Continuing entanglement after first measurement?

Question

Ok so if two particles are entangled and I choose to measure particle A, then the entanglement is finished. But what if I measure both A and B with photons of the exact same energy; is there now a new entanglement? What if I measure a particle, and then immediately change its spin (this can happen)? Does the entanglement continue? I am specifically looking for a way to continue entanglement after the first measurement. Thank you.

Answer 1

You cannot continue entanglement after the first measurement. Once they are measured, they are not entangled ever again.

You cannot use entanglement to send information.

Answer 2

In the "Feynman diagram" universe, after the first entanglement the two particles cease to exist, and begin new lives as identical replacements of themselves.  Otherwise the universe collapses into a predetermined series of inevitable events...  

Answer 3

Entanglement, in general, does not continue after the first measurement.  Now, there are a few points to explain here.  First, two non-entangled particles can be entangled (that's how you get them in the first place).  So, after you measure two particles, there is nothing prohibiting you from running the entangling procedure on them again to entangle them a second time, assuming you have enough control over the particles.  However, the first entanglement and the second entanglement would not be related in any way.  You can also maintain entanglement by undoing a measurement.  This is harder than it sounds.  Simply burning the results of the measurement is not sufficient.  You need to undo the measurement in a way that there is no possible way the results could have contacted the world outside the system.  Let me give you an example.  One way to measure the polarization a photon is with a polarizing beam splitter (PBS), where you pass a beam of light a PBS and the vertically polarized photons are sent in one direction while the horizontal photons go a different way.  This constitutes a measurement of the polarization of the photons and collapses entanglement between pairs of photons.  However, if you were to take both of these beams and re-combine them on another PBS, the output beams would have both polarizations and, since there was now way you could have measured the light between the two PBS's, the output beam will have the photons entangled just as if they had never gone through either PBS.  But as soon as you block one of the paths, the output photons will again lose their entanglement.

If you measure a particle and then immediately change its spin, nothing will happen to its previously entangled particle, since you measured it.  If you make a measurement and do not undo the measurement, the entanglement is lost.  Even if you don't look at the results of the measurement.  There is no way around it.  

Bryan is incorrect.  Entanglement can not be used to transmit information.  Quantum teleportation REQUIRES a classical channel upon which the information is transmitted.  However, a classical channel is not sufficient for quantum teleportation because coherence can not be transmitted via a classical channel.  Therefore, you need both a quantum channel (entanglement) and a classical channel (a phone line or something).

Answer 4

I think this goes against heisenbergs uncertainty principle.  That said - I somewhat disagree that entanglement can not be used to transmit information, otherwise quantum teleportation would be impossible.

Since experimentally teleportation has been achieved - it's possible to transmit the quantum information, at the expense of destruction of the first particle and losing the entanglement through the teleportative event.

I think.

Keep studying science - Let's solve the world's problems with insight.  Don't be like George W. Bush - A hater of science and the worst President in modern American history.