What is the meaning of "same direction" for spin measurement?

[Heidi]

Hi Pfs,
I read that pairs of entangled particles were measured on earth along a same direction
and that the distance between them was greater than 1000 kilometers.
What is the définition of a "same direction" on earth? is Is it related to the direction of
a very distant galaxy or somethig like that? (i think of the foucault pendulum)

 

[renormalize]

Hi Pfs,
I read that pairs of entangled particles were measured on earth along a same direction
and that the distance between them was greater than 1000 kilometers.
What is the définition of a "same direction" on earth? is Is it related to the direction of
a very distant galaxy or somethig like that? (i think of the foucault pendulum)

Please cite the reference where you read this.

 

[gentzen]

Take the measurement operators at the position where the measurements happened, and propagate them backwards (also in time) along the two particle paths towards the point where the particles were created. Then compare their directions at this common point in spacetime.

 

[PeterDonis]

Take the measurement operators at the position where the measurements happened, and propagate them backwards (also in time) along the two particle paths towards the point where the particles were created. Then compare their directions at this common point in spacetime.

While this might be a theoretical way of defining the ideal case, it is not going to be how "same direction" is actually done in an experiment. Without a specific reference to an experiment from the OP, we can't know how they did it.

 

[DrChinese]

The farthest I have a link for is 1400 km, a little less than 1000 miles. It is ground to satellite, so not the same as you mention. Anything like this is going to use photons, so the horizon is going to be an issue that would be quite difficult to overcome when you get that far apart.

Ground-to-satellite quantum teleportation

But check this out too. It is more distance and may address some of your issue. I don't think a Bell test is performed though.

Satellite-relayed intercontinental quantum network

 

[Heidi]

I foud 1200 kilometers here:
https://arxiv.org/abs/1707.01339
was there measurements on random directions and analyses after?

 

[Nugatory]

I foud 1200 kilometers here:
https://arxiv.org/abs/1707.01339
was there measurements on random directions and analyses after?

From the abstract: “We observe a survival of two-photon entanglement and a violation of Bell inequality by 2.37+/-0.09 under strict Einstein locality conditions.”

That seems to imply some amount of random choice of directions and after-the-fact analysis.

 

[Heidi]

A remark and a question:
When there are 1400 kms betweex the two laboratories, it seems that mountains or electric fields
in the middle play no role. nothing propagates between the laboratories.
And the question:
Coule "same direction" be defined by "always same resuts" with maximally entangles pairs?

 

[Fra]

Good and important question.

A remark and a question:
When there are 1400 kms betweex the two laboratories, it seems that mountains or electric fields
in the middle play no role. nothing propagates between the laboratories.
And the question:
Coule "same direction" be defined by "always same resuts" with maximally entangles pairs?

How does you compare results from two places without anything propagating?(ie propagating/communicating results from Alice to Bob?)

The generalisation of this question is how any two observations of different observers (wether distinguished by position or something else!) can be compared at all?

I'll quote Rovelli here which has a nice phrasing...

"First of all, one may ask what is the “actual”, “absolute” relation between the description of the world relative to O and the one relative to P. This is a question debated in the context of “perspectival” interpretations of quantum mechanics. I think that the question is ill-posed. The absolute state of affairs of the world is a meaningless notion; asking about the absolute relation between two descriptions is precisely asking about such an absolute state of affairs of the world. Therefore there is no meaning in the “absolute” relation between the views of different observers. In particular, there is no way of deducing the view of one from the view of the other.
...
Does this mean that there is no relation whatsoever between views of different observers? Certainly not...
...
There is an important physical reason behind this fact: It is possible to compare different views, but the process of comparison is always a physical interaction"
-- Carlo Rovelli, Relational Quantum Mechanics

Here "communicating the results between observers" qualifies as a physical interactions as well.

/Fredrik

 

[Heidi]

There are two levels where we have physical processes:
A) When Alice interacts with her particles to in a given direction (always the same)
and when Bob do the same in random directions (ans writes HIS results).
B) there is another type of physical interaction if Alice sends to Bob (by internet or another classical method) all her up or down results.

Using these two processes Bob will be able to know whar is the "same direction"

 

[Heidi]

Has my question an answswer in QFT book?
Is there an equation of motion for Bell pairs?
thanks