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Suppose I meet with my friend on the New Year's and we entangle two qubits in a Bell state. The qubits are physically realized as some solid state devices that can be transported easily. I take a qubit with me and they take it with them.

For the whole year, I put my qubit on my shelf. I live in a jungle and there is rain, a thunder hits the qubit, there is also an earthquake and the device falls into the mud. On the next New Year's I take my qubit to the lab and now try to do some quantum teleportation with it or whatever that requires me to have an entangled pair with my friend to send him good wishes for the upcoming year.

The question is: Has the entanglement survived throughout the year? My first take on this is yes, as local unitary evolution does not destroy entanglement. On the other hand, perhaps the environment does some "effective" measurement spontaneously.

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    $\begingroup$ in ideal conditions, yes. In practice, it is extremely difficult (as in, near impossible with the state of the art) to preserve quantum correlations for long times, due to the hardness of shielding the system from the environment, as you point our yourself $\endgroup$
    – glS
    Commented Apr 15, 2022 at 0:15
  • $\begingroup$ It is precisely the question: do I have to preserve the qubit in the fridge isolated from anything else or not? Why would the environment ever spoil the entanglement, if it only corresponds to local operations? $\endgroup$
    – xletmjm
    Commented Apr 15, 2022 at 1:36
  • $\begingroup$ oh, I see. Yes, you can imagine the environmental noise happening after the qubits are spatially separated to amount to "local operations". However, these are local operations not on the qubits themselves: the information is spread in the environment surrounding the qubits. So in principle, if you were able to perform a coherent measurement of each qubit with its surrounding environment, you'd still be able to see the entanglement (not that this is in any way feasible). But by only looking at the qubit, you won't. $\endgroup$
    – glS
    Commented Apr 15, 2022 at 8:30
  • $\begingroup$ @gIS please post your answer and I will tick it off as answered and you can get points. $\endgroup$
    – xletmjm
    Commented Dec 20, 2022 at 15:38
  • $\begingroup$ If you store your information in some very isolated degree of freedom - e.g. some nuclear hyperfine state - then it can survive for very long times, without much shielding. (On the other hand, then it is also very hard to store it there, and to get it out from there.) A year is really pushing it, though ... (e.g.: doi.org/10.1126/science.1239584). $\endgroup$
    – Norbert Schuch
    Commented Dec 20, 2022 at 17:03

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Yes, you can imagine the environmental noise happening after the qubits are spatially separated to amount to "local operations". However, these are local operations not on the qubits themselves: the information is spread in the environment surrounding the qubits. So in principle, if you were able to perform a coherent measurement of each qubit with its surrounding environment, you'd still be able to see the entanglement (not that this is in any way feasible). But by only looking at the qubit, you won't.

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  • $\begingroup$ Of course, if it rains on it, and the rain drains to a river, which goes to the ocean, there is quite a bit of environment to keep track of after a year. $\endgroup$
    – Norbert Schuch
    Commented Dec 20, 2022 at 17:07
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Will entanglement survive interaction with classical environment?

In quantum mechanics measurement is defined as interaction with classical environment. So the answer is known - it depends on what is measured.

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