View Full Version : Quantum teleportation

AriAstronomer

2010-Nov-21, 12:11 AM

Hey everyone,

So I'm not sure if everyone is familiar with quantum teleportation, but in essence how it works is as follows: "The Caltech experiment involved three photons (labeled A, B and C) in which the latter two are entangled. The scientists extracted some information from photon A, and the remaining information is passed to photon B through entanglement and then, from B to C. The information from photon A is therefore passed to photon C which replicates A by combining information from B. In the process, photon A is unalterably changed and disappears." (from http://www.tech-faq.com/quantum-teleportation.html).

However, a lecture I heard by The Teaching Company on quantum entanglement and cloning said that information copying is impossible, for if the exact replication of a particle was possible, FTL communication would become possible. Who is right?? I can narrate the exact experiment proving that quantum cloning is impossible if people are interested...

AriAstronomer

2010-Nov-21, 12:23 AM

I figured to be thorough I'll put the experiment as to why quantum cloning (i.e. copying) is impossible:

Firstly to explain diagonal filters: If a photon is vertically or horizontally polarized, when passed through a diagonal filter (only permitting diagonal polarization through), half the time the photon will go through, and half the time rejected (i.e. Half the time the photon will become diagonally polarized and pass through the filter, and half the time it will be rejected).

Now to the experiment:

Let's say I have two entangled photons, and send one of them to one end of the milky way galaxy where Alice is waiting with a detector, and one to the other side where Bob is waiting with a detector. If Bob has a cloning device, he can clone his incoming photon 100 times if he wishes before it hits his detector.

Situation 1: If Alice has a diagonal filter and Bob has a horizontal filter:

1) If the photon makes it through Alice's diagonal filter it will be diagonally (45°) polarized, and Bob must therefore have 100 anti-diagonally (-45°) polarized photons. If he has a horizontal filter, half will go through, half will be rejected.

2) If the photon does not make it through Alice's filter it will be anti-diagonally (-45°) polarized, and Bob will therefore have 100 diagonally (45°) polarized photons. If he has a horizontal filter, again, half will go through, half will be rejected.

Situation 2: If Alice has a vertical filter and Bob has a horizontal filter:

1) If the photon makes it through Alice's filter, it will be vertically polarized, and Bob will have 100 horizontally polarized photons. If he has a horizontal filter, all will go through.

2) If the photon doesn’t make it through Alice’s filter it will be horizontally polarized, and Bob will have 100 vertically polarized photons. If he has a horizontal filter, none will go through.

Alice now has control of the situation, and can send messages to Bob. If she has a diagonal polarizer, Bob will receive 50% of the light every time. If she has a vertical polarizer, Bob will either get all of the light, or none of the light every time. She could therefore send yes/no messages, based on Bob receiving 1) all/none of the photons, or 2) 50% of the photons.

The only way to make FTL communication impossible is if quantum cloning is in itself impossible, and at the quantum level, particles cannot be perfectly replicated. How then is quantum teleportation possible?

cjameshuff

2010-Nov-21, 01:47 AM

However, a lecture I heard by The Teaching Company on quantum entanglement and cloning said that information copying is impossible, for if the exact replication of a particle was possible, FTL communication would become possible. Who is right?? I can narrate the exact experiment proving that quantum cloning is impossible if people are interested...

Might be helpful to cite the exact lecture, where you heard the claim, etc. Cloning is possible, but does not make FTL communication possible...it's possible this was garbled at some point along the line.

Your thought experiment first fails at "he can clone his incoming photon 100 times". He can do so, but he'll only end up with a single 100th generation copy. As the description of the Caltech experiment says, quantum cloning is destructive to the original.

For the rest, I'm unclear as to how Alice's filter is supposed to send a signal via entanglement. It seems like a roundabout way of expressing the misconception that entanglement means you can wiggle one particle one way and observe it on the other side: there seems to be an assumption that the filter can set the polarization of both halves of the pair without disturbing the entanglement, when it can actually only observe the polarization. If a photon makes it through her diagonal filter, Bob did not necessarily receive an antidiagonal photon, it could have been a vertical or horizontal one, or none at all. If it does not make it through her diagonal filter, Bob may have gotten a horizontal, vertical, or diagonal one, or none at all. Similar if she makes her filter diagonal or horizontal.

No matter what orientation the filter is in, there's 2 cases where a photon has a 50% chance of getting through, 1 where the photon will always get through, and 1 where it will never get through, on either side. Alice's observations of the entangled photons she receives would prove to be consistent with those that Bob receives when compared afterward, but there's no way to do such a comparison without communicating information through some other channel. She can flip the filter around as much as she likes, Bob won't be able to tell.

EigenState

2010-Nov-21, 02:00 AM

Greetings,

To quote the link within the original post, with emphasis added:

In 1998, teams from Caltech and Europe were able to successfully teleport a photon: reading the atomic structure of the photon ...

Given that statement, it is difficult to lend any credence to that specific article. Perhaps links to the real research papers could be provided.

Best regards,

EigenState

a1call

2010-Nov-21, 02:10 AM

Hi AriAstronomer,

I raised a somewhat similar question here (http://www.bautforum.com/showthread.php/109422-The-Actuality-Of-The-Past?p=1816991#post1816991http://).

It seems that there is a misleading language involved here. The article that you link to fails to mention that the entangled photons are a rare subset of a pool of un-entangled photons. Because of this, the only way to determine the effect of one on the other is through the "noise" elimination (disregarding the un-entangled portion of the photons) by using a coincidence counter which means the information of the influence (spooky action at a distance) is only available after the received photon pairs are acknowledged at a central point where the coincidence counter is located without which the noise of un-entangled photons will make the signal of the entangled ones obscure.

So Entanglement by itself can not be used to achieve FTL. However teleportation aka quantum tunneling, AFAIK is a FTL (instantaneous) phenomenon. So theoretically it can be used for FTL, but it was originally described as effective across microscopic distances/barrier only. Then it came the equivalence of tunneling and Evanescent wave coupling (http://www.witricpower.com/wireless-energy-transfer/5-moderate-distance-methods/5-1-evanescent-wave-coupling.htm).

You can see the board responses to the issue in this thread (http://www.bautforum.com/showthread.php/81328-Regarding-Quantum-Tunneling):

tommac

2010-Nov-21, 03:01 AM

From what I understand the information can be passed but there would be no way for the observer to distiguish the information from the normal state of affairs.

So yes they are entangled the data gets passed. But there would be no way to interpret it as a signal.

AriAstronomer

2010-Nov-21, 04:54 PM

Might be helpful to cite the exact lecture, where you heard the claim, etc. Cloning is possible, but does not make FTL communication possible...it's possible this was garbled at some point along the line.

Your thought experiment first fails at "he can clone his incoming photon 100 times". He can do so, but he'll only end up with a single 100th generation copy. As the description of the Caltech experiment says, quantum cloning is destructive to the original.

For the rest, I'm unclear as to how Alice's filter is supposed to send a signal via entanglement. It seems like a roundabout way of expressing the misconception that entanglement means you can wiggle one particle one way and observe it on the other side: there seems to be an assumption that the filter can set the polarization of both halves of the pair without disturbing the entanglement, when it can actually only observe the polarization. If a photon makes it through her diagonal filter, Bob did not necessarily receive an antidiagonal photon, it could have been a vertical or horizontal one, or none at all. If it does not make it through her diagonal filter, Bob may have gotten a horizontal, vertical, or diagonal one, or none at all. Similar if she makes her filter diagonal or horizontal.

No matter what orientation the filter is in, there's 2 cases where a photon has a 50% chance of getting through, 1 where the photon will always get through, and 1 where it will never get through, on either side. Alice's observations of the entangled photons she receives would prove to be consistent with those that Bob receives when compared afterward, but there's no way to do such a comparison without communicating information through some other channel. She can flip the filter around as much as she likes, Bob won't be able to tell.

The lecture is from 'The Teaching Company' - Impossible - Physics Beyond the Edge, Lecture 21 - Entanglement and Quantum Cloning.

Although there is no transcript, I have dictated the experiment exactly as it has been stated in the lecture, so if what you claim is true, then the narrator (Benjamin Schumacher), has screwed up. I feel like he hasn't, because of what he has done shown here: http://en.wikipedia.org/wiki/Benjamin_Schumacher

The way it is really stated is how I have written it. Schumacher was saying if Alice has a diagonal filter, and the photon makes it through, she therefore has a diagonal polarized photon (45 degrees), and then Bob would therefore have an anti diagonal (-45 degrees) photon. If he then copied this photon 100 times and passed it through his horizontal filter, half would make it through, and half would not. Similarly if Alice has a vertical filter and the photon makes it through, Bob's photon is therefore horizontal, and all 100 of his now copied photons will make it through his horizontal filter. That's really how he states it. Maybe he did make a mistake, but then he makes this mistake on a recorded and distributed lecture.

The conclusion they draw from this is that perfect copying of a photon is impossible, or else this thought experiment would be possible. Maybe that simply means you cannot copy a photon and leave the other one intact, that wasn't explicitly stated. Upon further thinking, I guess that's the only thing that can resolve my question, leaving teleportation possible and not contradicting Schumacher.

Therefore, if B and C are entangled particles, quantum teleportation operates by measuring particle A (and destroying it's wavefunction), and then applying this information *somehow* on an entangled particle B, instantly changing C? Is there a reliable source perhaps online that isn't too mathy that I could follow? I'm an undergrad student, and have as far as my vector calculus. I'm taking qm right now, but it obviously wouldn't cover advanced topics in detail like the physics behind quantum teleportation (I don't think, anyway).

tommac

2010-Nov-25, 01:57 AM

How would you be able to detect that something was manipulated from a distance?

The lecture is from 'The Teaching Company' - Impossible - Physics Beyond the Edge, Lecture 21 - Entanglement and Quantum Cloning.

Although there is no transcript, I have dictated the experiment exactly as it has been stated in the lecture, so if what you claim is true, then the narrator (Benjamin Schumacher), has screwed up. I feel like he hasn't, because of what he has done shown here: http://en.wikipedia.org/wiki/Benjamin_Schumacher

The way it is really stated is how I have written it. Schumacher was saying if Alice has a diagonal filter, and the photon makes it through, she therefore has a diagonal polarized photon (45 degrees), and then Bob would therefore have an anti diagonal (-45 degrees) photon. If he then copied this photon 100 times and passed it through his horizontal filter, half would make it through, and half would not. Similarly if Alice has a vertical filter and the photon makes it through, Bob's photon is therefore horizontal, and all 100 of his now copied photons will make it through his horizontal filter. That's really how he states it. Maybe he did make a mistake, but then he makes this mistake on a recorded and distributed lecture.

The conclusion they draw from this is that perfect copying of a photon is impossible, or else this thought experiment would be possible. Maybe that simply means you cannot copy a photon and leave the other one intact, that wasn't explicitly stated. Upon further thinking, I guess that's the only thing that can resolve my question, leaving teleportation possible and not contradicting Schumacher.

Therefore, if B and C are entangled particles, quantum teleportation operates by measuring particle A (and destroying it's wavefunction), and then applying this information *somehow* on an entangled particle B, instantly changing C? Is there a reliable source perhaps online that isn't too mathy that I could follow? I'm an undergrad student, and have as far as my vector calculus. I'm taking qm right now, but it obviously wouldn't cover advanced topics in detail like the physics behind quantum teleportation (I don't think, anyway).

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