Scientists Achieve Direct Counterfactual Quantum Communication For The First Time (sciencealert.com)

schwit1 shares an article from ScienceAlert: Quantum communication is a strange beast, but one of the weirdest proposed forms of it is called counterfactual communication -- a type of quantum communication where no particles travel between two recipients. Theoretical physicists have long proposed that such a form of communication would be possible, but now, for the first time, researchers have been able to experimentally achieve it -- transferring a black and white bitmap image from one location to another without sending any physical particles... It works based on the fact that, in the quantum world, all light particles can be fully described by wave functions, rather than as particles. So by embedding messages in light the researchers were able to transmit this message without ever directly sending a particle.
It's different than quantum entanglement (which Einstein described as "spooky action at a distance.") The article describes it as "a pretty cool demonstration of just how bizarre and unexplored the quantum world is."

Comments

No FTL from this

[JoshuaZ]

Note that this does not allow for any form of FTL signaling. The No Communication Theorem https://en.wikipedia.org/wiki/No-communication_theorem is not violated.

Re:No FTL from this

[skids]

It's a reasonable cautinary comment to make, obviously based on the observation that some people may assume that FTL limitations are based on the limitations of particle transmission, and lacking any particle transmission, those limits might not come into play. Because not everyone reading this site have taken courses in physics.

An answer

[Okian Warrior]

You can define a particle as something that has energy - either rest energy as mass, or energy in motion as a photon.

Most of the time you need some sort of transfer of energy to transfer information. A photon is sent from one place to another, it interacts with a sensor, and information is exchanged.

One interpretation of this effect is that the photon itself doesn't travel down the path, it's the *probability* of the photon that travels down the path. When a particle is emitted, the universe takes the particle and puts it on a shelf somewhere (outside the universe) and sends out an instantaneous probability wave. As the wave evolves, the universe checks it for interactions with things along the path, and when the probability indicates an interaction it replaces the probability wave with the particle.

The probability wave takes all possible paths from the source to the destination, including non-straight paths. Most of the time most of the paths cancel out, leaving one path (the straight line) or two (beamsplitter mirror) or a few more, depending on the configuration.

I haven't found a non-paywalled version of the frikkin' paper yet, so I can't comment on what they're doing, but it seems like they are using the probability that the particle might be at the destination to affect particles at the destination without actually sending the particle.

There was an article in Scientific American that talked about taking a picture of Medusa without receiving any photons from Medusa. The presence of a photon in a cavity will alter the resonance frequency of a cavity which can be detected (IIRC - the article was many years ago).

If what the paper claims is true(*), it means that information can be transferred from one location to another without the transfer of energy, which is a very interesting philosophical statement.

(*) Many times in physics, especially QM, experimental outcomes turn out to be different than expected, so it's good to do the experiments. (Viz. Popper's experiment.)

Further answer

[Okian Warrior]

I've found the paper.

Using beamsplitters, Alice sets up one probability path that goes out to Bob and back. Bob can either insert a mirror, reflecting the probability path back to Alice, or not, making the probability path end there. Bob's mirror will change the phase of the Alice's photon in a way that can be detected by Alice, even though the photon doesn't actually go out to Bob.

A good simple example of what they're doing is the quantum bomb detector, where you can determine whether a bomb attached to a single-photon detector would explode if given a photon... without actually giving it a photon.

In the bomb example, you are getting information about the bomb without actually transferring energy to or from the bomb.

The experiment in the paper is somewhat similar.

Re:3*10^8 m/s.

[wonkey_monkey]

Is there any idea why Lightspeed == Radio speed ?

Yes, it's because radio is light.

Re:So...

[slew]

You are misunderstanding quantum counterfactual communications.

Quantum counterfactual communications uses the principle of interaction-free measurement. Although the information was phase encoded in photons, the information is transmitted in photons that are not actually sent to the receiver. Strangely (and this is hard to understand) photons are sent with some probability to the receiver, so it's possible that the receiver receives some photons, but those photons actually sent don't actually encode the information, but the information is conveyed by the photons that don't actually get sent. That way any photons that incidentally get sent can be "observed" without consequence of wave function collapse of the information.

This is similar effect to the quantum bomb detector thought experiment.

One interesting multi-verse interpretation of the multi-path bomb detectors is that in the universes where the photon took the path where the bomb went off aren't our universe, so we get to experience the consequence of the photon taking the other path. Similarly in this quantum counterfactual communication, we get to experience the consequences of the photons that were not transmitted.