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Question from a computer scientist (not a physicist). Imagine two nodes on a quantum network. Alice sends Bob a qubit in some state of superposition over a quantum channel. Is Bob able to sense that he's received a qubit without measuring it? If so, how?

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    $\begingroup$ The intuitive answer is "no," but you'll probably be interested in interaction-free measurement, especially this version by Elitzur and Vaidman $\endgroup$ Sep 21 at 18:57
  • $\begingroup$ what do you mean with "sense he's received a qubit without measuring it"? How can anything be "detected" if not via a measurement? Or do you mean whether it can be detected with some sort of indirect measurement? Could you be a bit more specific about this? $\endgroup$
    – glS
    Sep 22 at 6:52
  • $\begingroup$ @glS - that's really my question; there's all this talk of the quantum Internet and quantum repeaters which rely on qubit state swapping to bridge long distances so it seems almost implied that a machine needs to be able to "know" when it's received a qubit but I've not found anything that says that. If it's measured on arrival, it kinda defeats the point of sending qubits in a superposition state. $\endgroup$ Sep 22 at 15:32
  • $\begingroup$ @QuantumSPCACatProtectors if you want to "know" anything, you have to measure something at some point, that's a given. From your comment, I'd say the confusion comes from understanding what the carrier of information really is. If a qubit is encoded in the polarisation of a photon, knowing that a photon arrived at a given relay station doesn't have to collapse its polarisation state. You can simply know this from standard considerations. You cannot maesure the polarisation of said photon, but you can apply gates/operations on it and send it somewhere else $\endgroup$
    – glS
    Sep 22 at 16:57
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If you are talking about the idea that the quantum state is encoded on a physical system (perhaps an atom), and that system can be sent from Alice to Bob, then yes, you can detect the presence of the atom without measuring the state of the atom.

To make the point, I'm going to go a bit crazy. I'm not claiming this is exactly a physical scenario.... Imagine Alice has a single superconducting qubit in her lab. This needs a whole bunch of machinery to keep it cold. She can prepare this qubit in any way she wants. To send it to Bob, she has to package up essentially the whole lab (and power supply). Bob will undoubtedly know when the shipping container has arrived, but the lab will still be there, working properly, preserving the state of the qubit.

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  • $\begingroup$ of course, that makes sense if there's a lot of surrounding hardware. What if it's a single photon with no out-of-quantum-band signaling? $\endgroup$ Sep 22 at 15:34
  • $\begingroup$ I'm not an experimentalist and cannot tell you the details. The difficulty with photons is that detection is usually destructive, even if you're not looking at the polarisation. But not always. You might look, for example, at nature.com/articles/s41586-021-03290-z $\endgroup$
    – DaftWullie
    Sep 23 at 6:45
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Bob can't know in any sense that Alice has sent a message, until and unless Bob receives a classical message from Alice confirming that she has sent a qubit.

Even for the sake of argument, let's suppose he somehow knew that Alice has sent a qubit, then arises two scenarios - one, Bob knew it instantly and second, Bob knows only after the time it takes for a light ray to reach from Alice to Bob.

Clearly the first scenario is against the principles of Special relativity, therefore it is not possible. And the second scenario, Bob still has to know what kind of measurements he has to do to get useful information out of it. Therefore if you think hard, you will realise that your question is a redundant one!

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  • $\begingroup$ That is actually the conclusion I've been reaching (that out-of-quantum-band signaling is required) but that seems to be contrary to the "quantum repeater" literature. $\endgroup$ Sep 22 at 15:37

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