Questions tagged [quantum-measurements]
533 questions
58
votes
14 answers
Why is it impossible to measure position and momentum at the same time with arbitrary precision?
I'm aware of the uncertainty principle that doesn't allow $\Delta x$ and $\Delta p$ to be both arbitrarily close to zero. I understand this by looking at the wave function and seeing that if one is sharply peeked its fourier transform will be…
catmousedog
- 728
33
votes
7 answers
Why do we use Eigenvalues to represent Observed Values in Quantum Mechanics?
One of the postulates of quantum mechanics is that for every observable $A$, there corresponds a linear Hermitian operator $\hat A$, and when we measure the observable $A$, we get an eigenvalue of $\hat A$ as the result.
To me, this result seemed to…
Ishan Deo
- 2,101
33
votes
8 answers
The importance of the phase in quantum mechanics
In introductory quantum mechanics I have always heard the mantra
The phase of a wave function doesn't have physical meaning. So the states $| \psi \rangle$ and $\lambda|\psi \rangle$ with $|\lambda| = 1$ are physically equivalent and…
Jannik Pitt
- 1,062
32
votes
5 answers
Am I in a superposition?
Someone looks at me. Now, they know my position and my momentum, with some uncertainty.
Therefore, they haven't measured either my position nor my momentum, since neither is known perfectly. They measured some other observable $O$, and found me in…
Juan Perez
- 3,082
28
votes
2 answers
Why are quantum effects of the apparatus ignored in quantum experiments?
When reading about various quantum experiments the apparatus often includes things like "semitransparent mirrors", "regular mirrors", "polarization filters" and others. Usually a photon or an electron is emitted, bounces around in the device, and is…
Vilx-
- 3,501
22
votes
1 answer
How is conditional probability handled in quantum mechanics?
In ordinary probability theory the conditional probability/likelihood is defined in terms of the joint and marginal likelihoods. Specifically, if the joint probability of two variables is $\mathcal{L}(x,y)$, the marginal likelihood of $x$ is…
Sean E. Lake
- 22,927
19
votes
1 answer
Is my interpretation of the underlying idea behind this 2022 Nobel Prize story “How physicists proved the universe isn’t real” more or less accurate?
I’m not very good at math but I frequently watch PBS Spacetime on YouTube to try to satisfy my curiosity about physics despite that.
In a recent video by Dr Ben Miles titled “How Physicists Proved the Universe Isn’t Real”, the discussion is of…
J.Todd
- 1,841
19
votes
2 answers
Are observables in QFT actually observable?
Consider some interacting QFT on a lattice (just to avoid infinitely large momentums). The size of the lattice is assumed to be much smaller than the size of the emergent particles (like in our world). Lets assume there are scientists living in such…
Pavlo. B.
- 2,605
16
votes
3 answers
What does the quantum eraser experiment tells us?
I am a beginner in this quantum-mechanics stuff. I understand the quantum eraser only from an experimental view. So I didn't understand the formalism that describes the quantum eraser. But what does the experiment tells us? Does the photon know that…
kame
- 949
15
votes
2 answers
How do they measure quantum observables in general?
In most introductory quantum mechanics books they mention as an experimental example, which demonstrate the quantum nature of measurement, the Stern-Gerlach experiment and how they measure the spin. But is this only quantum experiment in this…
Physor
- 861
15
votes
3 answers
Why are wavefunction collapses instantaneous?
From my understanding of quantum mechanics, when a wavefunction is observed, it collapses into a single state instantaneously (or at least in the length of a Planck time.) Is there a reason it has to take no time? Could it be briefly observed in a…
TLink
- 346
14
votes
3 answers
Why are there no clear experiments describing the exact boundary between classical and quantum sizes?
Why are there no clear experiments describing the exact boundary between classical and quantum sizes? Assuming we believe in interpretations of quantum mechanics (QM) that state during measurement (interaction with a classical object), the wave…
David
- 1,116
13
votes
6 answers
Does the collapse of the wave function depend on the observer?
Reading various articles on quantum physics paradoxes, I found the following interesting statement:
When a measurement is carried out inside of a closed lab, such a measurement leads to a collapse for inside observers, but it does not lead to a…
Matteo Brini
- 361
12
votes
6 answers
Why are expectation values of an observable important in QM?
I've been reading that expectation values of an observable is all what we can get and are the key quantities of the theory, but performing the same experiment many times would generate a distribution probability for the possible values of the…
user536450
- 303
12
votes
5 answers
Can the collapse of the wave function be modelled as a quantum system on its own?
Imagine I have an observer $\mathcal O$, a quantum system $\mathcal S$ with Hilbert space $V_{\mathcal S}$, a Hamiltonian $H$, a self-adjoint operator $A$ acting on $V_{\mathcal S}$. The system is in the (normalized) state $|\psi_0\rangle$ at time…
Lorenzo Pompili
- 385