# Is there a way to determine if one qubit of an EPR pair has collapsed after the other was measured?

My friend on the other side of the universe and I share the respective qubits of 2 EPR pairs. My friend measures one of the qubits, so one of my qubits will collapse.

Must I measure this collapsed qubit to determine that it has collapsed? And if I measure both my qubits, is there a way to determine that one has already collapsed and the other was entangled just before measurement?

• What would it mean to "determine that it has collapsed"? Measurement itself involves collapse of the wavefunction (according to the interpretation of QM that you appear to be using here). Commented Jul 14, 2018 at 18:13
• What I mean is, my friend measured one of the qubits, collapsing the wave function of one of the epr pairs. So the qubit I have is now in one of the base states. I dont know that my friend has done this. The question is, can I know that the wave function is already collapsed, or did it just collapse when I measured? Hope it makes sense. Commented Jul 14, 2018 at 18:18
• Then the answer is no, you cannot know if your qubit is in one of the base states or not, because in general you cannot distinguish, with a single measurement, between these two outcomes: a) the qubit was already in that pure state, or b) the qubit collapsed into the measured state from a superposition. Commented Jul 14, 2018 at 18:22

Suppose you and your friend shared a bunch of identically-prepared entangled qubits. You then went to opposite sides of the universe and measured all of them. Each time you measured them, you and your friend would get a random result each time (for example, for the entangled pair of spins $\frac{1}{\sqrt{2}}(|\uparrow\uparrow\rangle + |\downarrow\downarrow\rangle)$ you would each get approximately equal numbers of spin-up and spin-down results). It is only when you compare your measurement results afterward that you find that (for our example case) whenever one of you measured spin-up, the other did too. (Other types of correlation can happen too - for example, the pair $\frac{1}{\sqrt{2}}(|\uparrow\downarrow\rangle + |\downarrow\uparrow\rangle)$ will give you the opposite results from your friend.) It doesn't matter which one of you measured theirs first, and it's impossible to tell.