How long can a set of entangled particles be expected to stay entangled? Is it a half life situation or a probability? If you take care of them do they stay entangled longer?
Once entangled, always entangled. It would be very difficult to unentangle particles. You’d have to run them backwards through whatever interactions took place to entangle them.
Then, is every particle entangled with each other throughout the universe, via the big bang? Kinda loses its meaning then…
No. What do you think the Big Bang was? There were no particles involved until well after. Like, whole picoseconds after, or some such prefix. First the particles had to condense, then the various entanglement effects could happen.
It seems that you believe every particle that exists has always existed, but that isn’t the case. Particles are created and destroyed all the time. Every time you turn on a light, you create photons, for instance. Experiments with entangled particles usually involve the creation of two entangled photons in the lab. The Big Bang doesn’t really play a role in this in any way.
What action disentangles two particles is one of the great debates in Quantum Mechanics.
The common version of the Copenhagen Interpretation says that once you observe one of the particles, that’s it. They are disentangled.
Observe? So it takes humans? No, but … then things get very debatable.
Take two oppositely circularly polarized entangled photons. You don’t know which one is polarized which way. You can send these photons quite a distance and preserve the entanglement. Two years ago a new record of 750 miles was set sending to a satellite. Not so far but a good distance has been achieved through fiber optics.
But note there are limits (which keep getting longer as tech gets better). So if you are sending a pair down a really long fiber optic cable there will be a point where Something Happens and they disentangle. No observer necessary. One of them just passes by an atom just right or goes through a quantum fluctuation or something and poof, bye-bye entanglement. (Note the “uncertainty” doesn’t disappear. It just is attached to something else. And since you don’t know what thing that is you can’t “observe” that something else and decipher the polarization.)
Photons are easy compared to big stuff like electrons, never mind atoms. Quantum computing relies on such entanglement but these computers have to be maintained in a special state for just long enough for them to complete their task. If things aren’t just about perfect, the state collapses early and you have to start over. Again, no observer. The uncertainty is lost in the system. And this is with a few dozen qubits. To scale up the precise conditions get harder and harder to maintain so the tech has to advance to make progress.
(Google’s recent record claim uses 54 qubits and took 3+ minutes. But there’s some questions about some of they stuff they claim regarding this.)