. coherence is (almost) restorable when only a few degrees of freedom are involved