Quantum Entanglement on two particles

[Jalo]

Hi.

Imagine a system of two particles, A and B, where they are entangled.
I've been studying a little bit of quantum entanglement and I understand how measurement of one property of a particle A leads us to find indirectly the value of that property to the particle B. My question is: if I act on one of the particles, let it be A, so that the particle changes in some way, will the particle B also suffer that same change? Or is quantum entanglement strictly related to measurements?

Thanks.
D.

 

[Naty1]

If a pair of photons are prepared with entangled polarization states entanglement persists as long as there is nothing in the way of either photon which disturbs the polarization state. Such a disturbance might be a collision of either of the photons with another particle or absorption by a measuring device.

 

[Jalo]

And is there anyway to send a message via morse code with a pair of entangled particles? Let's imagine we're dealing with two particles of symmetric spins separated by a large distance. If I manage to change the spin of one of the particles will the other one's spin also change?

 

[Nugatory]

And is there anyway to send a message via morse code with a pair of entangled particles? Let's imagine we're dealing with two particles of symmetric spins separated by a large distance. If I manage to change the spin of one of the particles will the other one's spin also change?

There is no way of sending a message.

You measure the spin, you get a spin-up result, you know the other guy's measurement will yield the opposite result. But when he gets spin-down, for all he knows he's made the first measurement, you haven't made a measurement yet, and when you do you'll get spin-up. Likewise, when you get your spin-up result, you have no way of knowing whether he's sending you a message ("I just got a spin-down result") or whether you're making the first measurement.

 

[jtbell]

You measure the spin, you get a spin-up result, [...]

And there's no way to control in advance whether you get spin-up or spin-down, and thereby originate a signal, any more than you could with a monkey pounding randomly at the key of a Morse-code telegraph machine.

 

[Naty1]

And there's no way to control in advance whether you get spin-up or spin-down

yes, the first measure is pure chance, 50/50 you'll get either one. But the entangled measurements ALWAYS correlates to the first measure...

See the first two sections here for some introductory insights...and some of the controversies surrounding this phenomena:

http://en.wikipedia.org/wiki/Quantum_entanglement