Do un-entangled photons exist?

[San K]

in the various QM experiments:

after select event(s) have occurred, we assume collapse of wave function and the photon becomes un-entangled from its twin.

however does this un-entangled photon almost simultaneously get entangled with some other part of existence/universe/experimental apparatus?

is there any such thing as being un-entangled?

 

[StevieTNZ]

in the various QM experiments:

after select event(s) have occurred, we assume collapse of wave function and the photon becomes un-entangled from its twin.

however does this un-entangled photon almost simultaneously get entangled with some other part of existence/universe/experimental apparatus?

is there any such thing as being un-entangled?

Usually we would say the photon is entangled with the apparatus used to measure it, and had the other photon not been detected yet, entanglement between the apparatus and partner photon would exist.

 

[San K]

Usually we would say the photon is entangled with the apparatus used to measure it, and had the other photon not been detected yet, entanglement between the apparatus and partner photon would exist.

thanks StevieTNZ, are you suggestion b (not a) below?

a) the photon gets entangled with the apparatus upon detection and looses its entanglment with its twin photon

b) the photon is already entangled with the apparatus and get dis/un-entangled upon detection

 

[StevieTNZ]

Yes, I would be going with (b).

The entanglement between the two photons (A and B), although a photon has been destroyed, has not been broken. Thereafter, entanglement exists between photon B and the apparatus used to measure photon A.