Is there a generator of entangled photons with fixed polarization?

[prokopcio]

Hello everyone!
I don't know where to look for information - maybe here it will work :)

Is there a generator of entangled photons with fixed polarization?
If not, is it theoretically possible to build or is it against the laws of physics.

I need this knowledge for further computer simulations.
Unfortunately, no one can answer this question for me :(

Can you help me?
Lukasps.AI chat (GPT) told me that such generators exist but I'm not sure how he would know about it :)

 

[DrClaude]

Theoretically, you can make the state preparation whatever you like. It might not be easy to implement in reality, but experimentalist can be quite clever!

 

[prokopcio]

Tkanks, for reply.

here :
https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion

i found it :

This suggests to me that it is possible.

But !!!

According to my computer simulations, such a device can be easily used to send information at superluminal speed, which is ruled out by the fundamental laws of physics.

I can present my simulations here, maybe someone will point out where I am making a mental error.

EDIT :
now I see that the entangled photons in this picture have no definite polarization. My oversight.
So... still waiting for reply if is posible it with definite polarisation ( i think that not ).

 

[Structure seeker]

Have you considered the no communication theorem?

The no-communication theorem thus says shared entanglement alone cannot be used to transmit any information. Compare this with the no-teleportation theorem, which states a classical information channel cannot transmit quantum information. (By transmit, we mean transmission with full fidelity.) However, quantum teleportation schemes utilize both resources to achieve what is impossible for either alone.

 

[prokopcio]

Yes, I am well aware of this theorem.
I directly agree, which is why I thought such entanglement was impossible.
Still, many people try to use entanglement indirectly.
Me too :).

 

[DrChinese]

Is there a generator of entangled photons with fixed polarization?
If not, is it theoretically possible to build or is it against the laws of physics.

Regardless of how you might interpret the previous comments:

It is an absolute contradiction of terms for a pair of particles to be polarization entangled AND have a fixed polarization. If particles are polarization entangled, they are in a superposition of polarization states - and therefore have no specific polarization value until they are measured.

Yes, it is possible to have a pair of particles that are entangled and have fixed polarization values. There are a variety of ways do accomplish this with crystals (similar to your diagram in post #3). However, such pairs are NOT polarization entangled.

 

[PeterDonis]

This suggests to me that it is possible.

No, it doesn't. You are misinterpreting the diagram. What the diagram is actually saying is that, in the vast majority of cases, the two photons are not entangled. But, in a small minority of cases, where the two photons come out along the lines labeled "entangled photons", then they are entangled--and neither of them have definite polarizations (all you know is that the polarizations are perpendicular).

 

[prokopcio]

Regardless of how you might interpret the previous comments:

It is an absolute contradiction of terms for a pair of particles to be polarization entangled AND have a fixed polarization. If particles are polarization entangled, they are in a superposition of polarization states - and therefore have no specific polarization value until they are measured.

It's what i needed , now all is clear and i know where was my important knowledge gaps....
Thank You !!!

One question more if can.... about this:

Yes, it is possible to have a pair of particles that are entangled and have fixed polarization values. There are a variety of ways do accomplish this with crystals (similar to your diagram in post #3). However, such pairs are NOT polarization entangled.

You are misinterpreting the diagram. What the diagram is actually saying is that, in the vast majority of cases, the two photons are not entangled. But, in a small minority of cases, where the two photons come out along the lines labeled "entangled photons", then they are entangled--and neither of them have definite polarizations (all you know is that the polarizations are perpendicular).

yes, I wrote that I misunderstood it earlier. But the question is if in this particular case the 2 entangled photons are polarization entangled, but we know it's a vertical+horizontal superposition, I mean (0 degrees)+(90 degrees) and not e.g. +45 degrees -45 degrees.

I'm sorry, my english isn't good but hope that you understand my question.Thanks Again - You are the best !!!

 

[Structure seeker]

If I add two vectors with 45 and -45 degrees of equal size I get horizontal polarization. If I subtract the -45 degrees from the 45 degrees I get vertical polarization. So superposition in horizontal and vertical equals superposition in 45 and -45 degrees, they are equivalent under the unitary transformation ## \frac{1}{\sqrt(2)} \begin{pmatrix}
1 & 1 \\
1 & -1
\end{pmatrix}##
There is such a transformation for any two directions with 90 degree direction difference.

 

[PeterDonis]

if in this particular case the 2 entangled photons are polarization entangled, but we know it's a vertical+horizontal superposition

No, you don't. All you know is that the polarizations are perpendicular.

 

[DrChinese]

But the question is if in this particular case the 2 entangled photons are polarization entangled, but we know it's a vertical+horizontal superposition, I mean (0 degrees)+(90 degrees) and not e.g. +45 degrees -45 degrees.

I'm sorry, my english isn't good but hope that you understand my question.

Thanks Again - You are the best !!!

When a pair of those entangled photons are in a vertical/horizontal superposition, they are also in a superposition on all polarization bases. That would include +45 degrees and -45 degrees. Both @Structure seeker and @PeterDonis are also correct in their answers, each coming at your question from a slightly different perspective.

 

[prokopcio]

All you know is that the polarizations are perpendicular.

It was clear "always".

For me todays reply is :

It's imposible to generate pair of photons with known polarizations...

Second learn is ( very good and simply explained by all of you ) :
0 and 90 = -45 and +45
i saw it difference before, now after calculation understand how it works.

Before talking to you, everything was exactly the same for me in the simulation, just a different point of view led me to a dead end.