understanding loopholes: what are the loopholes in entanglement swapping?

San K

what are the loopholes in entanglement swapping (that would say you cannot say for sure quantum entanglement happens/happened)?

DrChinese

The local realist would probably cite the same ones as with Bell tests, since it takes a Bell test to see the effect.

Of course, that really misses the beauty of the experiment, and it really is no "explanation" at all. This type of coincidence shouldn't happen at all when you have different sources.

San K

well put DrChinese.

On a separate note: Does the discovery, of the phenomena, of entanglement swapping, as well as two-photon interference at a distance (i.e. without having the photons arrive at the beam splitter at the same time), rule out hidden variable theory?

zonde

You mean why there is no "spooky action at a distance" or you mean why there is no "spooky action into the past"?

San K

Hi Zonde, interesting question.

the loopholes are arguments that are saying you cannot conclude with 100% confidence that action is happing non-locally i.e. QE exist.

the focus is generally on "spooky action at a distance" however

to answer your question ......we could look at both...if you wish.........space (at a distance) and time (into the past).

however the "spooky action into the past" is not a major concern/focus because its easily resolvable because:

all experiments, that seem to suggest "spooky action into the past" (for example DCQE etc) are unambiguously explainable by other explanations such as superimposition of data/patterns

the authors of such papers/experiments have themselves reiterated that there is no need to assume spooky action into the past and provided the real explanation...i.e. it depends upon which (filtered) data set you are looking at.

the sentence in bold below clarifies that -


"If one views the quantum state as a real physical object, one could get the seemingly paradoxical situation that future actions appear as having an influence on past and already irrevocably recorded events. However, there is never a paradox if the quantum
state is viewed as to be no more than a “catalogue of our knowledge”

Then the state is a probability list for all
possible measurement outcomes, the relative temporal order of the three observer’s events is irrelevant and no
physical interactions whatsoever between these events, especially into the past, are necessary to explain the
delayed-choice entanglement swapping. What, however, is important is to relate the lists of Alice, Bob and
Victor’s measurement results. On the basis of Victor’s measurement settings and results, Alice and Bob can
group their earlier and locally totally random results into subsets which each have a different meaning and
interpretation. This formation of subsets is independent of the temporal order of the measurements. According
to Wheeler, Bohr said: “No elementary phenomenon is a phenomenon until it is a registered phenomenon.”

We would like to extend this by saying: “Some registered phenomena do not have a meaning unless they are
put in relationship with other registered phenomena.”

zonde

It's good we can focus on "spooky action at a distance".

Now what we can claim based on (ideal) results of entanglement swapping? We can split all the photons into subsets of differently entangled photons. So the question could be about transferring enough information from two sources to the place of Bell-state measurement in order to do the sorting. But this doesn't require any loophole.

So I think that entanglement swapping is "spooky action at a distance" if you accept entanglement as "spooky action at a distance". But if you don't then entanglement swapping does not add any additional mystery of it's own.

Hmm, seems rather short argument. Maybe you want more detailed arguments about some part?

StevieTNZ

I honestly don't understand the delayed-choice entanglement swapping scenerio. Entanglement swapping is teleportation of a particular photon's state onto another photon. But if photons 1 and 4 are actual, how can a state get teleported onto it? Entanglement ceases to exist between 1 and 2, and 3 and 4, before the bell-state measurement of 2 and 3.

Question to Anders Sørensen:
Is it a requirement that in entanglement swapping experiments, including in the delayed-choice scenario, entanglement must exist between photons #1 and #2, and #3 and #4, at the time of the bell-state measurement of #2 and #3?

His answer: They have to be entangled before otherwise 1 and 4 will not be entangled after. It wouldn't violate quantum predictions, it would just not be entangled.

But isn't the whole purpose of delayed-choice entanglement swapping also to entangle 1 and 4. Otherwise, they would be separable states mimicing entanglement - not actually entangled - violating the CHSH inequality.

Of course all this goes away if we follow what QM says when we measure 1 and 4, the photons get entangled with the apparatus and aren't in any definite polarisation. Entanglement still exists between 1 and 2 before the bell-state measurement, just between 2 and the apparatus that absorbed photon 1.

San K

Hi Zonde, looks like we might be a bit out of phase... so let first move into coherence ...;) ....unto same wavelength/page....

In short, I am saying that -

entanglement swapping is another proof/phenomena/discovery that further weakens the hidden variable theory and strenghtens the idea that QE (i.e. non-local action) exists.

Also I am asking - are some of the loopholes (in earlier quantum experiments such as Bell's tests, Delayed choice etc) closed/weakened after the discovery/demonstration of QES (quantum entanglement swapping)?

zonde

I think we are not so much out of phase.

I am trying to understand how entanglement swapping goes further that Bell tests. In LHV we have to introduce additional variables that govern photon/measurement equipment interaction besides polarization HV (in order to exploit detection loophole in Bell tests). Let's say we have hidden variable "X" that we "measure" when some photons are absorbed somewhere along the way from PBS to detector (including non-detection at detector).
Now entanglement swapping would weaken LHV position further if it would require LHV model to introduce yet another variable "Y" to explain that experiment. And I do not see that this is so.

San K

agreed

i think it does require us to introduce another (hidden) variable(s) because when Victor/Charlie entangles A2 and B2 he is entangling particles which were not entangled with each other before.

(A1 & B1 are with Alice and Bob respectively and A1 is entangled with A2 and B1 with B2)

zonde

You make it sound like Bell-state measurement is kind of active process in respect to photons not just passive sorting of photon pairs.
Is this the reason why you say that we have to introduce another variables?

I think that we can speak about Bell-state measurement as a process where we look at existing variables and depending on their values we sort photon pairs in subsets.

San K

can passive sorting, completely, explain the high degree of correlation between alice and bob ('s photons), even though they never met, at all?