Linked Questions
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Why aren't particles constantly "measured" by the whole universe?
Let's say we are doing the double slit experiment with electrons. We get an interference pattern, and if we put detectors at slits, then we get two piles pattern because we measure electrons' ...
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53
votes
5
answers
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Why doesn't a typical beam splitter cause a photon to decohere?
In many experiments in quantum mechanics, a single photon is sent to a mirror which it passes through or bounces off with 50% probability, then the same for some more similar mirrors, and at the end ...
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22
votes
3
answers
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Why doesn't superselection forbid almost every superposition?
A superselection rule is a rule that forbids superposition of quantum states. As stated by Lubos here, one cannot superpose states with different charges because of the conservation of charge:
An ...
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8
votes
8
answers
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How do “many worlds” theorists explain particle double-slit interference patterns?
In the past couple of weeks I’ve re-watched Sidney Coleman’s wonderful 1994 lecture “Quantum Mechanics” in Your Face” and Sean Carroll’s 2019 Google talk “Something Deeply Hidden: Quantum Worlds & ...
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14
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3
answers
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When light reflects off a mirror, does the wave function collapse?
This question is specific to the Copenhagen interpretation, which states that the wave function collapses on interaction. If we have a beam of light reflected off a mirror, whether you see this light ...
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7
votes
2
answers
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In the double-slit experiment, why don't slits destroy interference pattern?
In the double-slit experiment with electrons, if a detector is placed in one of the paths to show where the electron is passing, the wave nature of the particle is lost and you have no interference.
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4
votes
2
answers
203
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Are superpositions contagious?
Does quantum mechanics really predict that a particle prepared in a state of superposition of spin will result, after being measured by an appropriate instrument (Stern-Gerlach device), in a ...
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6
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1
answer
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How does a reflected photon keep its entanglement [duplicate]
I'm curious how a photon from an entangled pair, travelling through say a delayed choice quantum eraser experiment, can bounce around between mirrors but remain entangled with its partner photon, when ...
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3
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Meaning of phase relationship for a superposition of states
I have studied an introductory course in quantum mechanics, and yet I still do not understand the significance of a phase difference between quantum states that a system is in a superposition of. In ...
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5
votes
3
answers
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What does Copenhagen interpretation tell about Mach-Zehnder interferometer with single photons?
Reading many sources on the net it is not clear what Bohr et al. says concretely about MZI. Maybe they say that the particle doesn't exist during this flight and appears only on the detectors? Maybe ...
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3
votes
3
answers
202
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How do photons interact with a very fine edge?
Suppose you have some material which has a low reflectance and is opaque to an incomming photon when the angle of incidence is small.
Now take that material and make a narrow (20:1 width:length ratio ...
7
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1
answer
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When does Quantum Mechanics Measurement lead to Mixed State
Consider the context of the Stern-Gerlach experiment. As is stated on numerous sources (e.g. Feynman Lectures, MIT Lecture), a silver atom in the state of $\vert+z\rangle$ that is put through a Stern-...
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0
votes
2
answers
501
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Is an atom's photon recoil on absorption affected by a beam splitter?
Consider a photon with momentum p approaching a half-reflective mirror. After encountering the mirror it is in a superposition of momentum +p and -p and the mirror (we presume) is in a superposition ...
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4
votes
3
answers
236
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Why is quantum measurement not happening all the time? [duplicate]
I have a question which may be very naive yet I have no answer. I studied undergraduate quantum mechanics 4 years ago now and even if I studied more advanced stuff like QFT I feel like I don't ...
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3
votes
2
answers
306
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Why can’t macroscopic objects like apples be isolated but a photon can?
I read that the reason that macroscopic superpositions decohere is because it’s difficult/impossible to isolate macroscopic objects from the environment. Why is this so difficult/impossible and why ...
-1
votes
1
answer
303
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Double slit experiment: why does the barrier between the slits not collapse the wave function?
During the double slit experiment the electron passes through both slits, how come the material between the two slits does not collapse the wave function and we get an interference pattern? On the ...
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1
vote
1
answer
193
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Will the mirror reflection make the interference pattern disappear?
I have read this question:
When light reflects off a mirror, does the wave function collapse?
where dmckee says:
Now, even giving a precise statement of what makes a "measurement" is non trivial, ...
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1
vote
1
answer
194
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Stern-Gerlach apparatuses in Feynman lectures
The figure above is taking from the Feynman lectures.Here $S$ ,$T$,$P_1$ and $U$ are Stern-Gerlach apparatuses.
$S$ and $T$ measures the spin in the $z$ direction where $z$ is pointing out of the ...
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0
votes
1
answer
119
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Why don't quantum gates get entangled with the qubits they operate on?
I am currently taking a first course in quantum information theory and having trouble understanding how quantum gates are possible in practice. How does a unitary quantum gate evolve the qubit without ...
0
votes
2
answers
109
views
The wavefunction's knowledge of its surroundings
This seems to me it must be trivial, but I have not been able to grasp it.
As I understand it, the wavefunction crucially depends on its immediate surroundings, whether it be a nucleus, a box, etc. ...
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4
votes
0
answers
67
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Why does a photon crossing a beamsplitter end up in a coherent superposition? [duplicate]
A single photon, passing through a beamsplitter (BS), ends up in the state $\frac{1}{\sqrt2}(a_1^\dagger + a_2^\dagger)\lvert\operatorname{vac}\rangle$, which is a coherent superposition of the two ...
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0
votes
1
answer
54
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Time variable in a quantum eraser
Hello,
On the image above you'll find the display for the double slit "delayed choice quantum eraser" by Scully. I use this experiment for my question as it cames through trying to understand it.
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2
votes
0
answers
58
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Why does the imparting of momentum to a mirror not count as "observation" in Wheeler's delayed choice experiment?
Presumably light pressure would change the momentum of the beam-splitting mirrors in the interferometer version of Wheeler's delayed choice experiment. And it's not until the photon encounters the ...
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2
votes
0
answers
45
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Heisenberg’s Formulation of Quantum Mechanics and Particle Trajectories [duplicate]
In the article, under the history section, Heisenberg recounts a discussion with Einstein regarding Heisenberg’s matrix mechanics: “ He pointed out to me that in my mathematical description the notion ...
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