[Pincho (OP)]

So two photons become entangled. You then separate them by a vast distance. Then an observer examine one of them, and if it has a left handed spin, the other will have a right handed spin?

Is that right?

Anyway, I wondered about the experiment, and don't have much actual detail on the subject. For example.. how are the photons collected, and moved? What is the observer?

I don't actually believe in Action At A Distance. I believe in the hidden variable. But unless I have every single detail on the experiment I can't figure out the hidden variable.

Thanks!

[Pmb]

So two photons become entangled. You then separate them by a vast distance. Then an observer examine one of them, and if it has a left handed spin, the other will have a right handed spin?

Is that right?

Yes.

Anyway, I wondered about the experiment, and don't have much actual detail on the subject. For example.. how are the photons collected, and moved? What is the observer?

Although I'm sure that this will be subject to some debate, the observer is usually a piece of equipment which detects and records events. It could be something like a  phosphorecent screen which detects a photon by emitting light. Not sure though. I'm not an experimentalist but I'm eager to find the answer to this myself.

I don't actually believe in Action At A Distance. I believe in the hidden variable. But unless I have every single detail on the experiment I can't figure out the hidden variable.

Good luck. As I recall that's pretty much been dismissed by experimentation. Experiments relating to Bell's inequality were done and the results were consitent with quantum theory.

[Pincho (OP)]

Good luck. As I recall that's pretty much been dismissed by experimentation. Experiments relating to Bell's inequality were done and the results were consitent with quantum theory.

I can solve it in less than 1 week, I promise you. I just need the details, and there could be a hidden variable in Bell's inequality (there is definitely hidden information in this experiment).

[JP]

Although I'm sure that this will be subject to some debate, the observer is usually a piece of equipment which detects and records events. It could be something like a  phosphorecent screen which detects a photon by emitting light. Not sure though. I'm not an experimentalist but I'm eager to find the answer to this myself.

Often when you have a small number of photons, you use a photomultiplier tube, which is a vacuum tube which converts a photon to a cascade of many electrons (via the photoelectric effect to get the initial electron, which is then turned into many electrons by the electrodes).  It performs well under low light conditions: http://en.wikipedia.org/wiki/Photomultiplier

Detecting spin of photons is equivalent to detecting polarization, so you can use a polarizer in front of your detector. 

Often you can use a polarizing beamsplitter and two detectors so that you don't waste photons.

[Pincho (OP)]

Good information thanks. Do they use fibre optic cable to move the particles? How do they create a no spin on the photons before the experiment? Oh wait, I remember reading that they can create both spins at once.. how do they do that?

[JP]

The photons move through the air or a vacuum from source to detector. 

A process used to create pairs of entangled photons is parametric downconversion:
http://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion

Because of conservation laws in physics, the entangled spins of the photons are determined from the polarization of the incident light. 

[Pincho (OP)]

The photons move through the air or a vacuum from source to detector. 

A process used to create pairs of entangled photons is parametric downconversion:
http://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion

Because of conservation laws in physics, the entangled spins of the photons are determined from the polarization of the incident light.

What do they mean by cones?

[JP]

From what I recall (it's been a while since I studied this), in order to satisfy all the conservation laws (energy and momentum) in the crystal, the index of refraction seen inside the crystal by the emitted photons has to be of a different value than that of the incident photon.  (Index of refraction has to do with the energy and momentum of photons in a material).

In these crystals, the index varies with angle, so that you can find the right index for the output photons at a fixed angle with respect to an axis through the crystal.  If you align the crystal properly, you get a cone of output photons at a fixed angle--which means a fixed index. 

Index also varies with polarization state, so if you have different polarizations of output photons, you get two output cones.

[Pincho (OP)]

From what I recall (it's been a while since I studied this), in order to satisfy all the conservation laws (energy and momentum) in the crystal, the index of refraction seen inside the crystal by the emitted photons has to be of a different value than that of the incident photon.  (Index of refraction has to do with the energy and momentum of photons in a material).

In these crystals, the index varies with angle, so that you can find the right index for the output photons at a fixed angle with respect to an axis through the crystal.  If you align the crystal properly, you get a cone of output photons at a fixed angle--which means a fixed index. 

Index also varies with polarization state, so if you have different polarizations of output photons, you get two output cones.

That's interesting. Then they are transported down a vacuum tube. Then observed, and then the equality is supposed to remain so that 1 + -1 =0. Which gives you two opposite spins. Is that right?

[JP]

Not quite.  The reason for 2 opposite spins is the laws of quantum mechanics, which says that the state at your detector is a combination of both possible sets of opposite spins.  It isn't as simple as 1+-1=0.

[Pincho (OP)]

Not quite.  The reason for 2 opposite spins is the laws of quantum mechanics, which says that the state at your detector is a combination of both possible sets of opposite spins.  It isn't as simple as 1+-1=0.

The third law doesn't apply? Or it applies at a distance? Or somebody doesn't know how it applies to a photon... how can it store its momentum from the crystal?

[Pmb]

I can solve it in less than 1 week, I promise you. I just need the details, and there could be a hidden variable in Bell's inequality (there is definitely hidden information in this experiment).

What in the world gives you the idea that you can solve it in less than a week? What details are you talking about? If there reallly were hidden variables then in principle electrons would have classical trajectories moving around inside of atoms and in doing so they'd be accelerating and giving off radiation and therefore energy and then atoms would soon collapse.

[Pincho (OP)]

I can solve it in less than 1 week, I promise you. I just need the details, and there could be a hidden variable in Bell's inequality (there is definitely hidden information in this experiment).

What in the world gives you the idea that you can solve it in less than a week? What details are you talking about? If there reallly were hidden variables then in principle electrons would have classical trajectories moving around inside of atoms and in doing so they'd be accelerating and giving off radiation and therefore energy and then atoms would soon collapse.

No they wouldn't. Only in the standard model which still includes some things from 1720. For example. If you put an asteroid in space, and give it a push, how does it know which direction it is going in, and at what speed?

[JP]

Not quite.  The reason for 2 opposite spins is the laws of quantum mechanics, which says that the state at your detector is a combination of both possible sets of opposite spins.  It isn't as simple as 1+-1=0.

The third law doesn't apply? Or it applies at a distance? Or somebody doesn't know how it applies to a photon... how can it store its momentum from the crystal?

No momentum is stored in the crystal.  Photon momentum in = sum of photon momentum out.  I'm not sure where you're getting spin and 1-1=0 from.

[Pincho (OP)]

Not quite.  The reason for 2 opposite spins is the laws of quantum mechanics, which says that the state at your detector is a combination of both possible sets of opposite spins.  It isn't as simple as 1+-1=0.

The third law doesn't apply? Or it applies at a distance? Or somebody doesn't know how it applies to a photon... how can it store its momentum from the crystal?

No momentum is stored in the crystal.  Photon momentum in = sum of photon momentum out.  I'm not sure where you're getting spin and 1-1=0 from.

I didn't write the same as your post. I said that the photon can store momentum, and I said that Newton's 3rd law says that 1 + -1 = 0. Energy cancels out, so two opposite spins cancel each other out. Anyway you answered it correctly the first time so lets just go with your first reply.

[JP]

Energy cancels out, so two opposite spins cancel each other out.

That's not true.  The spins cancel out because you chose two photons of opposite spin.  You could choose two photons of the same spin. 

I realize you're trying to develop a theory to explain all this, but you're getting very basic physics wrong here.  I'd strongly recommend going back to basics and learning about conservation of energy and momentum before jumping in to trying to explain quantum mechanical phenomena.

[evan_au]

Spontaneous parametric down-conversion has a very low efficiency in converting incoming photons to entangled pairs - around 1 in a trillion (1 in 10¹²).

You can improve the % of entangled photons by:

• Ignoring the central beam, ie those photons that go straight through with no deflection
• Ignoring the entire outgoing light cones, and just collecting photons from the region where the cones overlap. This is where the entangled photons are concentrated
• So current detection schemes have to cope with a few entangled photons embedded in a stream made primarily of non-entangled photons
• ...and detectors can't reliably detect 100% of the incoming photons, either.
• Some form of feedback to the receiver is typically used to identify which photons they can use for the quantum communication. With current success rates on entangled communications, this back-channel typically uses non-entangled communication.
• With these very low rates of entanglement and the need for a back-channel, the useful data rate is very low, more useful for encrypting secret keys (which are then used for encrypting a longer message), rather than for encrypting the entire message itself.
• Understandably, scientists and the military are looking for more efficient generators, detectors and transmission systems for entangled photons..

Transporting entangled photons can be done:

• Through the air (the current published record is 144km, achieved in 2010)
• Through optical fibre
• But most successfully through a vacuum, since any interaction with a dust particle in the air, or a scattering event in a fibre will cause the two photons to lose entanglement before they reach the detector

[Pincho (OP)]

Energy cancels out, so two opposite spins cancel each other out.

That's not true.  The spins cancel out because you chose two photons of opposite spin.  You could choose two photons of the same spin. 

I realize you're trying to develop a theory to explain all this, but you're getting very basic physics wrong here.  I'd strongly recommend going back to basics and learning about conservation of energy and momentum before jumping in to trying to explain quantum mechanical phenomena.

Newton's third Law has to be part of the setup. How is it not? If you have two photons with the same spin the opposite energy has to be expelled twice, that's the third law? It goes somewhere. Can we just forget this, I can't post my answer anyway so it's a waste of time.

No momentum is stored in the crystal.

I think your reply needed a comma.

[yor_on]

The idea is that you get photons of a opposite spin, but you might have meant that Pincho. From a point of symmetry it should be a symmetric arrangement, and the spin taking itself out in the 'photon' getting down converted to two, maybe? As far as I understand you can't get two down converted photons of a same spin? Can you? Whenever you find them to have the same spin then you can be assured, as I get it, that you are measuring on two uncorrelated photons as the definition should be that you by measuring one set the other state (polarization/spin) But it is a really deep subject, and you need to start from the beginning to understand how they have thought there. After that you're free to smash it to smithereens with logic, and mathematics

EPR Paradox.

And this guy too, he have a very clear mind, at least as I found reading him. Einstein on the Completeness of Quantum Theory.

[Pincho (OP)]

Being as my reply swings away from the standard model, I have posted it here...

http://www.thenakedscientists.com/forum/index.php?topic=47087.new#new

If you like Columbo, there is an episode where a young boy explains to Columbo how to solve a magic trick. He says something like....

"You remember that it's a trick, and so long as you remember that it's a trick you can figure out how it's done."

So remember that there is no Action At A Distance, and solve the illusion.