Quantum eraser and time travel

[black hole 123]

i don't understand, how does quantum eraser not imply time travel? i firmly don't believe in retrocausality, but it really does seem to imply it . can someone please explain in layman terms?

in here paul davies says observers now can "constrain" the past, does this mean our perception of linear time like a river is wrong? maybe there's only "now" containing our knowledge of the possible pasts and possible future? (the "now" is very strange tbh, problem of now, in relativity there's no special "now" moment in time).

 

[.Scott]

There's a slightly more elaborate experiment called the Delayed Choice Quantum Eraser:
https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser
When it comes to reversing cause and effect, it has the potential of being even more convincing.

But if you look at these experiments carefully, past events are never actually changed - there are only statistics that are open to interpretation. Clearly, something unusual is happening, but describing it as a changing the past is not the conventional way of explaining it - nor does the experiment allow one to change the past in any useful way.

 

[DrChinese]

Quantum mechanics is called a "contextual" theory. The full context of an experimental setup must be considered to obtain the proper predictions. This is especially true with the quantum eraser. Whether or not erasing occurs, that becomes part of the context. What is unusual, then, is that the erasure (or not) takes place in future spacetime. So, elements from a variety of points in time make up a context. (This is true, one way or another, in almost any quantum interaction. All Bell tests, for example, feature a quantum context.) This has the appearance of the future changing the past when you look at it from a certain perspective. Whether or not you interpret this as evidence of retrocausality is more a matter of interpretation. The outcomes themselves are still random.

 

[DrChinese]

Another excellent example of the quantum context is Entanglement Swapping. The swapping operation can be done before or after the entangled pair is detected, making causal ordering "difficult" to assign.

https://arxiv.org/abs/quant-ph/0201134

Experimental Nonlocality Proof of Quantum Teleportation and Entanglement Swapping
Thomas Jennewein, Gregor Weihs, Jian-Wei Pan, Anton Zeilinger (2002-2018)

"Quantum teleportation strikingly underlines the peculiar features of the quantum world. We present an experimental proof of its quantum nature, teleporting an entangled photon with such high quality that the nonlocal quantum correlations with its original partner photon are preserved. This procedure is also known as entanglement swapping. The nonlocality is confirmed by observing a violation of Bell's inequality by 4.5 standard deviations. Thus, by demonstrating quantum nonlocality for photons that never interacted our results directly confirm the quantum nature of teleportation."

...

"Such a delayed-choice experiment was performed by including two 10 m optical fiber delays for both outputs of the BSA. In this case photons 1 and 2 hit the detectors delayed by about 50 ns. As shown in Fig. 3, the observed fidelity of the entanglement of photon 0 and photon 3 matches the fidelity in the non-delayed case within experimental errors. Therefore, this result indicate that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres [4]: this paradox does not arise if the correctness of quantum mechanics is firmly believed."

 

[Strilanc]

The apparent "time rewriting" aspects of the delayed choice experiment are really just confusing correlation with causation (or rather, reverse causation).

I explain this in more detail in this blog post: http://algassert.com/post/1720 "A Classical Delayed Choice Experiment". Quote:

Bob has a six-sided die, a two-sided coin, and a small empty box to put the coin in. He rolls the die, gets a result between 1 and 6, and places the coin in the box in a way that depends on the die roll. If the die roll was even, the coin is placed heads up. If the die roll was odd, the coin is placed tails up. Bob then writes down his die roll, and carefully hands the box to Alice.

Alice now has to decide to either a) just open the box or b) shake the box before opening it. (Shaking the box randomizes the coin.) Once the box is open, she writes down whether the coin was face up or face down, and also writes down whether or not she shook the box.

Later, after repeating the above process many times, Alice and Bob get together and compare notes. They notice something a bit confusing: Alice's choice seems to have affected Bob's results, even though Bob wrote down his results before Alice's choice!

To be specific, Alice's choice seems to determine whether or not Bob's die rolls were "banded"! Look for yourself:

In a sense, the function of the "quantum" in "quantum delayed choice eraser" is to make the correlation-vs-causation paradox harder to see through. It is not fundamental to the paradox that the system be quantum. The "quantum" does become important if the choice is simultaneous (spacelike separated) instead of delayed, but that's Bell tests not a delayed choice eraser.

 

[black hole 123]

The apparent "time rewriting" aspects of the delayed choice experiment are really just confusing correlation with causation (or rather, reverse causation).

I explain this in more detail in this blog post: http://algassert.com/post/1720 "A Classical Delayed Choice Experiment". Quote:
In a sense, the function of the "quantum" in "quantum delayed choice eraser" is to make the correlation-vs-causation paradox harder to see through. It is not fundamental to the paradox that the system be quantum. The "quantum" does become important if the choice is simultaneous (spacelike separated) instead of delayed, but that's Bell tests not a delayed choice eraser.

hi,

it seems i misunderstood the experiment. i thought if the first 2 detectors of the idler photon (to determine the slit the idler came through) were shut off, then the TOTAL pattern would show banding, and if the detectors were turned on, then the total pattern would not show banding. according to wikipedia https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser, the total pattern NEVER shows banding regardless whether u turn the first 2 detectors on or off, but doesn't this contradict the double slit experiment? imagine removing every detector/mirror/beam splitter to do with the signal photon, according to wikipedia there would still no be banding, contradicting the double slit experiment.

basically i thought the experiment's outcome depended on human choices made after the signal photon hit the detector, as that video seemed to imply. but reading ur link and wikipedia it's just a selective subset of the total results that shows banding. still this contradicts the double slit experiment...

 

[Strilanc]

The total pattern never shows banding in the delayed choice experiment.

The double slit experiment shows a banding pattern because it doesn't split the photon passing through the slit into a signal photon and an idler photon. It's the splitting that takes away the banding, because the photon hitting the screen is no longer coherent.

The choice in the delayed choice experiment either gives you measurements that allow you to post-process the data in a way that recovers the interference pattern despite the splitting or else gives you useless data that doesn't do that.

 

[Cthugha]

[...] imagine removing every detector/mirror/beam splitter to do with the signal photon, according to wikipedia there would still no be banding, contradicting the double slit experiment.

How does this contradict the double slit? The majority of all light sources out there will show no interference in a double slit experiment. You need spatially coherent light to achieve that. For example, you will not see any interference if you direct sunlight towards a double slit. This is why Young had to place a pinhole in front of the double slit in his historical double slit, which is probably the single most forgotten and underrated piece of equipment in the history of experiments in optics.
By the way, spatial coherence and entanglement in the conjugate variable (momentum) are mutually exclusive. You cannot have a light field with a full degree of both simultaneously.

 

[Demystifier]

i don't understand, how does quantum eraser not imply time travel?

See Sec. 5.1 of my http://de.arxiv.org/abs/1703.08341

 

[entropy1]

i don't understand, how does quantum eraser not imply time travel?

https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser

The pattern, interference pattern or lump, is formed by a correlation between the signal photon and the idler photons. So they are both needed to form the pattern. So, the idler photons may be changed by the path they take and the pattern of the correlation with the signal photon will change. That would be a causal explanation. In fact, there is no causality involved.

 

[vanhees71]

You can just measure all photons (idler and signal) in the quantum-eraser setup provided you make the measurement protocols such that you know which measured photon pairs are from one entangled state (usually by making sufficiently accurate time stamps). Then you can after establishing the measurement protocol erase the which-way information by just looking at partial ensembles, i.e., the pattern of the registration of signal photons only from such pairs where the idler photon has one particular polarization state. That ensemble has no which-way information and thus you get an interference pattern. If you choose the other polarization state for the idler you also get an interference pattern complementary to the former one. Both together give of course the photon-position pattern of the total ensemble, which shows no interference pattern.

There is nothing mysterious in this and the facts are established once and for all by taking the measurement protocols. The correlations and thus the possiblity of "erasing" is due to the preparation of the photon pairs in (maximally) entangled state, socalled Bell states. There is no causality violation in just choosing the one or the other partial ensemble. It doesn't even change anything in the outcome of the measurements, it just reads off an established measurement protocol. That choice of the partial example makes it indeed a "delayed choice", which is a choice after establishing the facts by measuring the photons and store the result in the measurement protocol.