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?

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    $\begingroup$ 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). $\endgroup$ Commented Jul 14, 2018 at 18:13
  • $\begingroup$ 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. $\endgroup$ Commented Jul 14, 2018 at 18:18
  • $\begingroup$ 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. $\endgroup$ Commented Jul 14, 2018 at 18:22

1 Answer 1


When two qubits are entangled, the only thing that means is that the measurement outcomes are correlated.

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.

  • $\begingroup$ If I use a quantum eraser as described here en.wikipedia.org/wiki/Quantum_eraser_experiment then the observer would know if I used a linear polarizer or not by whether or not he sees the interference. Would he not? (Although this setup requires a source of entangled photons located in the "middle" of the universe.) $\endgroup$
    – safesphere
    Commented Jul 14, 2018 at 20:12

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