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Quantum Entanglement: Spooky Action or Incomplete Info?

Quantum Entanglement: Spooky Action or Incomplete Info?

Postby EthanNAustin » Wed Jul 12, 2017 1:30 am

First want to apologize if this kind of question has been asked before.
I searched the forum a couldn't find what I was looking for, so I thought I would ask.

Below is what I've been able to comprehend about Quantum Entanglement with a photon.

Quantum Entanglement with photon is created by shooting a laser into a crystal.
That crystal splits the photon's Quantum wave energy in two.
Creating 2 photons at half the energy of the original photon.
Each Photon is the opposite of the other photon.
One photon is up and the other photon is down.
We do not know which is up or which is down unless one of photons is measured.
We measure one of the two "entangled" photons.
After measuring one, we instantly know the up or down state of the other photon.

Is my explanation correct?
EthanNAustin
 
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Re: Quantum Entanglement: Spooky Action or Incomplete Info?

Postby Good Elf » Thu Jul 13, 2017 6:27 am

Hi Ethan,

Welcome!
EthanNAustin wrote:First want to apologize if this kind of question has been asked before.
I searched the forum a couldn't find what I was looking for, so I thought I would ask.

Below is what I've been able to comprehend about Quantum Entanglement with a photon.

Quantum Entanglement with photon is created by shooting a laser into a crystal.
That crystal splits the photon's Quantum wave energy in two.
Creating 2 photons at half the energy of the original photon.
Each Photon is the opposite of the other photon.
One photon is up and the other photon is down.
We do not know which is up or which is down unless one of photons is measured.
We measure one of the two "entangled" photons.
After measuring one, we instantly know the up or down state of the other photon.

Is my explanation correct?
It is very reasonable what you have said. A lot has been said recently about quantum entanglement and also quantum teleportation. I get a little "picky" about these points because of the fog of misunderstanding that bedevil these matters. So I hope you will not think of this as a criticism, only I add these points for completeness so we really understand each other... OK? It is also for the benefit of others who may also be reading too.

While the original photon is truly "split" using that BBO crystal.... it is not converted at that instant into "up and down" photons. A lot of testing has gone into this point and it is crucial to understanding this process. In fact the most important point to understand is while there are "two photons" there are not two wavefunctions... just one wavefunction existing in two separate places, This is called quantum non-locality, something we are not usually familiar with when speaking of "common sense" interpretations of things that are not on the quantum scale. The two photons are in a superposition of states. If you were to measure the current state of one of those photons... then and only then do both photons become either an "up" or "down" photon.... what is "up or down" depends on the property being measured... it must be a property that is capable of being in a superposition of quantum states such as polarization or spin. Once one state is measured, the other state only then becomes it's complement. No matter how far the photons are apart when the measurement is made, the state of the second entangled photon is decided for all subsequent time. Up till that time the state of either photon is a totally random variable and cannot be "forced" to be one or the other state. After that time it becomes a known state. It is like putting a stick through a hole in the wall, pushing on one end of the stick into the wall causes the other end of the stick to move out of the other side of the wall. The only odd feature of this event is these two sides of the wall could be on the opposite sides of the Universe and this outcome would be the same and simultaneous. The "stick" is not moving faster than the speed of light or anything like that, it's just a very funny "hole"... OK? It is like those Loony Tune cartoons where Elmer Fudd, all dressed up in his "Wabbit" Hunting Gear, pokes the barrel of his shotgun down Bugs Bunny's rabbit hole and Fudd finds what appears to be a very similar barrel from an almost identical gun, poking up his b**t from a similar "wabbit hole" from behind. Jiggling his end of the gun in and out of the rabbit hole seems to indicate the barrel of the gun from behind is also his gun, the one and the same gun. But it is so peculiar or freaky that Fudd just does not buy into it and he pulls the trigger, with obviously hilarious consequences... well that is "quantum mechanics" and quantum entanglement. There is one limitation to this scheme, no information can be sent using "pure" quantum entanglement" because you can't fiddle with that state to "work out" the physics behind it. But this is not all that can be done here.

This ensures that no information can be sent by means of "pure" quantum entanglement since the initial measurement cannot force the end state. On the other hand there is a protocol called quantum teleportation. These two photons can be held in a photon trap in different places without reading their state (that last bit is of utmost importance) so that they still both remain in that common state of superposition. A third photon can be prepared in a known state (lets say "up") then the first of the two entangled photons may be allowed to interact with the photon... and that entire initial state is destroyed but in doing so the state of the prepared photon is instantly transferred to the second of the two entangled photons, such that it will attain the state of the prepared photon ( a known state). The underlying process is linked to the Quantum Zeno Effect. See this NASA Cartoon illustration following...
Image
Researchers Advance 'Quantum Teleportation' - NASA - 20 Nov 2014
You may notice a few "weasel words" about sending two analog bits of information. You don't "absolutely" need to send these bits after the teleportation event. Actually this is simply the orientation of the photons in relation to each other relative to their respective apparatus... for example UP and DOWN and LEFT and RIGHT... are two analog bits defined on a common benchtop. You don't need to send this information strictly "after" the teleportation event, you can send or just "hold" this information before the event... it is the same two bits all the time whenever you measured them as long as the two photons derived from the same resource remain in the same geometric configuration (or at least you can work it out if they are, more generally, in some relative motion). You don't need to send it over an optical fiber since there are so many ways to send information nowadays... you could as easily just write that information down on paper... OK? Once Bob interprets this information correctly, along with his reading then it tells him the way to interpret information Alice has sent him.

Interestingly it is theoretically possible to send unlimited amounts of information using only two photon qubits as a resource, provided the entanglement is anchored in some "firm" store for these qubits. What is needed is advanced technology to only weakly measure these "memories" and reuse then by occasionally resetting them.
PC Mag wrote:Dutch Scientists Achieve Quantum Teleportation Breakthrough
Direct Information Transfer over a distance of 3 meters using a qubit dislocation in diamonds - Delft University.jpg
[click to enlarge] Dutch Scientists Achieve Quantum Teleportation Breakthrough
By Damon Poeter 31 May 2014, 5:47 p.m.
Direct Information Transfer over a distance of 3 meters using a qubit dislocation in diamonds - Delft University.jpg (168.19 KiB) Viewed 118 times

https://uk.pcmag.com/news/32803/dutch-scientists-achieve-quantum-teleportation-breakthrough
Scientists at the Delft University of Technology's Kavli Institute of Nanoscience managed to transfer information contained in one quantum bit, or qubit, to another "entangled" qubit about 10 feet away "without the information having travelled through the intervening space," Prof. Ronald Hanson, head of the research group wrote in a summary.

In other words, as Hanson noted, "teleportation."

The Dutch researchers reported that there was no degradation in the information transferred between the two quantum bits, potentially a major advance in the pursuit of quantum teleportation. In the past several years, other scientists around the world have devised ways to better measure the information contained in qubits without altering them in the process and managed to teleport information "instantaneously" across distances as great as 90 miles. ... read more at link above
The subtlety is the teleported information does not travel through the intervening space so a whole series of messages could (in theory) be "sent" without anything passing through the space between the source and destination (shown as two diamonds here) and therefore could not even be intercepted, or for that matter... blocked or jammed... a truly disruptive technology. It is all connected to the radical nature of the special "hole". Perfect communication security without even the slightest hint of even sending anything at all. On the other hand knowing this quantum process and utilizing it, it becomes possible to also attain information from places where no signals have ever gone. The problem reduces to distributing this "resources" and making it stable for extended periods. The resource is simply two entangled photons that are sent to these two locations but of themselves contain no special information. There are even ways to ensure that the two photons are linked without actually "reading" them in any way called "entanglement witnesses". This, of course, opens up amazing new fields for future technologies.

Here is a youtube video about quantum entanglement (not quantum teleportation) that may assist you or other readers to understand...
Quantum Entanglement & Spooky Action at a Distance lifted from this reference:
Physicists May Have Discovered One of the Missing Pieces of Quantum Theory Futurism - Big Think - 06 Jul 2017

Cheers
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PS: Sorry for being "so picky" but I am sure others would want to know too, but it was an excellent question.
Aa' menle nauva calen ar' ta hwesta e' ale'quenle
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