Quantum ‘spookiness’ passes toughest test yet

My understanding is that entanglement doesn't actually allow for transfer any information faster than the speed of light but is more like having two coins sealed in a box and given to two parties, that "God" always keeps the same way up until they're opened. You can manipulate them by flipping the coins over, but you don't know what it was originally. You can open your box and see that your coin is heads, and send a conventional message to the other person that says "if your coin is heads, then attack at dawn", but you don't know what the coin is until you open it, so are still reliant on sending the message over means that obey the speed of light in order to transfer the information.

Is that an accurate (given that it's an analogy) description of how entanglement works, or does this work disprove that idea and is what was mentioned as Einstein's idea?

Re: Quantum ‘spookiness’ passes toughest test yet

#5

Post by lclemente » Sun, Aug 30, 2015, 8:54 AM UTC

fryguy wrote: post #4

My understanding is that entanglement doesn't actually allow for transfer any information faster than the speed of light but is more like having two coins sealed in a box and given to two parties, that "God" always keeps the same way up until they're opened. You can manipulate them by flipping the coins over, but you don't know what it was originally. You can open your box and see that your coin is heads, and send a…

I think the problem with your analogy is that it doesn't include the measurement basis. Consider polarization of a photon: you can either distinguish between horizontal or vertical polarization with a measurement, so your basis looks like a "+". Or you can distinguish between diagonal polarizaitons, so your basis looks like an "X".

Now, considering two photons, you don't know which basis Alice or Bob will choose in advance. If you just say "the photon is polarized horizontally", then their outcomes will be random if they measure in the "X" basis. If you include so-called "hidden variables" for all possible measurements, then you're basically at Einstein's idea.

(I've simplified some things above, so please don't quote me on it ;)

Re: Quantum ‘spookiness’ passes toughest test yet

#6

Post by crishoj » Sun, Aug 30, 2015, 9:21 AM UTC

Can anyone explain (in layman-like terms) how one goes about transporting a single photon over a distance of 1.3km? I really like to have a general idea of the technology involved.

"The researchers started with two unentangled electrons sitting in diamond crystals held in different labs on the Delft campus, 1.3 kilometres apart. Each electron was individually entangled with a photon, and both of those photons were then zipped to a third location."

Re: Quantum ‘spookiness’ passes toughest test yet

#7

Post by dools » Sun, Aug 30, 2015, 11:04 AM UTC

fryguy wrote: post #4

My understanding is that entanglement doesn't actually allow for transfer any information faster than the speed of light but is more like having two coins sealed in a box and given to two parties, that "God" always keeps the same way up until they're opened. You can manipulate them by flipping the coins over, but you don't know what it was originally. You can open your box and see that your coin is heads, and send a…

What you've quoted there is a good analogy of an entangled state. In reality there are several differences, and the biggest piece of misinformation is that measurement of one system somehow "collapses a wave function" in the other system which isn't the case.

The phenomenon of entanglement is that measurements of certain things are either perfectly correlated or anti-correlated. Eg. Measurement of spin along some axis on electron A will always be -1 when measurement of spin along the same axis of electron B is +1.

The similarity to 2 coins is: if i have a 1972 coin and a 1973 coin, and give one to you randomly, then you go a million light years away and look at your coin and find you have the 1973 coin, you will know I have the 1972 coin. Although this is only a simplified analogy of entanglement the important point is that the state was prepared before we separated, and no information has been communicated. When you try to understand this in terms of classical physics, it's weird. But it's not classical physics. I wrote more about it here:

https://news.ycombinator.com/item?id=10133674

Also check out the leonard susskind messenger lectures for a quick overview and his stanford continuing ed lectures on youtube for the long version :)

Re: Quantum ‘spookiness’ passes toughest test yet

#8

Post by gus_massa » Sun, Aug 30, 2015, 11:34 AM UTC

crishoj wrote: post #6

Can anyone explain (in layman-like terms) how one goes about transporting a single photon over a distance of 1.3km? I really like to have a general idea of the technology involved. "The researchers started with two unentangled electrons sitting in diamond crystals held in different labs on the Delft campus, 1.3 kilometres apart. Each electron was individually entangled with a photon, and both of those photons were th…

They use a optic fiber. This version has mare technical details, but is still understandable: http://arstechnica.com/science/2015/08/experiment-confirms-t... (HN: https://news.ycombinator.com/item?id=10141658 (4 points, 12 hours ago, 1 comment))

> To get around this, the researchers started with the diamonds over a kilometer apart in two different labs at the Delft University of Technology. They entangled each electron spin with a photon, then sent the photons over fiber optic cable to a third lab somewhere in between. There, the photons were entangled, which in turn caused the electrons in the diamonds to be entangled as well.

Re: Quantum ‘spookiness’ passes toughest test yet

#9

Post by coldtea » Sun, Aug 30, 2015, 11:40 AM UTC

dools wrote: post #7

fryguy wrote: post #4

My understanding is that entanglement doesn't actually allow for transfer any information faster than the speed of light but is more like having two coins sealed in a box and given to two parties, that "God" always keeps the same way up until they're opened. You can manipulate them by flipping the coins over, but you don't know what it was originally. You can open your box and see that your coin is heads, and send a…

What you've quoted there is a good analogy of an entangled state. In reality there are several differences, and the biggest piece of misinformation is that measurement of one system somehow "collapses a wave function" in the other system which isn't the case. The phenomenon of entanglement is that measurements of certain things are either perfectly correlated or anti-correlated. Eg. Measurement of spin along some axi…

>The similarity to 2 coins is: if i have a 1972 coin and a 1973 coin, and give one to you randomly, then you go a million light years away and look at your coin and find you have the 1973 coin, you will know I have the 1972 coin. Although this is only a simplified analogy of entanglement the important point is that the state was prepared before we separated, and no information has been communicated. When you try to understand this in terms of classical physics, it's weird.

If that was the case (the coin analogy) it would be perfectly understandable by classical physics.

In fact it wouldn't even need physics to understand it, just common sense.

If that was the case there wouldn't be any EPR paradox etc.

The coin analogy is very misleading in this way -- and doesn't convey the problems physicists have had with explaining this phenomenon.

Re: Quantum ‘spookiness’ passes toughest test yet

#10

Post by jblow » Sun, Aug 30, 2015, 11:44 AM UTC

dools wrote: post #7

fryguy wrote: post #4

My understanding is that entanglement doesn't actually allow for transfer any information faster than the speed of light but is more like having two coins sealed in a box and given to two parties, that "God" always keeps the same way up until they're opened. You can manipulate them by flipping the coins over, but you don't know what it was originally. You can open your box and see that your coin is heads, and send a…

What you've quoted there is a good analogy of an entangled state. In reality there are several differences, and the biggest piece of misinformation is that measurement of one system somehow "collapses a wave function" in the other system which isn't the case. The phenomenon of entanglement is that measurements of certain things are either perfectly correlated or anti-correlated. Eg. Measurement of spin along some axi…

I don't see how you can claim that the idea of wave function collapse here is a misunderstanding.

The mathematical meaning of the statement "two things are entangled" is that you cannot factor the states into the product of two separate quantities. So there is only one equation for the two particles. So once you "collapse" this for one particle (whether or not collapse is a physical action) it is by necessity collapsed for the other particle, because there is no other state that could not be collapsed.