Following collapse of the waveform

[rroubaix]

A few new student questions: following collapse of the waveform, does the waveform ever reform? If so, and if entangled particles are involved, does the entangled twin reform as well? And if so, does this require forward progression in time, in other words, might the entangled twin decollapse in anticipation of its twin's decollapse? I've searched the archive, and found a thread that discusses this, but it seems to assume that it's accepted that waveforms may reform.

 

[DrChinese]

A few new student questions: following collapse of the waveform, does the waveform ever reform? If so, and if entangled particles are involved, does the entangled twin reform as well? And if so, does this require forward progression in time, in other words, might the entangled twin decollapse in anticipation of its twin's decollapse? I've searched the archive, and found a thread that discusses this, but it seems to assume that it's accepted that waveforms may reform.

Welcome to PhysicsForums, rroubaix!

Theoretically, you can "reform" states when nothing irreversible has occurred. In practice, some states are easier to re-create than others. If you take the outputs of a beamsplitter, conceptually you can re-combine to end up with the original state. And that could be with an entangled partner.

Eberly discusses this:
http://www.optics.rochester.edu/~stroud/cqi/rochester/UR19.pdf
We employ an arrangement of polarization analyzer loops to derive several simple Bell inequalities
and then discuss the violation of one of them in light of quantum and classical interpretations of the data recorded.

Here is an paper I wrote which is based on Eberly's work:
http://www.drchinese.com/David/EntangledFrankensteinPhotonsA.pdf
Abstract: The H> and V> outputs of a Polarizing Beam Splitter can be combined to restore the original input superposition state, as long as no knowledge is obtained regarding the path taken through the PBS. Using this principle, it should be possible to create entangled photons from the identical H> and V> components of different polarization entangled photons. These “Frankenstein” photons will also be polarization entangled and should violate a Bell Inequality.

There are experiments in which observations are "erased", so you might look at some of the Quantum Eraser papers for some good treatments.