I'm reading chapter 2 of Preskill's notes on quantum information. In explaining the delayed choice, he says that out of the two observers Alice and Bob,

Alice could measure all of her spins today (say along x) before Bob has made his mind up how he will measure his spins. Next week, Bob can decide to “prepare” Alice’s spins in the states $\left| n+ \right>_A$ and $\left| n- \right>_A$ (that is the “delayed choice”). He then tells Alice which were the $\left| n+ \right>_A$ spins, and she can check her measurement record to verify that $\langleσ_1\rangle_n = n.x$

I did not understand since Alice measured her spins along x axis, how did she verify the equation $\langleσ_1\rangle_n = n.x$ since she did not measure along n-axis. I mean, how that fact that Bob made a delayed choice is being used here? It would be helpful if someone provides a detailed explanation of this.

EDIT: Link to lecture notes: http://theory.caltech.edu/~preskill/ph219/index.html#lecture See chapter 2, section 2.5.4, pg 70

  • $\begingroup$ Hi @AninditaSarkar, please share a link to these lecture notes. $\endgroup$ Jul 5, 2023 at 13:44
  • $\begingroup$ Please add the lecture notes to the post $\endgroup$ Jul 5, 2023 at 13:47
  • $\begingroup$ @YaronJarach done, check the post now $\endgroup$ Jul 5, 2023 at 15:18
  • $\begingroup$ @MarkSpinelli yeah edited accordingly. $\endgroup$ Jul 5, 2023 at 15:18


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