Understanding Entanglement: The Impact of Magnetic Fields

[fargoth]

just checking:
when one particle of an entangled pair enters a magnetic field, the entanglement is gone as it starts to re-align in the magnetic field's direction, right?

 

[RandallB]

Of course

You can only detect or measure the entanglement on a particle once. If you don’t record it, trying to measure the same particle a second time will be of no use.
See Stern-Gerlach devices, they use magnetic fields to detect spin entanglements.

 

[fargoth]

thought so, just wanted to make sure.
my friend though he could make one spin turn at his end while making the other to turn too.
i told him i think he'll just break the entanglement...

 

[vanesch]

thought so, just wanted to make sure.
my friend though he could make one spin turn at his end while making the other to turn too.
i told him i think he'll just break the entanglement...

Be careful! The entering itself into the magnetic field is not going to destroy the entanglement irreversibly. (however, what RandallB writes is correct: it is not because you do not conserve the result that the measurement is not irreversible) It will modify the relationship between the entangled constituents. It is only if an irreversible measurement is made that way that the entanglement is gone. This is the idea of delayed-choice quantum erasers.
If you don't believe this, spin echo is exactly an illustration of this phenomenon.

 

[zekise]

The entering itself into the magnetic field is not going to destroy the entanglement irreversibly. ... It will modify the relationship between the entangled constituents. It is only if an irreversible measurement is made that way that the entanglement is gone.

How do you then do an "irreversible" measurement, as opposed to a reversible one? What is the difference between a reversible and irreversible measurement (if it is not related to entering a magnetic field and/or conserving the results)?