Down convertor 2 photon entanglement - explanation?

  • #1
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20

Main Question or Discussion Point

Just watched many times (specifically @ 20:13 - 25:04) Ron Garret's 'Google Tech Talk'


All was going well - the 2 photon experiment & concept visualised - only that the finality of comprehension wasn't delivered...... perhaps because Ron was merely using this for his own purpose.

This seems a shame because it looks to offer a very understandable explanation of entanglement.
So...... have I understood this correctly?

.... a UV photon enters the crystal, excites an electron to jump..... as it falls back down, during the descent, at:

Point 'x' a photon is emitted

a short time later, the electron descends further to:
Point 'y' a second photon is emitted.

I get this.... it sounds similar to night vision scopes (but probably isn't), but I get how the two photons are produced, and of course, I believed it when he states that these are somehow paired up - presumably entangled.
Sadly he then began to ramble on.... but my interpretation is that we now have:

the 'x' produced photon, and the 'y' produced photon, head off on two different bearings.
One photon (say x) heading leftwards...... the other photon (say y) heading rightwards.


Have I understood correctly?

If we place a photon detector in the path of (say) the left photon (x), and absorb it....... the right photon (y) instantaneously either:

ceases to exist
or
changes it's state

The concept being that, even if the two photons are separated by a million kilometres, the right photon will change instantly ie. indicating faster than light transmission of data.


This last element is stated clearly, and is apparently incontrovertible, only....
..... Ron then again rambles on about communicating through time, so this is impossible etc etc.
The explanation ends @ 25:04

Note: Ron points out that this can't be used as a means of communication due to randomness, but the explanation 'fizzles out' with: the supposed detector guy in space having seen a photon, and we've seen a photon, but we've not transmitted any information and.......................... (ha ha ha, nudge nudge, wink wink)

The final remarks are 'kind of' thrown away with a shrug, like as if it is anyway obvious.
Okay... so I believe him.... we can't use this as a means of transmitting data.
Only that the rambling closure to this explanation clouds the concept of something physically happening:

Two photons, created as described, can influence each other instantaneously regardless of distance.

Is this the case, as a physical fact?
---------------

Thoughts:
If this IS the case, it follows, as a physical fact, that there is a 'data comms connection' between x & y photons.

That being a comms connection of type.........

I think I'll stop there and stick to what we do know :nb)
 
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Answers and Replies

  • #2
DrChinese
Science Advisor
Gold Member
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It is true that down conversion can produce entangled photon pairs, and their entanglement is not affected by distance (separation).

You could say that there is an instantaneous influence, but no one can really say what the nature of that influence is. As far as anyone knows, there is no transmission of any useful information as part of the process of disentangling 2 such photons.

As to electrons absorbing/re-emitting photons: that is generally NOT an explanation for down conversion. This is a complex process, and is essentially the same as time-reversed up conversion. I would either recommend the black box approach (1 in, 2 out that meet certain conservation criteria and otherwise forget the details); or learn a whole lot about field theory (QFT).

Here is something that may help a bit regarding SPDC, if you are hungry for a bit more:

http://arxiv.org/pdf/quant-ph/0103168v1.pdf
 
  • #3
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20
Thank you for that confirmation.

I take en board your comment re the photon production mechanism.
I personally have no gripe with simplification of non-pertinent elements.... in this case, that would be:
"We have a mechanism that ejects two photons that are entangled."

The question, I believe, is whether the simplification is 'detailed to the point of outlining the process incorrectly'.
IE. Do I repeat to my son, the 'excited electron explanation' (or state the 'unexplained paired photon ejection mechanism')?
The problem being that, he in turn, will recount this explanation to his friends..... who will in turn recount......

Ron has got over 1m views, and 5k likes (1k don't like it)
Certainly influential at a general knowledge level.

I did check out http://arxiv.org/pdf/quant-ph/0103168v1.pdf
To be fair.... it's for another day.
When I climb a mountain, I like to hang around for a while, and enjoy the view.
The key for me was to grasp the unimaginable.... that a photon could influence another photon, regardless of distance.

I do accept your statement:

As far as anyone knows, there is no transmission of any useful information as part of the process of disentangling 2 such photons.
But my goodness, this statement is pregnant!

The very fact that disentanglement occurs, instantaneously, at any distance.
A particle can immediately influence another particle that might be 100 light years apart.

You confirm that this is true.
In which case..... communication outside the realm of space and time exists.
 
  • #4
berkeman
Mentor
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Thread closed for Moderation...

Edit: it will remain closed.
 
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