If two photons are entangled If the wavelength of one photon changes is the wavelength of the other affected - by Dan Stahlke and by Viktor T.Toth - Quora Question Review

This document contains a review of the answers by Dan Stahlke and by Victor T. Toth on the question in Quora: "If two photons are entangled If the wavelength of one photon changes is the wavelength of the other affected"
To order to read all the answers select: https://www.quora.com/If-two-photons-are-entangled-If-the-wavelength-of-one-photon-changes-is-the-wavelength-of-the-other-affected

Contents

Reflection

1. Answer Review - Dan Stahlke

No. When two particles are entangled, and are at a distance, it is impossible to affect one particle through any action performed on the other.
That is true. When any particle is measured, the state of any other particle (at a distance, correlated or not) is not affected
Quantum entanglement is thus said to be a non-signalling correlation.
However this raises a more basic question: how do 'we' know that two particles are entangled?
This can only be established by performing the same experiment many times and by measuring both particles . The results should show that the particles are correlated.
You may have heard that for an entangled pair, a measurement of one half determines the state of the other half.
That is not true. When particles are correlated, established by performning the experiment many times, the correlation established at the moment when the two particles were created, as part of the original reaction.
True, but this sort of misses the point of entanglement because you don’t even need entanglement to get such correlations.
Suppose someone flips a coin and gives it to Alice (who doesn’t peek yet).
At that moment the state of Alice's coin is established, fixed, determined (Alice doesn’t peek yet).
They then give a second coin to Bob.
At that moment the state of Bob's coin is also established (Bob doesn’t peek yet).
Alice and Bob don’t know if it’s heads or tails, but they are told they are either both heads or both tails.
That means there is a third observer involved, who has observed both coins.
As far as Alice and Bob are concerned, the coins are in an indeterminate state.
NO, definitely not Both coins are each in a certain state, however neither Alice nor Bob know what that state is.
Now, if Alice looks at her coin and sees that it is heads, then she instantly knows that Bob’s coin is heads.
Yes SHE knows that.
So her measurement has immediately caused Bob’s coin to no longer be unknown.
To her, but Bob does know that. Alice action does not cause any change in Bob's coin. Neither coins are entangled.
You may be wondering what the point was of this silly story. The point is that it is not at all mysterious or spooky for the measurement of one particle to determine the state of a second distant particle.
Alice's action does not determine the state of her coin nor the state of Bob's coin. Each state is determined the last time when the coin is flipped. The state of each card, in a deck of cards, is determined the last time when any card is moved to a different position.
The wonder of entanglement is that it can produce correlations that go beyond what you can have with classical things like coins in envelopes.
Please explain in more detail which your actions are. Wonder's don't happen in the physical world.
This necessarily involves Alice and Bob choosing to make one of many incompatible measurements (e.g. position or momentum, which by Heisenberg’s uncertainty principle can’t both be known).
This depends about the definition of incompatible measurements. Measurements are tricky if what you want to measure is directly influenced by your measurements. This is like measuring the temperature of water, while the temperature of the thermometer is higher than the temperature of the water.
You can look up “Bell’s inequality” to learn more. This is subtle enough that it wasn’t noticed until 1964 (by Bell), even though Einstein, Podolsky, and Rosen were writing about entanglement already in 1935.
And just like how Alice turning her coin from heads to tails doesn’t affect Bob’s coin, manipulating one half of an entangled pair doesn’t affect the other half. Such manipulation may break entanglement, or may not, depending on what you do.
But it will never affect the remote particle.
You give an example of changing the wavelength of a photon, which as others pointed out may not be convenient or even possible (I’ll leave it to the experimentalists to answer that). But you could ask the same question about polarization, which certainly can be manipulated. And the answer is the same: it does not affect the second photon.
BTW: Popescu and Rohrlich proposed a thought experiment involving correlations that go even beyond what is possible in quantum mechanics, yet is still non-signalling. This is useful because you can glimpse a bit of the subtlety of entanglement without needing too much math. Unfortunately, I can’t find any good non-academic articles on it. This piece by the Atlantic is decent, but covers too much ground while devoting only a single paragraph to the meat of it:

Page 3

2. Answer Review - Viktor T. Toth

Allow me to respond to this question by first clarifying two common misconceptions.

First, when we speak of entangled particles, what we really mean is that we created a set of particles that, at least temporarily, are not entangled with everything else. Because that is the normal state of things: everything is entangled with everything else almost all the time. In these laboratory experiments, we manage to isolate, e.g., a pair of photons from everything else in the world, so that we can study entanglement under such artificially created, “clean” circumstances without the unpredictable, random environment.

Second, the wavelength of a photon is not an intrinsic property. It is observer-dependent. An observer that is running after the photon will see a longer wavelength. An observer that is running in the opposite direction will see a shorter wavelength.
That is a wrong conclusion. The fact that two observers measure a different wavelength of the same photon, assuming that, that is possible, indicates they are moving relative to each other. If we want to measure the wavelength of photons we should use one reference frame.
OK, with that in mind… if the photon’s wavelength changes because an observer changes reference frames (e.g., a previously stationary observer begins to move) than obviously, the other photon’s wavelength would change the same way.
When I move towards or away from an object and if what I observe changes (hear, see, feel, smell, measures etc) does not mean that the object changes. It means only that what I observe changes. When you close your eyes, the Sun is still there.
They do not even need to be entangled; what changed, after all, is not the photons but the observer’s reference frame.

The concept "to be engtangled" has nothing to do with this.
If the photon’s wavelength changes because it interacts with its environment (e.g., it enters a refractive medium) then it really is no longer entangled only with its counterpart. There is now a complex interaction between this photon and the environment that caused its wavelength to change. As such, the “pure” entanglement (involving only the two photons, with the environment excluded) is broken, so we would no longer describe the pair as entangled.

3.

Reflection 1 - Question Review

The question: "If two photons are entangled If the wavelength of one photon changes is the wavelength of the other affected" is not clear
Two questions have to be answered:
  1. How are entangled photons created?
    The most detailed document is this one:     https://escholarship.org/uc/item/1kb7660q What this article demonstrates is that correlated photons are created, almost simultaneous from a common source.
  2. How is the wavelength of each photon measured?
    The wave length measured of the two photons created are 5517A and

Reflection 2 - Correlation versus Entanglement

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Created: 1 June 2023

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