[AlessandroF6587]

It wluld be interesting to send one beam alternatively throught a linear or circular polarizer. Then you can check the polarization on the other beam and see if you have faster than light information transmission. With all the implications about causality.

[n4r9]

Unfortunately you can't transmit information this way, even in theory. The polarizer has a 50-50 chance of testing one way or another, and the other beam will has the opposite polarization.

[orlp]

Well, yes and no. In certain games you do gain an advantage this way over a classical opponent, it's called quantum pseudo-telepathy: https://en.wikipedia.org/wiki/Quantum_pseudo-telepathy. It sounds like quackery but it's a real thing.

[adastra22]

If I understand it correctly, that's a really obtuse name for what is actually quite simple and has nothing to do with telepathy or information transfer. Basically the fact that both parties have access to the SAME random number generator can be useful in some schemes.

As an even simpler example, let's say Alice and Bob need to solve a really hard problem that can be cleanly split into two parts, A and B. In round 1 Alice and Bob can't communicate but they have access to an entangled photon. In round 2 they meet and compare notes. If they choose which part to work on randomly, then in only 50% of the outcomes are both parts A and B solved when round 2 starts. If, on the other hand, they use the polarity of the entangled photon to decide which part to solve, then quantum mechanics guarantees that they both solve different parts. It's random and unpredictable which part gets assigned to whom, but in 100% of outcomes when round 2 starts, Alice has solved one of A or B, and Bob has solved the other one.

[tonmoy]

I’m not sure if the simple example is faulty, but to me it seems like this can easily be done with a classical physics system. For example a white light beam passing through a randomly oriented prism and using mirrors to either send the purple or red light beam to Alice/Bob, don’t really need entanglement

[adastra22]

I believe you are correct. There is nothing new here that fundamentally needs quantum mechanics.

[n4r9]

The correlations achieved by separated measurements on entangled particles measurably exceed what is classically achievable. That's what's meant by "Bell inequality violations".

We can't transmit information instantaneously, but there are nevertheless certain distributed tasks we can do better at when we have a source of entangled states.

[n4r9]

Yeah, this is the field I worked in for my PhD. You can't transmit information this way though.

[adastra22]

It depends on what you mean by transmit information. It is possible for one party to read a true random number generator (the state of the photon) and transmit that reading faster than the speed of light to another party (the receiver of the entangled pair photon).

The physicist won’t call this transmitting information, but the information scientist has no qualms about acting on random data. And once you give data meaning, it is information. Maybe both parties pre-agree that N zero bits in a row from the digitized reading of the entangled photons is a start signal, and the bits that follow are used to make choices in whatever action is carried out. Now the first party has “sent” an instantaneous message informing of their actions.

To be clear I don’t think there is a clever gotcha here. But it is helpful in constraining what is meant by information locality here.

[n4r9]

Your scheme sounds pretty similar to something like "if I find the house key in my pocket, I instantaneously know that my wife doesn't have it".

That's fine and useful for some things, but as you say it's not fundamentally quantum and it's not what's exciting about this development imo.

[adastra22]

What is exciting about this development then? Because afaict this is the same thing.

[ynniv]

The CHSH experiment measures more pairs of entangled photons passing through two similarly oriented polarizers than two orthogonal ones, regardless of the source's angle of polarization, though the effect is small enough that it can only be seen statistically. Rotating one observing polarizer should then cause an immediate change in the number of photons passing the other observing polarizer, regardless of distance. It seems absurd, but that's the current understanding.

[n4r9]

In the standard CHSH experiment, there is one detector on side A and one on side B, each with two possible settings. No matter what settings are used, the partial probability distribution for each side is uniformly random. However, the correlations between the two sides' outcomes are dependent on which settings are used.

[AlessandroF6587]

That's not it if you discriminate between linear and circular polarization. You can use a phase inversion mirror to do it. That's the point of the experiment.

[n4r9]

There's a result called the no-signaling theorem; the choice of measurement on side A has no effect on the reduced quantum state on side B, therefore does not influence the outcome statistics of any measurement on side B: https://en.m.wikipedia.org/wiki/No-communication_theorem

A might know exactly what B is going to measure, after he's done his part, but that doesn't mean any information can be transmitted.

[schwoll]

They did an experiment like this called the Quantum Eraser Experiment. The outcome is quite strange and I still don't know if I fully understand the result. It suggests potential retro-causality or at least superdeterminism or some kinds of weird time independent action. People will claim I'm just misinterpreting the experiment but I've yet to hear any explanation that doesn't hand wave some critical things away.