The setup of "2012 entanglement between photons that (....)"

[Joao]

Hi all! Sorry for the bad english!

Please, can someone help me understand the setup from "entanglement between photons that never coexisted "?

So, we start with a bean from a laser, that hits a bbo crystal, makes two entangled photons (1 and 2), 1 gets measured (lets say its vertically polarised) and dies.

Photon 2 goes to (Where?), 2 more entangled photons are created there (how?) (Photons 3 and 4). Photon 2 gets entangled with they (how?), photon 4 gets detected and it shows its entangled with photon 1 (like it is measured as horizontally polarised ).

I Just want to know how it happens, please. Like "photon 2 hits another bbo cristal that generates the new two entangled pairs" or something like that! =)

Thanks Again! =)

 

[mfb]

There are many measurements with entangled photons that have been done in 2012. A reference would be nice.

Like "photon 2 hits another bbo cristal that generates the new two entangled pairs"

Sounds like it, but without seeing the experiment it is hard to tell.

 

[Joao]

There are many measurements with entangled photons that have been done in 2012. A reference would be nice.Sounds like it, but without seeing the experiment it is hard to tell.

Thanks for the reply! Here's the reference:

https://arxiv.org/abs/1209.4191

Thanks again!

 

[mfb]

Thanks.

So, we start with a bean from a laser, that hits a bbo crystal, makes two entangled photons (1 and 2), 1 gets measured (lets say its vertically polarised) and dies.

Right.

Photon 2 goes into the delay line. Later photon 3 and 4 are created independently from the laser. 3 and 2 hit the beam splitter at the same time. They are measured, and if they are in a suitable state ("We post-select the cases where each photon exits this PBS at a different port.") then it means photon 1 and 4 are entangled.

 

[Joao]

Thanks.Right.

Photon 2 goes into the delay line. Later photon 3 and 4 are created independently from the laser. 3 and 2 hit the beam splitter at the same time. They are measured, and if they are in a suitable state ("We post-select the cases where each photon exits this PBS at a different port.") then it means photon 1 and 4 are entangled.

Thanks a lot for your pacience! =)

So, photon 3 and 4 are created with the same laser pump and same bbo crystal, but after the photon 1 died , right? Then photon 2 meets photon 3 at the bean splitter and there they became entangled. Meanwhile photon 4 is in a delayed circuit. So far I am getting it right?

Thanks a lot! Really!

 

[mfb]

Right.

 

[DrChinese]

So, photon 3 and 4 are created with the same laser pump and same bbo crystal, but after the photon 1 died , right? Then photon 2 meets photon 3 at the bean splitter and there they became entangled. Meanwhile photon 4 is in a delayed circuit. So far I am getting it right?

I believe it is Photon 2 that is delayed, so it can sync up with Photon 3 a bit later. Photon 4 is entangled with Photon 1 (if all goes well).

 

[Joao]

I believe it is Photon 2 that is delayed, so it can sync up with Photon 3 a bit later. Photon 4 is entangled with Photon 1 (if all goes well).

I believe that Both photon 2 and 4 are delayed, photon 2 to sinc with photon 3 and photon 4 to do this:

So, after photon 2 and 3 becames entangled, they Both die, after that photon 4 gets measured and dies (is it correct, please?)

So, i didnt understood why they are talking about "nom time locality"... To me, it seens that it went like this (for example )

Photon 1 wavefunction crashed into vertical polarizarion, photon 2 became horizontal polarizated, when entangled with photon 3, photon 3 became vertical polarized, that made photon 4 became horizontal polarized...

Its awesome and wonderfull, but i don't see the need to say that it were "non time locally " entangled... it was Just info passing from photon 1 to photon 2, photon 2 to photon 3 and photon 3 to photon 4...

What am i missing?

Thanks! =)

 

[DrChinese]

I believe that Both photon 2 and 4 are delayed, photon 2 to sinc with photon 3 and photon 4 to do this:

So, after photon 2 and 3 becames entangled, they Both die, after that photon 4 gets measured and dies (is it correct, please?)

So, i didnt understood why they are talking about "nom time locality"... To me, it seens that it went like this (for example )

Photon 1 wavefunction crashed into vertical polarizarion, photon 2 became horizontal polarizated, when entangled with photon 3, photon 3 became vertical polarized, that made photon 4 became horizontal polarized...

Its awesome and wonderfull, but i don't see the need to say that it were "non time locally " entangled... it was Just info passing from photon 1 to photon 2, photon 2 to photon 3 and photon 3 to photon 4...

What am i missing?

Thanks! =)

First: photons 1 and 4 never interact. So for them to be entangled is quite amazing. Usually, objects must interact to become entangled.

Second, the interaction of 2 and 3 (projecting them into a Bell State can be done before, during OR after photons 1 and 4 are observed. In this particular experiment, they are projected before photon 4 is observed. But other experiments, they are projected AFTER.

Third: your explanation "somewhat" works if influences go backward in time. But forward in time does not work. In fact, it is impossible to say which "direction" polarization collapse occurs. Is it from 1 to 2, or from 2 to 1? You can't really say. All you can do is assume.

 

[Joao]

Thanks a lot DrChinese! =)

If I can bother you a little more, I still didnt understood it. Maybe I lack the background to do so... But I don't get this:

Second, the interaction of 2 and 3 (projecting them into a Bell State can be done before, during OR after photons 1 and 4 are observed. In this particular experiment, they are projected before photon 4 is observed. But other experiments, they are projected AFTER.

Please, to be in a Bell state means they are entangled, right?

So, in this experiment, they became entangled Before the measurement of photon 4, if I got it right...

Can the scientists choose when to entangle they? Like after the measurements occour, the scientists choose if they want to entangle photon 2 and 3?

By the way... how does this entanglement happens? Is it the mere fact that they are near each other that entangles they?

If you think its impossible to explain to me due to my lack of more background, its ok! =)

Thanks! =)

 

[DrChinese]

Thanks a lot DrChinese! =)

If I can bother you a little more, I still didnt understood it. Maybe I lack the background to do so... But I don't get this:
Please, to be in a Bell state means they are entangled, right?

So, in this experiment, they became entangled Before the measurement of photon 4, if I got it right...

Can the scientists choose when to entangle they? Like after the measurements occour, the scientists choose if they want to entangle photon 2 and 3?

By the way... how does this entanglement happens? Is it the mere fact that they are near each other that entangles they?

If you think its impossible to explain to me due to my lack of more background, its ok! =)

Thanks! =)

a. When 2 & 3 are in a Bell State, 1 & 4 are entangled. (Yes, it is confusing.)
b. 2 & 3 are projected into a Bell State. But not all pairs are so projected. Only some are. The conditions must be precise for this to happen. They must arrive at the same time.
c. The scientist may elect to attempt to project 2 & 3 into a Bell State at any time at which they can make the conditions work. That timing does not affect the results at 1 & 4.
d. Importantly: there are 2 key Bell States being analyzed in this experiment. One produces matching 1 & 4, the other produces mismatching 1 & 4. The basic description is seen in Fig. 3, (a) and (b) where is discusses photons 2 & 3 projected into the φ+ and the φ− states. This occurs randomly (whether φ+ or φ−) and cannot be controlled by the scientist.

Please note that this experiment is far less well known than the usual Bell tests. (There is a Bell test performed as part of this experiment too, that is what proves there is entanglement.)

 

[Joao]

a. When 2 & 3 are in a Bell State, 1 & 4 are entangled. (Yes, it is confusing.)
b. 2 & 3 are projected into a Bell State. But not all pairs are so projected. Only some are. The conditions must be precise for this to happen. They must arrive at the same time.
c. The scientist may elect to attempt to project 2 & 3 into a Bell State at any time at which they can make the conditions work. That timing does not affect the results at 1 & 4.
d. Importantly: there are 2 key Bell States being analyzed in this experiment. One produces matching 1 & 4, the other produces mismatching 1 & 4. The basic description is seen in Fig. 3, (a) and (b) where is discusses photons 2 & 3 projected into the φ+ and the φ− states. This occurs randomly (whether φ+ or φ−) and cannot be controlled by the scientist.

Please note that this experiment is far less well known than the usual Bell tests. (There is a Bell test performed as part of this experiment too, that is what proves there is entanglement.)

Thanks a lot! I will try to understand the difference between a Bell state and entanglement, then I will reply if I understood the experiment! =)

 

[Joao]

Hi again! So, the kind of bell state photon 2 and 3 are put (or even if they are in a bell state at all) influences the kind of entanglement between photon 1 and 4?

And suppose we make a very long delay circuit to delay the meeting of photon 2 and 3, so the photon 1 and 4 gets detected before photon 2 and 3 meets and gets into any bell state... it would not change the outcomes?

Thanks again! =)

 

[StevieTNZ]

Hi again! So, the kind of bell state photon 2 and 3 are put (or even if they are in a bell state at all) influences the kind of entanglement between photon 1 and 4?

Yes, that is correct.

A good reference for this is pages 63 & 64 of 'Six Quantum Pieces: A First Course in Quantum Physics' by Valerio Scarani (https://www.worldscientific.com/worldscibooks/10.1142/7965)

 

[Joao]

Yes, that is correct.

A good reference for this is pages 63 & 64 of 'Six Quantum Pieces: A First Course in Quantum Physics' by Valerio Scarani (https://www.worldscientific.com/worldscibooks/10.1142/7965)

Thanks a lot! I will read it! =)

About the second part, is it correct? If we delay the interaction of photos 2 and 3 for after 1 and 2 are detected, it wouldn't matter?

Thanks again! =)

 

[StevieTNZ]

About the second part, is it correct? If we delay the interaction of photos 2 and 3 for after 1 and 2 are detected, it wouldn't matter?

If I understand correctly, your experimental set up is similar to the Delayed Choice Entanglement Swapping experiment, published in Nature back in 2012.

https://www.nature.com/articles/nphys2294

 

[Joao]

Thanks a lot! I will read it! =)

About the second part, is it correct? If we delay the interaction of photos 2 and 3 for after 1 and 2 are detected, it wouldn't matter?

Thanks again! =)

Just correcting:

About the second part, is it correct? If we delay the interaction of photos 2 and 3 for after 1 and 4 are detected, it wouldn't matter?

 

[Joao]

If I understand correctly, your experimental set up is similar to the Delayed Choice Entanglement Swapping experiment, published in Nature back in 2012.

https://www.nature.com/articles/nphys2294

Yes it is! Thanks! Too bad that the article isn't free... =(

So, if I understood correctly:

Let's call the possible measurements of photon 1 - H1 and V1

Let's call the measurements of photon 4 - H4 and V4

Let's call the possible bell states of photon 2 and 3 - red, green and nothing (nothing is when there's no bell state between photons 2 and 3).

Is this kind of true:
IF V1 AND RED, THEN V4
IF H1 AND RED, THEN H4

IF V1 AND GREEN, THEN H4
IF H1 AND GREEN, THEN V4

IF V1 AND NOTHING, THEN H4 OR V4
IF H1 AND NOTHING, THEN H4 OR V4

?

Thanks again! =)

 

[DrChinese]

Just correcting:

About the second part, is it correct? If we delay the interaction of photos 2 and 3 for after 1 and 4 are detected, it wouldn't matter?

As far as can be determined experimentally, there is difference due to timing. You can therefore choose to delay the projection of 2 & 3 into a Bell state AFTER the 1 & 4 particles are detected. This seems nonsensical, but is fully in keeping with QM.

Here is a free version of the article StevieTNZ referenced:

https://arxiv.org/abs/1203.4834

"Motivated by the question, which kind of physical interactions and processes are needed for the production of quantum entanglement, Peres has put forward the radical idea of delayed-choice entanglement swapping. There, entanglement can be "produced a posteriori, after the entangled particles have been measured and may no longer exist". In this work we report the first realization of Peres' gedanken experiment. Using four photons, we can actively delay the choice of measurement-implemented via a high-speed tunable bipartite state analyzer and a quantum random number generator-on two of the photons into the time-like future of the registration of the other two photons. This effectively projects the two already registered photons onto one definite of two mutually exclusive quantum states in which either the photons are entangled (quantum correlations) or separable (classical correlations). This can also be viewed as "quantum steering into the past"."

 

[Joao]

Thanks a lot everyone! I'm reading the article and it's awesome! =)

I think it's very strange that the order of measurements doesn't affect the final outcome... because I guess the "normal causality order" should be: photon 3 and 2 gets in a bell state, then photon 1 and 4 gets entangled. The "retrocausality order" should be photon 1 and 4 gets measured, then 3 and 2 gets in bell state.

But if photon 1 is detected and dies , 2 and three gets in a bell state and photon 4 gets detected and dies, not even retrocausality would explain that! It's like this: time starts flowing normally, then does a little "jump" when photon 1 should be detected, photon 2 and 3 gets in a bell state, time goes backwards from when photon 1 should be detected and dies, then time jumps again to photon 4 getting detected and dying and flows normally!

Wonderful!

It's like time doesn't matter at all, it just has to comply with the relations that must happen in quantum mechanics.

Like in general relativity. The maximum speed of travel is c, and time and space has to adapt to it.

To me, it seems like there are some fundamental rules of the universe, and time and space just has to adapt to it.

Thanks a lot everyone! Now I'm more amazed with the universe then when I created this thread! Thanks again! =)

 

[mfb]

It's like time doesn't matter at all, it just has to comply with the relations that must happen in quantum mechanics.

That is the key result of experiments like this.

 

[DrChinese]

I think it's very strange that the order of measurements doesn't affect the final outcome... because I guess the "normal causality order" should be: photon 3 and 2 gets in a bell state, then photon 1 and 4 gets entangled. The "retrocausality order" should be photon 1 and 4 gets measured, then 3 and 2 gets in bell state.

Pretty good summary, I would more or less describe it that way myself.

Note that around here, most others *dislike* the thought of anything being considered retrocausal. So don't be surprised to hear disagreement on the point, because there are a number of ways to interpret QM that don't include anything retrocausal.

Another way to consider it: QM is considered a contextual theory. That is: a future measurement context must be known to provide the most accurate statistical predictions. So the present experimental context PLUS a future measurement context together define the overall context. In that view, this experiment is perfectly consistent regardless of the ordering of individual actions. Because the overall context never changes.

 

[StevieTNZ]

Do remember that photons 2 & 3 are detected/measured in a bell-state, so we cannot assign them any polarization thereafter (as they are 'destroyed' when measured). You can assign |H> and |V> (or |45>, |135>, |L>, |R> etc.) to photons 1 and 4.

 

[Derek P]

Do remember that photons 2 & 3 are detected/measured in a bell-state, so we cannot assign them any polarization thereafter (as they are 'destroyed' when measured). You can assign |H> and |V> (or |45>, |135>, |L>, |R> etc.) to photons 1 and 4.

Even if you did it with electrons, which are not destroyed, you would not be able to assign spins to them after they have become entangled. That's what entangled means.

 

[StevieTNZ]

Even if you did it with electrons, which are not destroyed, you would not be able to assign spins to them after they have become entangled. That's what entangled means.

Correct.

 

[Joao]

Thanks everyone!

That is the key result of experiments like this.

Pretty good summary, I would more or less describe it that way myself

Thanks a lot! =)

(Multi quotting isn't working well here)

I didn't understand any of these:

Another way to consider it: QM is considered a contextual theory. That is: a future measurement context must be known to provide the most accurate statistical predictions. So the present experimental context PLUS a future measurement context together define the overall context. In that view, this experiment is perfectly consistent regardless of the ordering of individual actions. Because the overall context never changes.Do remember that photons 2 & 3 are detected/measured in a bell-state, so we cannot assign them any polarization thereafter (as they are 'destroyed' when measured). You can assign |H> and |V> (or |45>, |135>, |L>, |R> etc.) to photons 1 and 4.

Even if you did it with electrons, which are not destroyed, you would not be able to assign spins to them after they have become entangled. That's what entangled means.

I will Google about it, and then I will se if I can understand it! Thanks again! =)