
Two-Way Communication Is Possible with a Single Quantum Particle - IntronExon
https://www.sciencenews.org/article/two-way-communication-possible-single-quantum-particle?tgt=nr
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yodon
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.

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cryptonector
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
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.

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

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randomerr
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.

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

~~~
sometimesijust
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
... 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.

