# Observing that some action has happened in an entanglement scenario

Hobbyist quantum programmer here and just getting back into the swing of things by speccing out some fun apps in my head and trying to build them to learn more about the whole ecosystem which is fascinating. I have a reasonable physics background but not the depth that some of the answers here go to.

I am getting some conflicting information and I suppose the crux of this is my understanding of the difference between observed and measured and what can actually collapse an entangled state Vs what can be understood. I am thinking through an Alice and Bob scenario where they share 2 entangled qubits and go about their separate ways. Alice goes and does something to her qubit (e.g a spin change or something similar). Does Bob know that something happened without making a measurement (which collapses the entanglement) or without Alice calling him to tell him what happened? My question here: Is Bob aware that something has happened? In the sense that he can detect/observe that something has happened but can't figure it out without one of the 2 scenarios above. I'm thinking of this as a pager kind of app, so Bob gets a notification that something has happened and he should call/expect a call from Alice.

Are there any possible entanglement scenarios where the above is feasible? I have read about weak entanglement, no-communication theorem and several articles on entanglement and measurement/observation. Every second search is giving me conflicting results, some say you can detect something. I'm less concerned about a perfect state being maintained, just that the overall entanglement remains in place.

• The way you've described it, Bob cannot determine, based on a local action on his qubit alone, any action taken on Alice's qubit. Alice and Bob could be more than $2$ light years away from one another. After $1$ year, Alice decides to either do a spin-change, or do nothing at all. Let the spin-change represent the classical bit $1$, and the "do-nothing" the classical bit $0$. If Bob could determine Alice's actions, then Alice would have sent a superluminal (faster-than-light) classical bit to Bob – Mark S May 30 '19 at 23:27