# Tag Info

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This is really a subtle point. You are right that in 25% of the cases, Bob will randomly chose the "correct" measurement basis and thus get the correct value. However, there is no way for Bob to know when he has actually chosen the right basis and when he has chosen the wrong basis, so his measurement outcome does not contain more information that a random ...

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Quantum entanglement is the property of two objects $A,B$ – more precisely two subsystems – or a relationship between these two objects whose quantities or observables aren't independent of each other. It means that there exist some quantities $a_j$ and $b_k$ describing $A,B$, respectively, such that the probability distribution for these observations ...

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This hand waving assumption you are making is the crux: At the mirrors A and B (and also at the half-transparent ones not considered) the photon interacts with one or more electrons of the mirror, transferring momentum The photon is not interacting with one or more electron on its way, it is interacting with the lattice of atoms. This means that the ...

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Here's how I understand your question: A and B are space-like separated and make a measurement on a single particle that has equal (or just non-vanishing) probabilities of being in A's or B's region. You now ponder how the measurement process works on a deeper level. Could the collapse be a dynamical (i.e. time dependent) process? I think it can not. If ...

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Engtangled photons: first you must understand that the photon is the particle obtained when the modes of the electromagnetic field are quantized, and that they are created and destroyed as discrete quanta of energy, in agreement with Planck's relation, $E=hf$, where $f$ is the frequency of the electromagnetic field corresponding to the quantized mode; that ...

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If you are happy with Bob being right only in some percentage of the cases, there is a much easier protocol which does not even require entanglement: Just let Bob guess Alice's bits. He will be right in 50% of the cases. Note that once the probability to guess the right result is above 50%, communication is possible, e.g. by doing majority voting. In ...

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Weak measurements don't let you learn about the system without disturbing it. They let you make tradeoffs, where you disturb/decohere/collapse less by revealing less, but you still have to pay for whatever the measurements do reveal. You can't combine many weak measurements into a "free" strong measurement. But say Alice weakly measures X, and learns ...

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Wave function collapse is not global, it is fictional. Let's suppose that the state is $\alpha|X=0\rangle+\beta|X=1\rangle$, where $|\alpha|^2+|\beta|^2=1$. When Alice measures the state, an operation is applied that correlates both Alice and the environment with the value of $X$, like so ...

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You can use circuit simulators to help with understanding small circuits like this, and to check your work. For example, here's your circuit in Quirk: The key things you need to understand for this circuit are: How to apply an operation on paper to an entangled state (for the H after the CNOT). Group by the uninvolved bits, and apply the operation ...

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A full quantum mechanical description of the interferometer is complicated because it's not an isolated system. But we can do a thought experiment where we imagine it to be made out of mirrors that are floating in free space. Then as Anna has explained, the photon will interact with the entire system, it will not excite vibrational modes of the lattice. This ...

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