# Quantum Entanglement on two particles

by Jalo
Tags: entanglement, particles, quantum
 P: 114 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.
 P: 5,632 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.
 P: 114 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?
Thanks
P: 3,458
Quantum Entanglement on two particles

 Quote by 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?
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.
Mentor
P: 11,619
 Quote by Nugatory 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.
P: 5,632
 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

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