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Nielsen and Chuang, QCQI, page 57, last paragraph, says "suppose Alice and Bob share between them a Bell state." I know how to prepare a Bell state, but what would be the physical process that would cause it to be shared by the two parties. Suppose Alice is in Pasadena and Bob is in Cambridge; one of them prepares the state. How does the other one get access to it?

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  • $\begingroup$ It doesn't seem to... people are saying that after Alice transmits the qubit to Bob, she no longer has access to it, so it seems like it is not shared. That is, either Alice has it or Bob has it but not both. $\endgroup$
    – Anna Naden
    Nov 2 '21 at 6:55
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    $\begingroup$ A Bell state is made of two qubits. By "sharing a Bell state" the authors mean that Alice has one of the qubits and Bob has the other. $\endgroup$ Nov 2 '21 at 9:47
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Depends on how the qubits are implemented. If the qubits making up the Bell state are encoded in the polarisation states of two photons, for example, then "sharing the state between the parties" simply means to send the two photons to Alice and Bob. This can be done via fiber optic cable, air, or something else. Encoding qubits in different ways will result in completely different physical substrata and thus different ways to "transport" the qubits from one hand to the other.

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