Two-Way Communication Is Possible with a Single Quantum Particle | Hacker News Reader

Two-Way Communication Is Possible with a Single Quantum Particle

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30 pointsIntronExon11 comments

This is really fun to think about from an FTL communications standpoint.

The experiment involves thinking of the photon as a wave interfering with itself between two experimental stations. The experimenters can adjust the phase of the waveform locally to determin where the particle manifests. The changes they make locally to the phase of the waveform should propagate towards the other experimenter at the speed of light because the waveform is an electromagnetic wave (I suspect this manifests as a slightly higher or lower energy photon briefly as the change in phase propagates because the phase change region is essentially a brief bit of shorter or longer wavelength waveform).

The experimenters can certainly each measure the photon’s position before the speed of light would allow for conventional information propagation (nothing stops them from doing so), but they won’t get the other party’s message if they do because the phase change won’t have propagated to their end yet.

Ok, so far so good - we’re safe, no FTL communications. But we just concluded that these two measurements are made on one photon and are essentially decoupled because the E/M wave hasn’t propagated the phase change yet. That means there could be scenarios where both experimenters get the photon or neither experimenter gets the photon. But there is only one photon and only enough energy for one photon (we change the phase slowly enough to make sure we don’t add a second photon’s worth of energy). So we’re cool on FTL but just violated conservation of energy, which is also kind of a big no no. It seems like you can either conserve energy or FTL but not both.

I’m sure this will all get worked out and submitted to Physical Review Letters and odds are good both FTL and conservation of energy will be safe from disruption, but it’s definitely fun to think about.

Conservation of energy is hard to think about in an infinite, expanding universe that has "weird" quantum effects. Apparent local violations of conservation of energy may not be meaningful.

In physics, local violation of conservation of energy due to quantum effects is pretty established by now. Unfortunately, these effects don't scale, that is, in larger scale conservation of energy does hold. So alas, you can't use these effects to make a power generator.

There is no FTL or energy conservation problem with the theoretical or the experimental experiment.

They explicitly ask for a clear path between A and B after they choose which message to send, so the "signal" can travel from A to B and from B to A. And they explicitly ask for enough time. So there are no time paradox or break of energy conservation problems.

The weird part is that in the experiment they use a single photon instead of two photons as usual. It's weird because you expect that it's clear if the photon is traveling from A to B or from B to A, but there are very similar experiments with the same problem.

For example there is the classic double slit experiment, where you send light through two slits and see the interference pattern. The weird part is that you can repeat the experiment using a very low power system that guaranty that you are using a single photon each time. So for each photon you have the question: Did it pass though the upper left slit or the right slit? The answer is that the question makes no sense in quantum mechanics. A popular answer is to consider that the photon passed through both slits, this is an useful idea in the calculations but it's misleading and prone to weird interpretations.

More info: https://en.wikipedia.org/wiki/Double-slit_experiment#Single_...

So in the double-slit single-photon it's not clear if the photon pass though one slit of the other, and in this experiment it's not clear if the photon goes from A to B or B to A. In both cases, the answer is that it's difficult to explain the quantum mechanics effect accurately with a good classical analogy.

<Edit: probably wrong> First they can use a single photon to send a message(bit) from Alice to Bob or from Bob to Alice, they can't use a single photon to send a message form Alice to Bob and from Bob to Alice. [note: exclusive or] </probably wrong>

Also, the article is not very clear, but IIUC after one of Alice or Bob decide to send a message to the other one, they must have a clear path to "send" the entangled-modified-photon to the other. (For example, if Alice is in submarine in a cave at the bottom of the sea, and Bob is in the surface, and there are 1 mile of rocks, mud, sand and dirty water, and they can't communicate in any direction, even if they have a magic box with the trapped entangled photon.)

But the quantum hype train must be fueled!

AFAIK the only value of quantum communications seems to be that it can provide a tap-proof channel to send one bit of information per entangled photon to a receiver, but only if they have already established the entanglement and have established the timing of the "quantum message" transmission.

This new discovery suggests that the two entangled parties can both encode those qubits with a message and both receive the other party's message before losing the entanglement?

What am i missing here?

There is no entanglement, it is a single photon. This appears to be a rare event of quantum physics be accurately reported.

Single wire communication does the same thing. The best example is the telegraph. You have a start, body and end protocol for your message on both ends. The quantum particle replaces the wire. Setting up the protocol and consistently pulsing that particle is the hard part.

I thought something like this was supposed to be impossible because it would allow FTL communication? Or is the communication at light speed?

The photon travels the full distance between Alice and Bob to interfere with itself and be detected. So not faster than light. You may be confusing this with entangled photons which this would also probably work for although this shows you only need the one photon so entangling multiple photons would be a waste of time.

... you’re right. I expected them to use entangled photons (which would have achieved the same but without the delay) and that shaped how I read the article.

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