Do all which-way attempts/techniques introduce phase difference between the waves ?

[San K]

do all which-way attempts/techniques introduce phase difference between the "waves"?

In a double slit experiment

One hypothesis (or fairly well accepted theory?) is:

If we try to find which-way via Quarter Wave Plates (QWPs), we add a phase difference (between the "waves that seems to have traveled" from each slit).

The phase difference results in the waves not being coherent which in turn spoils/distorts the interference pattern.

Interference pattern is the (neatest and) cleanest when the waves are coherent.

Even in the (modified) Mach–Zehnder experiments, attempts to get which-way cause phase difference between the "waves that seem to travel" both the arms simultaneously.

Are there any alternate techniques/methods (other than QWPs) to get which-way without causing any phase difference in the "waves that seem to travel" both/numerous paths?

Wheeler had as a very hypothetical/thought-experiment, suggested telescopes. But I guess telescopes cannot be used.

Also can we ever observe/detect a photon (position, spin etc) without disturbing it?

 

[eljose79]

I can confirm that the use of Quarter Wave Plates (QWPs) or any other method to determine the "which-way" information in a double slit experiment does introduce a phase difference between the waves. This is due to the fact that in order to determine the which-way information, we must interact with the particles/waves in some way, causing a disturbance and altering their original state.

However, there are alternative techniques that have been proposed and are currently being researched in order to obtain the which-way information without causing a phase difference. One such technique is called weak measurement, where the interaction with the particle/wave is very small and does not significantly alter its state. This allows for the determination of which path the particle/wave took without affecting the interference pattern.

Another approach is using entangled particles, where the which-way information of one particle is correlated with the other particle, but without directly interacting with it. This has been demonstrated in experiments using entangled photons.

As for Wheeler's suggestion of using telescopes, it is not a practical method as it would require a very large and precise telescope to detect the individual particles/waves in a double slit experiment. Additionally, the act of observing with a telescope would also cause a disturbance and alter the state of the particles/waves.

In terms of detecting a photon without disturbing it, it is not possible according to the principles of quantum mechanics. The act of measurement inherently disturbs the state of the particle/wave being measured. However, as mentioned before, techniques such as weak measurement or entanglement can minimize this disturbance.