The Mudcat Café TM
Thread #162429   Message #3865792
Posted By: Donuel
12-Jul-17 - 05:37 PM
Thread Name: BS: Quantum entanglement
Subject: RE: BS: Quantum entanglement
Au contraire mon frère

Instant means instant. That's partly why Einstein called it spooky action at a distant. Distance is entirely irrelevant with this phenomena.

Imagine a dimension that connects all points at once because a distance vector does not exist. In that through the looking glass reality the concept of instant can exist.

If you believe you can picture thought experiments in your head I would be glad to speak to you on this level.
If you believe nothing of value can come from this, there is nothing to explore with me.

To go a surprising step further, if the receiver particle were to travel at relativistic speed toward the base particle or away from the base, the message will either be received from the future or from the past. Brian Greene taught me how this works and I can teach others the same way. It still amazes me. Do you deny this too?

FTL communication problem needs to solve a very tricky thing
The trick is to not collapse the wave function or change polarity with a detector. - do not attempt to measure the message -
Neat trick if we an solve it.

Standard Intro

The basic scenario most people learn for entanglement-based communication looks like this: two people, traditionally named "Alice" and "Bob" share a pair of particles that can each be measured in one of two quantum states, which we'll call "0" and "1." These particles are prepared in an entangled state in which a measurement of the state of Alice's particle is correlated with the measured state of Bob's particle, no matter how far apart they are. That is, if Alice measures her particle in state 1 at precisely noon in Schenectady, she knows that Bob in Portland will also measure his particle to be in state 1, whether he's in Portland, Maine, Portland, Oregon, or Portland Station on one of the moons of Akenaten.

This seems like a perfect mechanism for sending information over vast distances, as Ethan notes:

So now to the question: could we use this property — quantum entanglement — to communicate from a distant star system to our own? The answer to that is yes, if you consider making a measurement at a distant location a form of communication. But when you say communicate, typically you want to know something about your destination. You could, for example, keep an entangled particle in an indeterminate state, send it aboard a spacecraft bound for the nearest star, and tell it to look for signs of a rocky planet in that star's habitable zone. If you see one, make a measurement that forces the particle you have to be in the +1 state, and if you don't see one, make a measurement that forces the particle you have to be in the -1 state.

This seems like a really obvious application, and in fact a bunch of people seized on this as a justification for ESP and various other schemes-- I recommend David Kaiser's How the Hippies Saved Physics for the fascinating history of this whole business. And, in fact, if the situation described above were possible-- if you could measure a particle's state in a way that forced a particular outcome-- you could absolutely send information this way. But you can't do that.

It's a brilliant plan, but there's a problem: entanglement only works if you ask a particle, "what state are you in?" If you force an entangled particle into a particular state, you break the entanglement, and the measurement you make on Earth is completely independent of the measurement at the distant star. If you had simply measured the distant particle to be +1 or -1, then your measurement, here on Earth, of either -1 or +1 (respectively) would give you information about the particle located light years away. But by forcing that distant particle to be +1 or -1, that means, no matter the outcome, your particle here on Earth has a 50/50 shot of being +1 or -1, with no bearing on the particle so many light years distant.

There's a subtle shift here from the impossible operation that would allow FTL communication to a different sort of operation, and it deserves to be spelled out. That is, in the original statement, you "make a measurement that forces the particle" to be in a particular state, while in the second you "force an entangled particle into a particular state" which breaks the entanglement. Those are not the same thing, though-- one is a measurement, the other is a change of state followed by a measurement.

It helps to think about a concrete implementation of this to make the distinction clear. So, imagine Alice's particle is one of the trapped ions that people regularly use to do quantum information experiments, which can be in one of two internal states. If her particle starts in a superposition of equal parts "0" and "1," how would she go about forcing a definite measurement outcome, let's say "1"?

The answer is to do an operation that we would describe in words as "If you're in state 0, flip the state, otherwise leave it alone." For a trapped-ion system, this is done using lasers to drive a transition from state 0 to state 1 by way of a third state (the jargon term for this is a "Raman transition"). If you choose your states carefully, you can arrange it so that an atom in state 0 will absorb the laser and flip its state, but an atom in state 1 won't interact with the laser at all. This sort of selective absorption is how they distinguish between states 0 and 1 in real trapped-ion experiments (state 0 absorbs a laser photon then re-emits the light, and repeating this a few million times a second gives you a bright spot on a camera pointed at the trap holding the ion), and a two-particle variant of it is how you entangle ions in the first place (the operation is "If Ion A is in state 1, flip the state of Ion B," and you give it an input state where B is definitely in state 0 and A is in a superposition of 0 and 1).

I do not know how to make a quantum detector or state changer that will always give me the original message. Do you have any ideas