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Two-Way Communication Is Possible with a Single Quantum Particle

30 points by IntronExon 9y ago

randomerr 9y ago
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.
gus_massa 9y ago
<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.)
- nick_ 9y ago
  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?
  - sometimesijust 9y ago
    There is no entanglement, it is a single photon. This appears to be a rare event of quantum physics be accurately reported.
croddin 9y ago
I thought something like this was supposed to be impossible because it would allow FTL communication? Or is the communication at light speed?
- sometimesijust 9y ago
  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.
  - ArneBab 9y ago
    ... 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.
yodon 9y ago
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.
- cryptonector 9y ago
  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.
  - vog 9y ago
    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.

30 points by IntronExon 9y ago

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.

gus_massa 9y ago

<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.)

nick_ 9y ago
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?
- sometimesijust 9y ago
  There is no entanglement, it is a single photon. This appears to be a rare event of quantum physics be accurately reported.

croddin 9y ago

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

sometimesijust 9y ago
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.
- ArneBab 9y ago
  ... 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.

yodon 9y ago

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.

cryptonector 9y ago
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.
- vog 9y ago
  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.