QUIP

[Nephew Twiddleton]

Well, I suppose it could still be useful. If I got the gist of it, the information can't travel faster than the speed of light because the particle in question (in this example a photon) can't go faster than that. Can't do shit with it now, but say if you were able to have a conglomeration of particles in some sort of analog to a radio on Earth and the same on Pluto. The particles in one would be entagled with the particles in the other and stabilized so they don't go flying off somewhere else. You could then send a message to Pluto instantaneously instead of a broadcast that would take 5 hours.

That won't work. The only time information is passed between the two particles is when they are "set up" on Pluto and Earth. Once they're in place, no more can go through. It's the fact of their being set up that is the information, in a way.

You really can't get around the speed of light.


Rat, if I understand you correctly; there isn't really much difference in the case where the particles are kilometres apart and when they're light years apart. You have to use the classical channel in both cases to get the information out. In the case of lightyears, it'll just take years instead of minutes.

Easier to use radio waves, really. They're as fast as you can get. The entanglement thing is really only useful for encryption.

You can't respin them? I was figuring a sort of binary system, but if it can't be done more than once, then, fair enough. But is there other research to suggest that?

[Igor]

Rat:
You could - theoretically - send a series of entangled particles. Up up down up down etc. So you've got binary and an arbitrary amount of information.

Twid:
I'm afraid I can't quote any research, this is mostly off the top of my head
But it's the process of generating the entangled particles is what causes them to be "linked" like this. That's the only way for the two particles to be linked. The only way to send more info is to entangle more particles and send them.

Or to borrow from LMNO: what we're dealing with here is an event and two detectors. Thus:
D1 --------  E ------------ D2

Once D1 goes bleep, D2 must go bloop. But once the detectors have sounded, no more info can be gained. Grabbing one particle and twisting would probably cause the superposition to decohere and the link to be lost.

This is fun!
Igor,
degree in theoretical physics,
temp job in an office cube.

[Nephew Twiddleton]

Rat:
You could - theoretically - send a series of entangled particles. Up up down up down etc. So you've got binary and an arbitrary amount of information.

Twid:
I'm afraid I can't quote any research, this is mostly off the top of my head
But it's the process of generating the entangled particles is what causes them to be "linked" like this. That's the only way for the two particles to be linked. The only way to send more info is to entangle more particles and send them.

Or to borrow from LMNO: what we're dealing with here is an event and two detectors. Thus:
D1 --------  E ------------ D2

Once D1 goes bleep, D2 must go bloop. But once the detectors have sounded, no more info can be gained. Grabbing one particle and twisting would probably cause the superposition to decohere and the link to be lost.

This is fun!
Igor,
degree in theoretical physics,
temp job in an office cube.

I originally wanted to be a scientist but decided to go a very different route, so all of this is hard for me to grasp. I get the fact that once they pass the detectors, one does this, the other does that.

That's why I said having a stable set of particles on Pluto entangled with a stable set of particles on Earth.

For example, in the binary example:

You have 8 particles that aren't going anywhere but are entangled with 8 particles that aren't going anywhere on Pluto. You want to express to Pluto 11010011 (whatever that actually means) which would be up-up-down-up-down-down-up-up. So you would program into this solid state (for lack of a better term) set of 8 entangled particles: down-down-up-down-up-up-down-down....

Oh wait. Ok, I see what you're saying, sort of. I just don't understand why those particles can't be reused. Like those same above mentioned particles can't be respun to send a different message.

Please make any responses as dumbed down as possible, because I apparently am not getting it...

[Igor]

It might help to consider how the particles are generated. A typical way is to make a nucleus (collection of protons and neutrons) decay. It sends out (say) two electrons in opposite directions.

So you do this 8 times and you have your setup on Pluto and Earth. The only reason that the electrons are entangled (and have transmitted the 8 bits of information) is that they both used to be part of that nucleus. It's that decay that generates the information, not the particle state at either end. To send more info you need more decays, not more changes to the particle state


If you did take one of the electrons and changed it from 0 to 1, the link would break. It's the same as looking into Schrodinger's box. A larger system of particles has interacted with the carefully-prepared quantum state and this causes it to collapse. So the cat is dead and the link is broken.

[Nephew Twiddleton]

It might help to consider how the particles are generated. A typical way is to make a nucleus (collection of protons and neutrons) decay. It sends out (say) two electrons in opposite directions.

So you do this 8 times and you have your setup on Pluto and Earth. The only reason that the electrons are entangled (and have transmitted the 8 bits of information) is that they both used to be part of that nucleus. It's that decay that generates the information, not the particle state at either end. To send more info you need more decays, not more changes to the particle state


If you did take one of the electrons and changed it from 0 to 1, the link would break. It's the same as looking into Schrodinger's box. A larger system of particles has interacted with the carefully-prepared quantum state and this causes it to collapse. So the cat is dead and the link is broken.

I still don't get why the change would cause a break in the entanglement. The way I understand it is that if you spin up the spin down, it will spin down the sipin up. I'll just have to take your word on it. I probably just read into this wrong, but I got the sense that if you changed the state the complementary particle would instantaneously take the opposite state...

Maybe I'm going to go with Captain Utopia's take and chalk it up to tricking hot research assistants.

[Golden Applesauce]

The reason they're entangled in the first place is conservation of spin.  It's analogous to conservation of momentum, so I'll try to explain it using that conservation law.

You're a space-flight controller on Pluto.  A rocket ship radios in, "Hello, this is the solid-propellant rocket Galileo making approach to Pluto spaceport; requesting permission to dock.  Our mass is 10,000 kilograms and our approach velocity is 10 meters per second towards Pluto, so our momentum is 100,000 kilogram-meters per second."  Now, since you know that momentum is conserved, you can deduce that somewhere in the solar system is a cloud of propellant gases with equal and opposite momentum as the rocket ship (in the reference frame of the rocket ship + propellant system.)  You know this because you know that rocket ships start out with the propellant inside fuel tanks and at rest (i.e., at zero momentum.)  As long as the rocket and it's propellant don't interact with anything else, the momentum of the system must remain at a constant zero kilogram meters per second.  So if you know the momentum of the rocket, you can calculate the momentum of the cloud of propellant gas it ejected, and if you know the momentum of the propellant, you can calculate the momentum of the rocket.

The same thing happens in the creation of the entangled pair of electrons.  A particle with zero spin (like a rocket with zero momentum that is loaded with fuel) is split into two particles with equal and opposite spin (like the rocket and propellant with equal and opposite momentum.)  If you know the initial condition (the spin of the originating particle, or the initial momentum of the rocket with fuel) and the condition of one of the two halves of the system (the spin of one particle, or the momentum of either the fuel or the rocket) you can calculate the condition of the other half.  If you were to then do something to one of the particles, the other particle would be unaffected in exactly the same way (and for the same reason) as the cloud of propellant gas ejected from the rocket doesn't suddenly reverse direction if you were to turn the rocket around from Pluto.

[Nephew Twiddleton]

The reason they're entangled in the first place is conservation of spin.  It's analogous to conservation of momentum, so I'll try to explain it using that conservation law.

You're a space-flight controller on Pluto.  A rocket ship radios in, "Hello, this is the solid-propellant rocket Galileo making approach to Pluto spaceport; requesting permission to dock.  Our mass is 10,000 kilograms and our approach velocity is 10 meters per second towards Pluto, so our momentum is 100,000 kilogram-meters per second."  Now, since you know that momentum is conserved, you can deduce that somewhere in the solar system is a cloud of propellant gases with equal and opposite momentum as the rocket ship (in the reference frame of the rocket ship + propellant system.)  You know this because you know that rocket ships start out with the propellant inside fuel tanks and at rest (i.e., at zero momentum.)  As long as the rocket and it's propellant don't interact with anything else, the momentum of the system must remain at a constant zero kilogram meters per second.  So if you know the momentum of the rocket, you can calculate the momentum of the cloud of propellant gas it ejected, and if you know the momentum of the propellant, you can calculate the momentum of the rocket.

The same thing happens in the creation of the entangled pair of electrons.  A particle with zero spin (like a rocket with zero momentum that is loaded with fuel) is split into two particles with equal and opposite spin (like the rocket and propellant with equal and opposite momentum.)  If you know the initial condition (the spin of the originating particle, or the initial momentum of the rocket with fuel) and the condition of one of the two halves of the system (the spin of one particle, or the momentum of either the fuel or the rocket) you can calculate the condition of the other half.  If you were to then do something to one of the particles, the other particle would be unaffected in exactly the same way (and for the same reason) as the cloud of propellant gas ejected from the rocket doesn't suddenly reverse direction if you were to turn the rocket around from Pluto.

Ok, that makes sense. But it negates my understanding of previous posts. I'll have to reread.

[Golden Applesauce]

I could be wrong.  I had to drop out of my QM class because I just couldn't wrap my head around bra-ket notation.  The physicist approach to math is bizarre if you come to it via a mathematician approach to math.  At at least one point the textbook flat-out said "This derivation is wrong.  If that bothers you, you should be change your major to mathematics."

It's like they think that just because they can show that their results are correct by doing an actual experiment they don't feel the need to bother with hundreds of lines of proof using nonlinear differential equations, matrices, and a vector space with the rigorous mathematical definition of "only the things a particle can do in real life."

[P3nT4gR4m]

The reason they're entangled in the first place is conservation of spin.  It's analogous to conservation of momentum, so I'll try to explain it using that conservation law.

You're a space-flight controller on Pluto.  A rocket ship radios in, "Hello, this is the solid-propellant rocket Galileo making approach to Pluto spaceport; requesting permission to dock.  Our mass is 10,000 kilograms and our approach velocity is 10 meters per second towards Pluto, so our momentum is 100,000 kilogram-meters per second."  Now, since you know that momentum is conserved, you can deduce that somewhere in the solar system is a cloud of propellant gases with equal and opposite momentum as the rocket ship (in the reference frame of the rocket ship + propellant system.)  You know this because you know that rocket ships start out with the propellant inside fuel tanks and at rest (i.e., at zero momentum.)  As long as the rocket and it's propellant don't interact with anything else, the momentum of the system must remain at a constant zero kilogram meters per second.  So if you know the momentum of the rocket, you can calculate the momentum of the cloud of propellant gas it ejected, and if you know the momentum of the propellant, you can calculate the momentum of the rocket.

The same thing happens in the creation of the entangled pair of electrons.  A particle with zero spin (like a rocket with zero momentum that is loaded with fuel) is split into two particles with equal and opposite spin (like the rocket and propellant with equal and opposite momentum.)  If you know the initial condition (the spin of the originating particle, or the initial momentum of the rocket with fuel) and the condition of one of the two halves of the system (the spin of one particle, or the momentum of either the fuel or the rocket) you can calculate the condition of the other half.  If you were to then do something to one of the particles, the other particle would be unaffected in exactly the same way (and for the same reason) as the cloud of propellant gas ejected from the rocket doesn't suddenly reverse direction if you were to turn the rocket around from Pluto.

Ok, that makes sense. But it negates my understanding of previous posts. I'll have to reread.

I could be way off here but I'm thinking that the important distinction vis a vis quantum encryption is that the qbits are not transferring information from one side to the other, rather the particle is acting like a random seed, generating a key simultaneously at both ends. The advantage of this would be that the key is only going to arrive at the two ends of the conversation and nowhere else, as opposed to the current system where it happens over wires or radio and is subject to potential eavesdropping scenarios.

[LMNO]

For Twid:  I might have left this out in the OP to save space.  Hope it helps.



It is easy to imagine apparatus that will produce clicks in two separate detectors that are correlated in some way (I mean for the pattern of clicks to be correlated, not the detector settings, which are always under our control).

For example, an excited nucleus might shed energy by radiating two electrons and nothing else. Imagine that two detectors, A and B, are set up to register them.

If the original nucleus were standing still (zero momentum), then the electrons would necessarily be detected moving with identical speeds in opposite directions, because momentum is conserved even in the quantum world (equal and opposite momenta add to zero).

So the momenta are perfectly correlated. Similarly, if the excited nucleus had no spin then the observed spins of the emitted electrons must also cancel (to conserve angular momentum).

If the spin detectors are both set at the same angle, we can be sure this apparatus of stationary radiating nucleus plus detectors will cause one detector to register "up" and the other one "down."

Therefore the detector registrations in opposite settings at the two different detectors will be perfectly correlated.

[Igor]

The conservation of momentum analogy explains the correlation part well, but it fails to emphasise the weirdness of the situation.

Like in GA's example, you would expect there to be a correlation even without quantum entanglement. The classical world already has correlations, but the quantum has even more.
So here's another analogy:

You have two balls, one is black and one is white. You put them in a bag. You blindly pick one out of the bag, let's say it's white. Then you know the next ball you pick out must be black. That's the classical situation.

In the quantum situation, you have several pairs of balls. Each pair is a different colour. Two white, two black, two red, etc. You reach into the bag and randomly pick out a green one. Now you know that the next ball you pick will be green. No matter how many balls there are in the bag.
That's how weird this is.

[Bebek Sincap Ratatosk]

So it would be like having two coins spinning, when you knock Coin A over, its Heads... so you know that the other coin will be tails. Whereas if you knock Coin A over and its tails, you will know that Coin B is heads?

[LMNO]

STOP USING METAPHORS, MOTHERFUCKERS.

[Kai]

The universe cheats.

[Cain]

STOP USING METAPHORS, MOTHERFUCKERS.

This.  Analogy has to be among the most facile forms of understanding.