Delayed choice experiment and FTL communication

[cala]

I've been thinking about a communication system that could allow faster than light communication. The basic principle that would make this system work come from the double-slit experiment and Wheleer's delayed choice experiment, taking advantage of the different patterns that would appear on a double-slit experiment if both slits are open (wave interference) or only one slit is open (particles).

• The double-slit experiment is closely related to wave-particle duality. Basically, it shows that the behaviour of quantum objects (photons, electrons, protons and even molecules) depends on the methods used to measure them. If we have a source of quantum objects, and put two very close slits on their way, they end creating an interference pattern when detected on a screen (even if the quanta arrive one at a time). This means they behaved as waves. But if we close one slit, then the pattern changes, interference dissapears, and they behave as particles.
• The Wheeler's delayed choice experiment states that we can not trick quantum objects "in flight" to be waves or particles before really detecting them, no matter what we do to the setup or how many times we change the number of slits until the final detection.

So the presence of one or two slits at detection time will decide if we end up detecting a particle or a wave pattern. We could say that all the detection screen impacts are instantaneously correlated with the presence of one or two open slits on the system, irrespective of distance.

The whole communication system (transmitter, channel and receiver) can be seen as a very big, elongated Mach-Zehnder interferometer.

The transmitter (Tx) is composed of:

• A coherent light source that will work continuously, creating the communication channel.
• A beam splitter that splits the light from the source into two perpendicular paths.
• A mirror at 45º that redirects light from one path, so both paths get parallel.
• An optical switch that enables or disables one of the parallel light paths.

The receiver (Rx) is composed of:

• A mirror at 45º that redirects the light on the path that was not redirected before, making both paths orthogonal and equal in length.
• A beam splitter that will receive the light coming from one or both paths.
• Two detectors at the other sides of the beam splitter, that will fire depending on light traveling one path (particles) or two paths (waves).

The two long parallel light paths between transmitter and receiver comprise the communication channel. One of those light paths will be switched on and off using the Tx optical switch:

• When one light path gets disabled, light at the receiver is detected as having traveled just the path that is now available, so the photons received must behave as particles. As the Rx beam spliter can equally transmit or reflect particles, both Rx detectors will fire with 50% probability.
• When both light paths are enabled, light is detected as having followed two equal paths, so photons reach the Rx beam splitter and behave as a wave interfering with itself, thus, one Rx detector will fire all the time (100%) while the other won't fire at all (0%).

Theoretically, there is no problem assuming a very long distance between transmitter and receiver, but practically, we must achieve a great level of accuracy placing all the elements for the system to work as intended. Also, when the transmitter light source is started for the first time we have to wait for the light to reach the receiver to stablish the communication channel, but once the channel is set, we have a way to know the transmitter state almost instantaneously at the receiver, irrespective of Tx/Rx distance.

The correlation between Tx switch and Rx beam splitter is, in fact, instantaneous, but as we have to wait for some statistics to build up on the detectors (to distinguish the states correctly), communication between Tx and Rx can not be instantaneous, but it can be superluminal. This is so because the time to discriminate between the particle/wave pattern depends just on the Tx light source emission speed, not on the actual transmitter-receiver distance. This means the time we have to wait at Rx to discriminate Tx states can be less than the time light really takes going from Tx to Rx. Weird, isn't it?

As an example, imagine the Tx light source fires 100 million photons per second (so each second you could have 100 million hits at the Rx detectors to distinguish between the particle/wave patterns). Suppose you only need 100 hits to decide between states. That means you could change the Tx switch at a microsecond rate and still be able to "decode" Tx switch states on the receiver. So you are able to know Tx states each microsecond... irrespective of Tx/Rx distance! If the transmitter is placed more than 300 meters away from the receiver, we get superluminal information. The actual distance (and time) it takes for the individual photons to reach the receiver is irrelevant, because the information the photons give when detected is encoded in their positions, in the instantaneous correlation between that photon impact and the actual state of the lightpaths present at detection time.

I mean, photons take time at lightspeed to arrive to the receiver, but when they reach the detectors, they give information about the current state of the Tx switch at detection time, not the state the switch had when photons left!

I think this experiment could be tested for real almost inexpensively with technology available today. What do you think?

 

[StevieTNZ]

The transmitter (Tx) is composed of: ...

• A beam splitter that splits the light from the source into two perpendicular paths. ...

The receiver (Rx) is composed of: ...

• A beam splitter that will receive the light coming from one or both paths. ...

I believe that at the beam splitter, the photon is in a superposition of being reflected and transmitted.

 

[cala]

Stevie, are you just being more specific on how the beam splitters work, or do you mean the beam splitters creating superposition states for the photons would not make the system work as expected?

 

[Strilanc]

Blocking or clearing the bottom path won't affect photons that have already passed by the place where you're blocking/clearing. There will be a light-speed delay before you see the detection probabilities switch between 50:50 and 100:0.

 

[DrChinese]

Blocking or clearing the bottom path won't affect photons that have already passed by the place where you're blocking/clearing. There will be a light-speed delay before you see the detection probabilities switch between 50:50 and 100:0.

Echoing what you said to cala. The "signal" moves at c, and is not FTL.

 

[cala]

I think everything depends on the answers to these questions:

• If there are two paths while a photon is "travelling", and we remove one of the paths just before detecting it, will it manifest particle behaviour?
• If there's only one path while the photon is "travelling", and we make another path available just before detecting it, will it manifest interference?

I understand you are saying that once the photon has past the switch, it has already decided between traveling as wave or particle,
but from the Wheeler's delayed choice experiment it's said the photon somehow decides retroactively to be particle or wave to be in synch with Rx beam splitter presence.

So if this FTL experiment doesn't work, then I think it means that this Wheeler's delayed choice experiment interpretation is wrong.
In that case, I think the Rx beam splitter is what allows the photon to express as wave. If it's not there, the potential to be wave is there, but as the element that allows us to get a wave pattern is not present, then the photons show as particles. Is that right?

 

[DrChinese]

So if this FTL experiment doesn't work, then I think it means that this Wheeler's delayed choice experiment interpretation is wrong.

The predictions of Wheeler are of course correct. The interpretation of what is occurring varies. There still is no FTL signalling going on.

 

[Strilanc]

I understand you are saying that once the photon has past the switch, it has already decided between traveling as wave or particle,
but from the Wheeler's delayed choice experiment it's said the photon somehow decides retroactively to be particle or wave to be in synch with Rx beam splitter presence.

No, I am not saying it has already decided to travel as a wave or a particle. Thinking about these experiments in terms of "is it a wave or is it a particle" is completely the wrong way to go about understanding them. Nature doesn't wildly oscillate between using lumps of stuff and using waves of stuff.

The photon is traveling down the paths in superposition. All parts of a superposed photon about to arrive at the destination have passed by the point where you are placing a blocker, and therefore are not affected by your actions.

(Even if the photon was switching from wave to particle, why do you think that would cause an FTL effect? Dipping your toe into the water at the center of a pool doesn't instantly perturb a wave that was about to hit the far end..)

 

[Boog Alou]

Sorry to reply to such an old thread, but I had the same idea as Cala. I was under the impression that the second beam splitter could be inserted into the path (where the beams cross) any time after the photon passed the first beam splitter, but before detection. In his excellent book "Beyond Weird" (2018), Philip Ball describes the results of the delayed choice experiment on page 94:

"The first experimental implementations of Wheeler's arrangement were made in the late 1980s. Many variations have been tried since. They all show that indeed it makes no difference when we intervene, so long as we do so before a measurement is made. Nature always seems to 'know' our intentions."

I also read an article in Scientific American about 15 years ago that seemed to make the same claim: The second beam splitter could be inserted and/or removed at any time (even multiple times) after the photon passed the first beam splitter, but before detection. Only the configuration present at the time of detection determined the result. Unfortunately, I can't search the Scientific American archives, as I don't have access.

Thank you for any further discussion on this topic!

 

[PeroK]

Sorry to reply to such an old thread, but I had the same idea as Cala.

I've read Cala's old posts. I would say he had a fundamental confusion about the nature of wave-particle duality and QM.

Thank you for any further discussion on this topic!

What is your question?

 

[Boog Alou]

PeroK, thank you for your reply. In the previous discussion above, Cala suggests an apparatus that leverages the results of a delayed choice experiment where the second beam splitter is inserted (or removed, depending on the design) just before a photon is to be detected. Someone monitoring the detector that transitions from 0% to 50% (or 50% to 0%) probability of detection would know quickly whether or not the second beam splitter had been added (or removed). If the photon flux was high enough and the distance to the detector far enough, someone monitoring the detector would know the state of the second beam splitter faster than the speed of light.

On 22-Dec-2016, Strilanc's response stated "All parts of a superposed photon about to arrive at the destination have passed by the point where you are placing a blocker, and therefore are not affected by your actions." That statement seems to be the opposite of Ball's (and Wheeler's) conclusion " . . . that indeed it makes no difference when we intervene, so long as we do so before a measurement is made."

Strilanc's and Ball's statements seem to draw opposite conclusions. How do we reconcile them?

Thanks, again!

 

[PeroK]

In the previous discussion above, Cala suggests an apparatus that leverages the results of a delayed choice experiment where the second beam splitter is inserted (or removed, depending on the design) just before a photon is to be detected. Someone monitoring the detector that transitions from 0% to 50% (or 50% to 0%) probability of detection would know quickly whether or not the second beam splitter had been added (or removed). If the photon flux was high enough and the distance to the detector far enough, someone monitoring the detector would know the state of the second beam splitter faster than the speed of light.

One problem with the idea is that if the photon flux is high enough, by the time you insert another device in the set, millions of photon events take place. Unless you can identify individual pairs of entangled photons, then you can't really say what's happening at the level of each individual pair of photons.

Let's try a different idea. We let one photon from an entangled pair be detected, say, while the other is still in the experimental aparatus somewhere. And, we do this with a significant number of photons. Let's say we see an interference pattern build up. Then, we insert a which-way measurement on the other set of photons. In other words, we try to outwit QM by waiting until we see the pattern before deciding what to do with the other set of photons.

It's interesting to think why this wouldn't work. Ultimately, while the pattern is building up, there is growing uncertainty about the which-way information that can be obtained. And, unless you can definitively tie a which-way measurement to a definite entangled pair, then you actually have no useful which-way information.

 

[Boog Alou]

One problem with the idea is that if the photon flux is high enough, by the time you insert another device in the set, millions of photon events take place. Unless you can identify individual pairs of entangled photons, then you can't really say what's happening at the level of each individual pair of photons.

I think it has been proven that you can't send a message using pairs of entangled particles. Instead, we can use a delayed choice apparatus, using single photons whose paths are controlled by the second beam splitter. Perhaps single photons wouldn't be practical for sending a message over long distances, but could be used to prove the concept in a lab experiment.

Let's try a different idea. We let one photon from an entangled pair be detected, say, while the other is still in the experimental aparatus somewhere. And, we do this with a significant number of photons. Let's say we see an interference pattern build up. Then, we insert a which-way measurement on the other set of photons. In other words, we try to outwit QM by waiting until we see the pattern before deciding what to do with the other set of photons.

With a delayed choice apparatus, we can use the second beam splitter to control whether photons take the 0% or 100% path or one of the 50% paths. If we monitor the detector on the expected 0% path and receive a photon (assuming ambient is zero photons), we know instantly the state of the second beam splitter. However, when the second beam splitter changes to the 50% - 50% state, we would need to wait a specific interval in order to determine with reasonable probability that it was in that state.

So, for example, we could define a protocol where we always send groups of 10 photons in a nanosecond while leaving the state of the second beam splitter unchanged. If the apparatus is configured as mentioned in the previous paragraph and we receive no photons at that detector in a nanosecond, then we know the second beam splitter is in the 0% state. If we receive any photons at that detector in a nanosecond, then we know the second beam splitter is in the 50% state. In this example, if the distance to the detector is 10 light nanoseconds and we wait 9 nanoseconds after the 10 photon (1 nanosecond) pulse passes the second beam splitter before toggling the second beam splitter in/or out of the path, then the state of the second beam splitter is known faster than the speed of light.

Yes, the first byte of information would arrive at the speed of light, but all subsequent bytes would only take two nanoseconds regardless of distance. If we send 10 photons every other nanosecond there would be 50 photons always on their way to the detector. The second beam splitter could then be toggled every other nanosecond for continuous communication. Once established, no byte of information would take longer than 2 nanoseconds to arrive at the detector, no matter the distance.

 

[PeterDonis]

I think it has been proven that you can't send a message using pairs of entangled particles.

No, it's been proven that you can't send a message faster than light using entangled particles.

 

[PeterDonis]

I've been thinking about a communication system that could allow faster than light communication.

This is not possible according to QM.

Sorry to reply to such an old thread, but I had the same idea as Cala.

Still not possible according to QM.

Thread closed.

 

[PeterDonis]

The two long parallel light paths between transmitter and receiver comprise the communication channel. One of those light paths will be switched on and off using the Tx optical switch

As a postscript: for a similar scenario and an analysis of why it can't be used for FTL communication (short version: because there will not in fact be any detectable change in the "receiver" channel when you toggle the Tx optical switch--that is what a correct QM analysis shows), see problem 9.6 of Ballentine.