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When the spin of a quantum entangled particle is measured, is it only possible to do an instantaneous measurement, or can a particles spin be held in a collapsed state by constantly observing it?

In other-words, can you inspect particle A's spin and continuously observe it while sampling B's spin multiple times?

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    $\begingroup$ You might be looking for a variation of the Zeno effect $\endgroup$
    – ACuriousMind
    Aug 4, 2014 at 22:35
  • $\begingroup$ A useful search phrase for this trick on ordinary states is "quantum watched pot". $\endgroup$ Aug 4, 2014 at 22:35
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    $\begingroup$ Once the spin of a particle has been measured, it is no longer entangled. $\endgroup$ Aug 4, 2014 at 23:10
  • $\begingroup$ It's not possible to sustain the entanglement by observing only a part of the system. It is possible to sustain entanglement by observing the whole system (For example - if you measure 2 qubits in a Bell basis - you will find both the particle entangled - whether they were so before or not). In quantum mechanics if you trying to find an entangled state by measurement - you will (create and) find one. $\endgroup$
    – Alexander
    Jul 14, 2015 at 5:35

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Measurements are never instantaneous, only idealizations of actual measurements are modeled as instantaneous.

Measuring one particle in a entangled pair destroys the entanglement, after a measurement they are no longer entangled.

Doing repeated measurements quicker and quicker can exert a quantum Zeno effect where the no-measurement evolution away from a measurement eigenstate is difficult to achieve because of the repeated measurements.

What happens in a measurement is you split the state into eigenstates, so an entangled state become not entangled but get the correlation that was encoded in the entanglement. After that first measurement they have that correlation, but are no longer entangled, you can further split the one and the other just is what it is (the same thing) for each of the splits after the first measurement.

And whenever you do a measurement, you have to say exactly what you are measuring, different measurements are incompatible, so you can't do them all, and doing one first and then the other might give different results than doing the other first and then doing the one. So just talking about "measuring" one is too vague. Even a very simple measurement such as spin that only produces two results (say $\pm \hbar/2$) still has many choices about which axis to measure it, and you can't measure them all and the order you measure them matters when you do measure different ones.

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