Questions tagged [quantum-measurements]
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546 questions
0 votes
0 answers
10 views
Why can't we measure non-linear operators in QM using single shot measurements?
Consider an operator like the modular Hamiltonian $K$ defined by $$\rho=e^{-K}$$ where $\rho$ is some (reduced) density matrix. So $K$ depends non-linearly on $\rho$ in general. It can be shown that ...
- 463
3 votes
1 answer
97 views
Interpretation of real scalar field, its propagator and causality [duplicate]
It seems this has been discussed here previously (e.g this and this post), but I still feel uneasy. Essentially, I think it all boils down to the popular interpretation of the scalar field operator in ...
- 1,158
2 votes
1 answer
44 views
"Remembering" the polarization in the BB84 protocol
I am trying to understand the essence of the BB84 protocol and by watching this video, https://www.youtube.com/watch?v=uiiaAJ3c6dM&t=171s I cannot manage to understand the following. Alice is ...
0 votes
1 answer
104 views
Can counterfactual measurements be used to measure incompatible observables?
Imagine we have a qubit system prepared by choosing one of two states as measured in one of two possible, incompatible bases, i.e. there are four total possible states, such as e.g. polarizations of 0,...
- 1,094
5 votes
4 answers
981 views
Energy Conservation for Quantum Systems
This seems like a basic question but I couldn't find the answer when looking online. I'm in an introductory quantum mechanics class and we learned how classical laws can be seen in QM when you look at ...
- 61
8 votes
3 answers
1k views
Why does the wavefunction collapse after no detection?
In a beam splitter experiment with a single photon, if I place a detector on one path and after some time it never clicks, why does the wavefunction still collapse to the other path even though I ...
- 254
-4 votes
1 answer
120 views
Classical Probability in Quantum Mechanics [closed]
I have been reviewing the foundations and formalisms of quantum mechanics in order to better understand some things, and one of my goals is to be able to clearly separate things which are physical (i....
2 votes
2 answers
319 views
What happen with wavefunction immediately after a measurement?
One of the postulates of quantum mechanics, introduced by Dirac, says that immediately after a measurement of an observable $\hat A$ the wavefunction abruptly becomes an eigenfunction of $\hat A$. In ...
- 2,407
1 vote
5 answers
439 views
When does a transition collapse to the final state?
I did my PhD research on device physics of optoelectronic devices. We take the square of $\langle f| H_{int} |i\rangle $ as the photon absorption probability or the theoretical efficiency of a ...
- 419
0 votes
4 answers
244 views
Is it possible that the square amplitude law is only approximately correct?
I would like to ask about the law in quantum mechanics whereby the measured probability is the square of the probability amplitude. Is it possible that the law is only approximately correct, i.e. it's ...
- 297
1 vote
0 answers
113 views
Transfer of displacement using photon number measurement?
Consider the following optical setup. Mode $A$ is prepared in a thermal state with mean photon number $\bar n$. Mode $B$ is in the vacuum state. The two modes are mixed on a lossless beam splitter ...
- 209
0 votes
1 answer
122 views
Has entropy been used to derive or bias the Born rule in quantum mechanics?
I’m interested in whether there are existing approaches in the literature that derive the Born rule, or modify quantum probabilities, using entropy-based arguments. In particular, I’m wondering about ...
3 votes
0 answers
148 views
Confusion about Choi-Jamiolkowski Isomorphism and supermaps
I am currently reading Transforming quantum operations: quantum supermaps. In short the paper is about transformations of quantum operations: Not only are the channels acting on quantum states, but ...
- 1,804
0 votes
0 answers
153 views
Alternative theories to traditional QM where the residual spectrum can be measured?
The spectrum of an operator $T$ may be decomposed into its point, continuous, and residual parts. The point spectrum is all $\lambda$ where $T-\lambda I$ fails to be injective, the continuous is where ...
- 169
3 votes
1 answer
133 views
Does fixing a $z$-axis and state also fix other coordinate axes in quantum mechanics?
Say we have an EPR pair $|\psi\rangle := (|00\rangle + |11\rangle)/\sqrt{2}$, where $|0\rangle, |1\rangle$ denote the Z-axis eigenstates of, say, a spin-1/2 particle. We measure both parts of the ...
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