Can a lens act as a quantum eraser?

[I_am_X']

I am just starting to study quantum mechanics, so I realize I am working with over-simplified examples, but that should work out since I'm hoping for simple answers...
Among other things, the claim that this eraser would work has me scratching my head.
http://sciam.com/media/inline/DD39218F-E7F2-99DF-39D45DA3DD2602A1_p95.gif

They say "the photon need not be detected".
I interpret this to imply that the detector could be switched off or removed entirely without changing the result. That being the case I hardly see why there is a detector in the diagram at all.

Now, the lens "makes it impossible to ascertain which slit the photon came from" _only at the focus point_. If the detector was placed a mm closer to OR further from the lens, the photons would arrive at distinct points corresponding to the slits.

And since the detector can be removed from the situation because the "photon need not be detected", All the lens has done is provide one point in space at which the information is unavailable. That seems to me very different from "erasing" it.

If someone could inform me of my specific mistakes in interpretation I would appreciate it, Thanks, X'

 

[eljose79]

Dear X',

Thank you for your interest in quantum mechanics and for your insightful questions. I can understand your confusion regarding the claim that this eraser would work. Let me try to explain it in a simpler way.

Firstly, in quantum mechanics, particles like photons can exist in multiple states at the same time, known as superposition. This means that a photon can pass through both slits simultaneously, and its state is not determined until it is measured or detected.

In the experiment shown in the diagram, the lens is used to create a situation where the photon's position at the focus point cannot be determined. This is because the lens causes the photon to spread out and interfere with itself, creating a pattern of light and dark bands on the detector. This interference pattern is a result of the photon's superposition state.

Now, the key to understanding the eraser is the use of entanglement. Entanglement is a phenomenon where two particles become connected in such a way that the state of one particle affects the state of the other, regardless of the distance between them.

In this experiment, the photon that passes through the lens becomes entangled with another photon that goes through one of the slits. This entanglement allows the information about which slit the photon went through to be "erased" or made inaccessible, even though the photon's state is not determined until it reaches the detector.

This means that even if the detector is removed or switched off, the interference pattern on the detector will still be affected by the entangled photon, and the information about which slit the photon went through will still be inaccessible.

I hope this explanation helps clarify the concept of the eraser in this experiment. Please feel free to ask more questions if you need further clarification.

 

[denian]

I can say that the concept of a lens acting as a quantum eraser is a complex and debated topic in the field of quantum mechanics. While it is possible to create experimental setups where a lens can act as a quantum eraser, it is not a straightforward or simple process.

Firstly, let's clarify the concept of a quantum eraser. In quantum mechanics, the concept of wave-particle duality states that particles, such as photons, can behave as both waves and particles. This means that a photon can pass through both slits in the double-slit experiment and create an interference pattern, but it can also be detected as a particle at a specific point on the screen. The "eraser" part comes in when we introduce a measurement or detection device that can determine which slit the photon passed through, thus "erasing" the interference pattern.

Now, coming to the lens in this scenario. The lens does not directly act as a quantum eraser, but it can be used in an experimental setup to achieve the same result. The lens is used to focus the light from the two slits onto a single point on the screen, creating an interference pattern. However, as you correctly pointed out, this only happens at a specific distance from the lens. If the detector is placed closer or further away, the interference pattern will not be observed.

So, how does this relate to the concept of a quantum eraser? The key here is the position of the detector. When the detector is placed at the point of focus, it can determine which slit the photon passed through, thus "erasing" the interference pattern. But, if the detector is placed before or after the point of focus, it will not be able to determine which slit the photon passed through, and the interference pattern will be observed.

In summary, a lens alone cannot act as a quantum eraser. Still, it can be used in an experimental setup to achieve the same result by controlling the position of the detector. This concept is still being studied and debated in the scientific community, and it is essential to continue studying and exploring quantum mechanics to fully understand it. I hope this helps to clarify the concept for you.