Quantum communication

[seer]

That's not quite right. One, it doesn't have to be us. Any interaction with the environment (and we're part of the environment) can cause the particle's spin to switch from indeterminate to determinate. And, since it's a single system composed of two particles, spin is conserved, so the other particle is compelled to adopt the opposite spin. And again, it's not changing its spin - its spin is just going from a superposition of all possible spins to a specific one.

Another analogy: you have two cards, the ace of spades and the ace of hearts. You don't look at the cards, but place one in a box and ship it to california. While it's shipping, both cards are in an indeterminate state - you have no idea which one is which. Once it arrives, you look at your card and find it's the ace of spades. The card in california has to be the ace of hearts then. That's similar to how this works, although the mechanism is entirely different.

Ok, say the other card is already in California, and I do something to my card, will that have an effect on the card in California?

[TheLurch]

Ok, say the other card is already in California, and I do something to my card, will that have an effect on the card in California?

Once you measure the spin, the spin on the other particle is defined, and the entanglement is over. There's nothing else that has an effect.

[Sparko]

If i had a dollar for every time someone said quantum mechanics does not make sense...


That's not quite right. One, it doesn't have to be us. Any interaction with the environment (and we're part of the environment) can cause the particle's spin to switch from indeterminate to determinate. And, since it's a single system composed of two particles, spin is conserved, so the other particle is compelled to adopt the opposite spin. And again, it's not changing its spin - its spin is just going from a superposition of all possible spins to a specific one.

Another analogy: you have two cards, the ace of spades and the ace of hearts. You don't look at the cards, but place one in a box and ship it to california. While it's shipping, both cards are in an indeterminate state - you have no idea which one is which. Once it arrives, you look at your card and find it's the ace of spades. The card in california has to be the ace of hearts then. That's similar to how this works, although the mechanism is entirely different.

so how do you know it is in an indeterminate state? if nobody is observing the particle it could have a specific spin but it is just unknown to any potential observer. basically both particles already have the same spin (A) when the original photon was split and entangled. we just didn't know it until we looked.

[TheLurch]

so how do you know it is in an indeterminate state? if nobody is observing the particle it could have a specific spin but it is just unknown to any potential observer. basically both particles already have the same spin (A) when the original photon was split and entangled. we just didn't know it until we looked.

I'm not sure about spin (i'm a biologist, and i'd have to talk to someone who really knows this stuff) but I know that in other quantum situations, a particle with a state that's indeterminate behaves as if it's in all possible states. The big example is an electron that could take either of two paths - unless you monitor which path it's in, it behaves as if it takes them both. i don't really know of the equivalent experiment for spin, but I suspect it's been done.

[Sparko]

I'm not sure about spin (i'm a biologist, and i'd have to talk to someone who really knows this stuff) but I know that in other quantum situations, a particle with a state that's indeterminate behaves as if it's in all possible states. The big example is an electron that could take either of two paths - unless you monitor which path it's in, it behaves as if it takes them both. i don't really know of the equivalent experiment for spin, but I suspect it's been done.

also is it possible to force a particular state/spin on the first particle as you measure? instead of just "rolling the dice" to see the random state? if so then you could indeed transmit information by assigning a value to various states (up=1, down=0)

[Sparko]

and if you cant use it to transmit information how are they using it for encryption? is it something like this?

1. entangled particles are sent to two places in superposition

2. team one measures the various particles and gets up down down down (and assigning 1 to up and zero to down) result is random number 1000

3, team two then measures the particles and gets 0111 (opposite spins)

4. they know that theirs is opposite so they encrypt data with the key 1000

5. they send (via conventional means) the encrypted info to team one who already has the key to decrypt the info

?

[TheLurch]

also is it possible to force a particular state/spin on the first particle as you measure? instead of just "rolling the dice" to see the random state? if so then you could indeed transmit information by assigning a value to various states (up=1, down=0)

There's no way of doing so that we're aware of. You're just seeing what's there, really.

and if you cant use it to transmit information how are they using it for encryption? is it something like this?

Yeah, basically, both sides make measurements and track the timing of when the photon originated. They then publicly share which times had photons that were successfully measured (single photon measurements don't always work). They then build an encryption key using the results of those measurements.

[JimL]

There's no way of doing so that we're aware of. You're just seeing what's there, really.

But what is there before we see it? I mean, are we just seeing whats there, or is our observation causing whats there? It seems as though, according to this, that the entire world is in some sort of super state of all possible configurations until someone, by looking, causes it to snap into one particular state.

[Sparko]

But what is there before we see it? I mean, are we just seeing whats there, or is our observation causing whats there? It seems as though, according to this, that the entire world is in some sort of super state of all possible configurations until someone, by looking, causes it to snap into one particular state.

Yeah I think that is what is so spooky

The universe requires observers

Makes you want to believe in God huh? The ultimate observer

[TheLurch]

But what is there before we see it? I mean, are we just seeing whats there, or is our observation causing whats there? It seems as though, according to this, that the entire world is in some sort of super state of all possible configurations until someone, by looking, causes it to snap into one particular state.

Well, yes and no. For something in a quantum superposition, it really does behave as if it's in two (or more) states at once. But having something end up in a quantum superposition is rare - we need some rather elaborate equipment to generate them. And the state is very fragile - "observer" is really a stand in for "any interaction." So, if something in a superposition happens to bump into a stray electron in the environment, it comes crashing out of the superposition.

As a result, most of the universe is not in a state of uncertainty.

[JimL]

Well, yes and no. For something in a quantum superposition, it really does behave as if it's in two (or more) states at once. But having something end up in a quantum superposition is rare - we need some rather elaborate equipment to generate them. And the state is very fragile - "observer" is really a stand in for "any interaction." So, if something in a superposition happens to bump into a stray electron in the environment, it comes crashing out of the superposition.

As a result, most of the universe is not in a state of uncertainty.

Is it your understanding that as we stand here in the present, the future of the universe somehow exist as well, but in a superposition. Is that what quantum mechanics amounts to?

[TheLurch]

Is it your understanding that as we stand here in the present, the future of the universe somehow exist as well, but in a superposition. Is that what quantum mechanics amounts to?

The universe isn't a quantum system, so i don't really understand that question.

[Sparko]

I thought the double-slit experiment pretty much shows that all particles ARE in superposition all the time unless measured?

[TheLurch]

I thought the double-slit experiment pretty much shows that all particles ARE in superposition all the time unless measured?

Not really. If you shot the electrons down a path that only had one option, they'd never enter a state where they were in a superposition. It's only because you have a specialized device that offers them two paths that creates the locational superposition.

NB: a superposition is a different thing from the inherent uncertainty about the electron's location/energy implied by the wavefunction.

[Sparko]

Not really. If you shot the electrons down a path that only had one option, they'd never enter a state where they were in a superposition. It's only because you have a specialized device that offers them two paths that creates the locational superposition.

NB: a superposition is a different thing from the inherent uncertainty about the electron's location/energy implied by the wavefunction.

are you sure? From what i have read, the whole idea of quantum mechanics pretty much relies on superposition and how particles act as waves at quantum levels. besides if when measuring a particle you take it out of any superposition, how would you even know that "not all particles are in superposition" when you are not observing them?