This tag is for questions about the quantum mechanical measurement problem, such as e.g. the wavefunction collapse/decoherence. DO NOT USE THIS TAG for non-quantum measurements.

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110 views

On a measurement level, is quantum mechanics a deterministic theory or a probability theory?

Quantum mechanics is a non-commutative probability theory. As such, it fundamentally behaves differently from classical probability theories. This manifests itself most pronouncedly in the uncertainty ...
-1
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0answers
21 views

What's role of signal-to-noise ratio in the quantum measurement? [on hold]

Recently, I've been watching videos of MIT opencourse (http://www.youtube.com/watch?v=TcvY8Nt0ZGA&list=PLUl4u3cNGP62FPGcyFJkzhqq9c5cHCK32&index=2) taught by Nobel laureate Wolfgang Ketterle. ...
66
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0answers
2k views

Experimental test of the non-statisticality theorem?

Context: The paper On the reality of the quantum state (Nature Physics 8, 475–478 (2012) or arXiv:1111.3328) shows under suitable assumptions that the quantum state cannot be interpreted as a ...
3
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1answer
217 views

Superposition and simultaneous observation

Trying to understand superposition. Ok, so double slit experiment. The multiple paths the particle simultaneously travels interfere with each other but as it is absorbed, it chooses one "actual" ...
13
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4answers
2k views

Is the collapse of the wave function inherently time asymmetric?

Schroedinger's equation, as we all know, is time symmetric. In quantum field theory, we have to come up with a more sophisticated CPT reversal, but the essential point remains unchanged. However, the ...
2
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1answer
57 views

Different postulates and statistical interpreations of quantum mechanics

Hi I have a query about the difference of two aspects of the statistical interpretation of quantum mechanics given in the popular introductory quantum mechanics books "Introduction to Quantum ...
0
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3answers
104 views

Why diaphragm in diffraction experiment using electrons is quantum object?

In the book Quantum Mechanics - Volume 1 written by Albert Messiah, page no. 142-143, author says: ...But the diaphragm is a quantum object, just like the electron. Its momentum is not defined to ...
42
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9answers
6k views

Extension of Schrödinger's cat thought experiment

My question is quite simple. In the thought experiment of Schroedinger's cat: When the scientist measures the state of the cat, its wavefunction collapses into either the alive or dead state. But ...
13
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1answer
473 views

Does measurement, quantum in particular, always increase the total entropy?

Measurement of a quantum observable (in an appropriate, old-fashioned sense) necessarily involves coupling to a system with a macroscopically large number of degrees of freedom. Entanglement with this ...
22
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6answers
3k views

Isn't the uncertainty principle just non-fundamental limitations in our current technology that could be removed in a more advanced civilization?

From what I understand, the uncertainty principle states that there is a fundamental natural limit to how accurately we can measure velocity and momentum at the same time. It's not a limit on ...
1
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1answer
48 views

Quantum state after change of magnetic field

I have the following conditions: $\lvert\psi(0)\rangle=\lvert+\rangle_x=\frac{1}{\sqrt{2}}\lvert+\rangle+\frac{1}{\sqrt{2}}\lvert-\rangle$. So the state at $t=T$ is $\lvert\psi(t)\rangle=\frac{1}{\...
0
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1answer
46 views

Want to measure entanglement of the state [closed]

Good day, I want to measure the state with concurrence and negativity. I do local unitary transformation with represented by $U\in SU(4)$ (Lie group). After the transformation (rotation of angle) ...
0
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1answer
40 views

Probabilities with the Density Matrix

The density matrix of the system is given by: $$ [\rho_{S}(t)]_{mn} = [\rho_{S}(0)]_{mn} e^{-i\omega_{0}(m - n)t} e^{-i \delta(t)(m^2 - n^2) - \gamma(t)(m - n)^2}, ...
0
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1answer
73 views

An example of a nonlinear but deterministic physical transformation in Hilbert space

Supposedly all physically realisable transformations are either linear or non-deterministic (measurements are not linear transformations, but they are non-deterministic, from the perspective of the ...
4
votes
2answers
328 views

Transition from one state to another in Quantum Mechanics

When we measure an electron's position we know that the wave function $\psi$ peaks at the measured position and the wave function as a function of momentum is a harmonic function. When it makes the ...
21
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11answers
2k views

Why can't the outcome of a QM measurement be calculated a-priori?

Quantum Mechanics is very successful in determining the overall statistical distribution of many measurements of the same process. On the other hand, it is completely clueless in determining the ...
3
votes
1answer
227 views

In quantum weak measurement, what kind of theory replace Copenhagen interpretation?

Here, I denote the initial states of device and quantum system as $|\Phi_\textrm{in}\rangle$ and $|\Psi_\textrm{in}\rangle$. The measurement interval is $[t_i,t_f]$, after measurement, the device and ...
5
votes
1answer
172 views

Our choice of basis surely cannot effect possible outcomes of a measurement?

Common sense says that, of course, the outcome of a measurement on a quantum system cannot be affected by what base we choose to represent it in. However, while studying QM text, it seems like they ...
0
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1answer
47 views

State and measurement after a sequence of measurements

Hi I just want to confirm my interpretation of the following question: Let the quantum state be given as $$|\psi_0 \rangle = [\sqrt{2}|\phi_1 \rangle + \sqrt{3}|\phi_2 \rangle + | \phi_3 \rangle + |\...
0
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1answer
63 views

Quantum entanglement and affecting the particles [duplicate]

I am trying to grasp some aspects of the quantum entanglement, but the existing resources (including some of the links here) seem a bit confusing. I am trying to find an answer to the following ...
3
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3answers
106 views

What happens when we manipulate the observer in the double slit experiment?

Consider the following alterations to the double slit experiment: turn the observer on and off rapidly move the observer further and further away, until the point at which it can no longer ...
2
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2answers
56 views

How can an electron's magnetic moment precess around the direction of an external magnetic field?

I am reading this article: The Nature of the Electron by Don Lincoln in The Physics Teacher, Volume 54 (2016), pg. 203, and I ran across the part where he talks about measuring the magnetic moment of ...
7
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1answer
67 views

Does spontaneous emission count as a measurement?

It is my understanding that in the density matrix formalism for open quantum systems the environment-induced measurements/collapses/projections are accounted for by the Lindblad superoperator (from ...
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1answer
67 views

Is superposition state of SHO ever observed? [closed]

Feynman says, "It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong." So, is superposition state of Simple ...
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2answers
115 views

Do conservation laws contradict quantum mechanics? [closed]

Take for example the double-slit experiment interpreted in the Copenhagen sense. The particle leaves as an object with mass, yet passes through the slits as a massless wave, only to collapse again as ...
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2answers
79 views

Variation of schrodinger cat replaced by quantum computer

In the "classical" imaginary Schrodinger's cat experiment, which seems to be no longer serious, or at least irrelevant, by many (some?) people, everything is explained away by decoherence. Now, let ...
4
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4answers
245 views

How does one describe a state with a density matrix after measuring position?

My question is about position measurement in non relativistic quantum mechanics. I've been taught that when you measure the value of an observable for some state of a system described by $|\psi\rangle$...
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1answer
58 views

Implications of weak measurement on entanglement

What are the implications of weak-measurement on entangled particles, and how does that resolve the problem of non-superluminal quantum "communication"? If I understand correctly, entangled particles ...
2
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0answers
57 views

Schrödinger's cat experiment in the context of decoherence

I am trying to visualize in more detail the Schrödinger's cat experiment in the context of decoherence. Please let me know if the description below makes sense or is somehow flawed. If the ...
0
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0answers
21 views

Precession or projection?

A spin of 1/2 in a magnetic field is known to rotate around the direction of the field. But then it is also stated that it must be projected spin up or down. Is this simultaneous or happens after ...
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1answer
60 views

Measurements in quantum mechanics

Why does measurement change things? I read that measurement changes things because we have to bounce photons off an object to 'see' it and that changes its position, momentum etc... But on the other ...
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2answers
86 views

Is the momentum of a particle both uncertain and, independently, also random?

Is momentum of a particle "random" because it is uncertain, or is it uncertain in addition to being random? Is the uncertainty principle and quantum randomness different names the same physical ...
7
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2answers
474 views

How can we know the state of a quantum system?

One of the postulates of QM states that given a system in a state $|\psi\rangle$ and given an observable $A$ whose eigenstates are $|\phi_i\rangle$, then the state of the system can be expressed as a ...
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5answers
2k views

Does the observer or the camera collapse the wave function in the double slit experiment?

Ok so if we setup a camera before the slit we will find a single photon and will follow through accordingly, likewise by having a camera setup after the slit, we can retroactivly collapse the wave ...
4
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3answers
310 views

Are the authors saying that the observer effect plays no role in Bohr's thought experiment of the Heisenberg uncertainty principle?

Here is an excerpt from Eisberg & Resnick's Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles. Here is introducing Bohr's though experiment to establish a physical origin for the ...
0
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1answer
36 views

Measuring compatible observables in quantum mechanics

Suppose a particle that is under a quantum oscillator potential and is, initially, in the state $\Psi(x,0)=\frac{1}{\sqrt3}\phi_1(x)+\sqrt{\frac23}\phi_2(x)$, where $\phi_1(x)$ and $\phi_2(x)$ are ...
5
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4answers
406 views

Is commutation relation an equivalence relation?

I'm now learning quantum mechanics with Liboff. In the book it deals with "a compete set of mutually compatible observables" in order to make a state maximally informative. How can one find such set? ...
4
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1answer
59 views

Born-like measuring rule in classical experiments

this 2011 paper "Born's rule from measurements of classical signals by threshold detectors which are properly calibrated" by Khrennikov investigates the theoretical possibility of Born-like ...
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2answers
65 views

Linear Combinations of Energy Eigenfunctions in 1D

Given that a particle is in a state defined by the wavefunction: $$\Psi (x,t) = \psi_0(x)e^{-iE_0t/\hbar}+\psi_1(x)e^{-iE_1t/\hbar}$$ where $\psi_0(x)$ and $\psi_1(x)$ are the energy eigenfunctions of ...
2
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1answer
54 views

Wavefunction Collapse

I believe my Lecturer and the textbook have contradicted one another. My lecturer gave the example that if the spatial part of the wavefunction of a particle is given by $\psi(x) = c_1\psi_1(x) + c_2\...
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2answers
81 views

If I repeated a quantum measurement, would it be the same? [closed]

I was thinking about the probabilistic nature of quantum mechanics and that if I measured the position of an electron twice in succession, the outcomes would depend on a probability. However, what if ...
0
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0answers
29 views

Detecting position of electrons [duplicate]

To detect particles like electrons, why would the accuracy of the position determined be affected by the wavelength of EM wave used?
4
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1answer
80 views

What is a weak value really?

There have been a lot of recent experiments performing weak measurements. Some of the conclusions seem to be quite surprising (e.g. this paper) and there is still debate if the weak measurement is ...
1
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1answer
57 views

Quantum non-locality with commuting measurements?

We consider the Bell scenario, in which Alice and Bob share an entangled pure quantum state $\mid \Psi \rangle_{AB}$. Alice gets an input in the set $\{1,2\ldots X\}$ and Bob gets an input in the set ...
10
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1answer
246 views

Did Leggett and Caldeira solve the measurement problem?

In 1983 Leggett and Caldeira published a paper (see also here) that shows the evolution of the density matrix in a dissipative system. Follow-up work by Zurek and others shows the relevance to ...
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4answers
2k views

Uncertainty in Uncertainty?

I was reading about how the Planck's Constant can be measured with LEDs, which made me think about this question. The Heisenberg Uncertainty Principle states that: $$\Delta x \Delta p \ge {\hbar\...
0
votes
1answer
28 views

Given any two quantum states and the information that the system is in one of these two states

Given any two quantum states and the information that the system is in one of these two states, one cannot reliably devise a single measurement which could determine with certainty which state the ...
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9answers
6k views

Hidden observers in Double Slit experiments - Do they matter?

I'm still struggling a bit with some ideas around double slit experiments. One that keeps cropping up for me is the role of observers. Imagine a classic double slit experiment with a hidden observer ...
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2answers
174 views

“Randomness” versus “uncertainty”

Highly rated PhysicsSE contributor @CuriousOne regularly makes the following claim about quantum mechanics (e.g. here): There is no randomness in quantum mechanics, there is only uncertainty. I ...
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0answers
55 views

How does the two slit experiment work?

Whilst going through 'A brief history of time', I faced difficulty in understanding the two slit experiment. How can an individual electron cause fringes on screen? I was unable to understand it? ...