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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."
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."
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.
"How is this different than radio?"
Signal strength. If implemented in the real world it would be harder to jam or disrupt. We already have used tight beam laser communications but they are line of sight and susceptible to interference by any particles between the source and target. It seems this new attempt at something else would provide better performance without the line of sight or environmental obstructions getting in the way. If any of this type of technology becomes practical look for it to show up in the military technology basket. This new technology also opens the door for advancements in encryption making interception and deciphering the content much more difficult if not impossible at this time.
Radio waves are photons too. Light is just photons that happen to have the right frequency for our eyes to pick up. Whenever electrons are changing magnetic fields that act on other electrons, deep down at the quantum mechanical level it turns out to actually be an interchange of photons.
In this latest experiment, no photons or any other kind of particles were exchanged between emitter and receiver. So it really is quite different.
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.)
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.
In this latest experiment, no photons or any other kind of particles were exchanged between emitter and receiver. So it really is quite different.
That's not actually quite true. Photons are sent (and in fact photon counters are used to detect the signal). However, the information itself isn't encoded in the photons, it's encoded in the phase of the photons. The abstract uses the comparison to holography: in normal photography, only the amplitude of the light is relevant. In holography, the phase of a laser is used to encode information as well (specifically, the 3D depth of the target) in addition to the amplitude. This technique encodes all of the information into the phase alone, and none of it stays in the amplitude.
TFA is, to put in bluntly, wrong. It claims
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.
But light is photons, and photons are light. You can't encode information in light without sending photons: it's like saying you talk to someone over the phone using words, but without using sound. It's just absurd. Of course, you can communicate over the phone using sound without using words, which is roughly analogous to what the scientists actually did.
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