The Double Slit Experiment

The Delayed Choice Quantum Eraser

The Delayed Quantum Eraser Experiment is essentially a continuance of the infamous double slit experiment. The Theory of observation was determined as an interaction that caused the wave form, an "infinite wave of possibility", to collapse producing the physical particles we know and understand as the makeup of the real world. The question proposed going further, "at which point does the wave function collapse? Ultimately, out of 2 entangled particles, does ones path and/or wave function collapse ultimately determine the path of its unobserved counterpart? In essence, can observations in the present dictate past events? Experimentation on this subject shows positive results for this hypothesis.

Previous Quantum experimentation demonstrated conventional wisdome

The most famous of which is that of Kim; who devised a method of implementation devoid of screens. Ultimately, the "quantum eraser" was later proposed in order to obtain which-path information while neither scattering the particles or exposing them to uncontrolled phase factors to them. Multiple experiments have demonstrated the validity of the so-called quantum "eraser" by implementing various lenses and prisms to direct the photons.

Instead, detectors connected to an electronic counter were set in their stead. Previously Which-way experiments demonstrated that if particle detectors are positioned at the slits, showing through which slit a photon goes, the interference pattern will disappear at the same time. The Delayed Quantum Eraser experiment has essentially succeeded the infamous double slit experiment in that it aimed to effectively study the behavior of quantum particles by avoiding standard observation techniques that otherwise would induce wave collapse. Wave collapse “occurs when a wave function—initially in a superposition of several eigenstates—reduces to a single eigenstate due to interaction with the external world.”. The conclusion of the experiment unveiled 1) displacement of either entangled photon is relative to its universe until the point in which its entangled Which-Way counterpart

1. Entangled Superposition: A photon arrives, encounters a wall with two slits, and passes through the slits. This puts the photon's position into a superposition. Special crystals then split the photon into two photons (with entangled positions, without breaking the superposition). One of the resulting photons does the normal double-slit thing, building up an apparent lack-of-interference pattern on the interference screen. The other photon embodies the which-way information. Its journey is represented by the right half of the diagram, and can be delayed as long as desired.

2. Choice: The "choice" to erase is performed by two beam splitters (the "choosers"), one for the top slit and one for the bottom slit. If the which-way photon passes through the choosers without being reflected, its impact point at the top or bottom of the which-way screen tells us which slit the original photon passed through. But if the which-way photon is reflected by the choosers then, regardless of the starting slit, an "eraser" beam splitter will distribute the photon equally between the two erased cases. Because both the top slit and bottom slit create a 50/50 split between the two erased cases (when erased), the which-way information is lost. It cannot be recovered.

3. Recovery: Grouping runs of the experiment into "where the which-way photon hit" buckets reveals some interesting patterns. Within the bucket of test photons whose which-way photon went to the top case, no interference pattern is revealed. The same happens for the bottom case bucket. But the erased-case buckets filter the apparent lack-of-interference pattern we saw into two complementary interference patterns!

the beam splitters and mirrors direct the idler photons towards detectors labeled D1, D2, D3 and D4. Note that: • If an idler photon is recorded at detector D3, it can only have come from slit B. • If an idler photon is recorded at detector D4, it can only have come from slit A. • If an idler photon is detected at detector D1 or D2, it might have come from slit A or slit B. • The optical path length measured from slit to D1, D2, D3, and D4 is 2.5 m longer than the optical path length from slit to D0. This means that any information that one can learn from an idler photon must be approximately 8 ns later than what one can learn from its entangled signal photon.

Detection of the idler photon by D3 or D4 provides delayed "which-path information" indicating whether the signal photon with which it is entangled had gone through slit A or B. On the other hand, detection of the idler photon by D1 or D2 provides a delayed indication that such information is not available for its entangled signal photon.

By using a coincidence counter, the experimenters were able to isolate the entangled signal from photo-noise, recording only events where both signal and idler photons were detected (after compensating for the 8 ns delay). Refer to Figs 3 and 4.

• However, when they looked at the signal photons whose entangled idlers were detected at D3 or D4, they detected simple diffraction patterns with no interference.

In Conclusion:

Which slits it could be taking, whether it behaves more as a particle or a wave, etc. are encoded in the wave function until the very moment of the measurement which is why they may always be "changed back" to the previous answers. For example, in quantum eraser, the photon is ordered to behave as a wave again, even though a premature argument could lead a sloppy person to think that the photon has already decided to behave as a particle forever.

you measure the photon, it is finally possible to think of its properties classically and the wave function allows one to calculate all probabilities that the outcome will be something or something else. In the case of the quantum eraser, we restore the interference pattern. But any attempt to "imagine" that the photon has obtained a classical property at any moment before it was measured would lead to wrong predictions.

The question proposed going further, "at which point does the wave function collapse? Ultimately, out of 2 entangled particles, does ones path and/or wave function collapse ultimately determine the path of its unobserved counterpart? In essence, can observations in the present dictate past events? Experimentation on this subject shows positive results for this hypothesis. What does this mean? Does the simulation theory make more sense in this regard that finding? The holographic universe?

In my opinion, it sounds more intuitive to imagine the simulation theory, suggesting that the program (the universe) displays data to the subject based on the data provided up to the point of interaction. But ultimately the wave function collapse could be an illusion of sorts via the effect of OUR entanglement with the quantum particles we intended to observe.

what does this mean? Does the simulation theory make more sense in this regard that finding? The holographic universe? In my opinion, it sounds more intuitive to imagine the simulation theory, suggesting that the program (the universe) displays data to the subject based on the data provided up to the point of interaction.

But ultimately the wave function collapse could be an illusion of sorts via the effect of OUR entanglement with the quantum particles we intended to observe.

A bit about photons, sometime around 2014 a program simulation was successful showing that it is theoretically possible for a photon to travel backwards through time and interact with itself. Cern in 2017 did some confirmation on this simulated theory, LHC finds direct evidence light interacting ITSELF. then nothing...

after that they just kept referring to it every so often like "remember that! that was cool". I’m under the impression that's because they implemented it successfully, this would technically enable communication with our future selves as well. Crazy implications. If there have been any actual advancements made public since 2017 I am unaware of it.

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Wiki Page

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Quantum Eraser explanation

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Displacement