Questions tagged [quantum-mechanics]

Quantum mechanics describes the microscopic properties of nature in a regime where classical mechanics no longer applies. It explains phenomena such as the wave-particle duality, quantization of energy and the uncertainty principle and is generally used in single body systems. Use the quantum-field-theory tag for the theory of many-body quantum-mechanical systems.

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1answer
60 views

Why the randomness in glass/water/air does not destroy coherence of light over fairly macroscopic scales?

When light passes through glass/water/air, photons are absorbed and re-emitted by the chemical bonds, so that the speed of light in medium is reduced. However, in these media, it would appear that the ...
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1answer
103 views

How much can a wave function tell us?

We can not predict the future by getting the velocity and position of particles since it’s not possible to get both of these together due to the uncertainty principle. But, according to Hawking’s book ...
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When an electron changes its spin, or any other intrinsic property, is it still the same electron?

I am not asking why an intrinsic property, like spin can have more then a single value. I understand particles (electrons) can come to existence with either up or down spin. I am asking why it can ...
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0answers
15 views

Why is the angular momentum of a 3D kicked rotor non-negative?

We know that for a 2D kicked rotor the angular momentum quantum number can be any integer from minus infinity to infinity. However, for a 3D kicked rotor this is not the case: it can only be positive ...
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Reversing the spin of quantum entangled particle

If we have a pair of quantum entangled electrons, does reversing the spin of them affect the other. I mean will that also reverse the spin of the other electron.
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1answer
49 views

What does a coordinate representation of density matrix mean?

A coordinate representation of density matrix $\rho$ is defined as $$ \rho (x, x') \equiv \left<x\right| \rho \left|x'\right> .$$ When $x = x'$, this expresses a probability where a particle ...
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0answers
13 views

Can all CFT state be prepared through scale invariant MERA

It is known that in numeric computation, scale invariant MERA is useful for representing a CFT vacuum state. Is the converse true? i.e. all CFT vacuum state (the quantum state with translation and ...
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3answers
129 views

Eigenvalue of a Hermitian operator are always real. A contradiction

f(x) = $e^{-kx}$ $P_x$f(x) = -kih$e^{-kx}$ Hence, eigenvalue = -ikh
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Can there be a mean of a quantum state

If $e_i$ is one of the basis, does $\langle e_i\rangle$ have a significance? The mean of a physical variable $a$ associated with the operator $A$ is $\langle\psi|A|\psi\rangle$, but can there be a ...
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2answers
92 views

Relationship between $\star$-products in phase-space QM and NC geometry

What exactly is the relationship between $\star$-products in phase-space quantum mechanics, i.e. $$ (f \star g) (x,p) = f(x,p) e^{\frac{i \hbar}{2} ( \overleftarrow{\partial_x} \cdot \overrightarrow{\...
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1answer
57 views

Question about a point in Srednicki's QFT book

On page 6, Sredniciki says (taking into account the erratum), that the "simplest possibility is for Alice and Bob to agree on the value of the wave function at a particular space-time point". This ...
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1answer
45 views

Beta decay of beryllium into boron: conservation of spin violated?

I found following reaction in my lecture: $$^{10}Be_{6} \rightarrow ^{10}B_5+e^{-}+\overline{\nu}_e$$ since Beryllium has an even number of protons and neutrons its nuclear spin is $0$. However, ...
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1answer
68 views

Why do photon only interact with atoms whose electronic orbitals are at very particular energies? [duplicate]

From https://www.sciencealert.com/physicists-made-a-new-part-matter-part-light-particle-with-a-shake-of-an-atom: Photons - those fundamental particles of light - have a slew of interesting ...
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1answer
39 views

Are individual atoms inside a molecule (covalent) still really EM neutral?

I am not asking about ionic bonds. I am in this case asking about covalent bonds. I have read this question: Why do atoms repel when closer but attract when farther apart? Where Swike says: ...
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3answers
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What does it mean that no two electrons with the same spin can occupy an atomic orbital?

First of all my notion of orbitals was that they are nothing more than the path traversed by an electron and more that it is the wave of the electron spread around the nucleus . So going by that ...
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0answers
72 views

Is resonance an energy eigenstate?

In the particle physics book of Martin & Shaw, they used QM to derive the decay distribution, namely, breit wigner formula. What confused me was that, here they assumed the resonance state $\psi_0 ...
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1answer
19 views

How does one determine the number of eigenstates of a system with a given spin? [closed]

I have had a true/false question in a practice exam stating: For a spin 3/2 system (S=3/2), there are only four spin eigenstates. which is true. (solutions) I do not understand how one can ...
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2answers
91 views

Is it correct, according to quantum mechanics, to say that an observer can get entangled with the measured system?

Entanglement applies to any physical system, observers included. However, what one typically means by the observer being entangled with the measured system seems not to comply with the requirements ...
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0answers
26 views

Constant term in the periodic Poisson equation [closed]

When we solve the Poisson equation with periodic boundary conditions we have to ensure that the integral of the right hand side is zero. That is, we neglect the constant term of the Fourier expansion ...
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0answers
39 views

What is the simplest circuit on a quantum computer that “proves” superposition/entanglement

I am a Physics Teacher at pre university level. Tunneling, wavefunctions and uncertainty are on the syllabus on a very vague way. Quantum superposition and entanglement are not but tie in well and I ...
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1answer
64 views

Gravitons (real) passing through a black hole

This is not a duplicate. I do not ask about the gravitational field of the black hole, or why it extends farther out then the event horizon. I do understand that the gravitational field is described ...
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2answers
50 views

Do the pure states in the decomposition of a density operator need to be orthonormal to each other?

So, I was studying quantum computation using the book Nielsen and Chuang and it stated a theorem known as "Spectral Decomposition theorem" $$A=\sum _{i}\lambda _{i} | i \rangle \langle i|$$ I infer ...
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How to obtain a unitary time evolution that leads to the Lindblad equation on tracing out?

It is known that given the time evolution of an open quantum system by Kraus Operators, one can rewrite it as a unitary time evolution of a bigger system. That is, given Kraus operators on the Hilbert ...
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1answer
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Stability/decay, are they boolean or not, or does QM probabilities overrule this?

This is not a duplicate, I am not asking whether the proton is a stable particle, or why it is. I am asking about the definition of stability/decay whether it is boolean or not. I have read this ...
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1answer
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Does the Breit–Wigner formula indicate “violation” of energy conservation?

In the quantum mechanical derivation of Breit–Wigner formula, for example, in the particle physics book of Martin & Shaw, we assume if the resonance particle $X$ is in an initial energy state with ...
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2answers
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What alternatives did Einstein ponder to supersede quantum mechanics? [closed]

It is well known that Einstein was unhappy with quantum mechanics and it is also well known that he worked on new ideas. Is there surveys or books or research notes or lecture notes that talk about ...
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2answers
22 views

Orbital wavefunction for a system of two electrons

I am new to this forum! I write here hoping someone can help me. I have found a statement in my quantum mechanics book that I really don't understand. We have a system of two electrons. If both are ...
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1answer
32 views

Questions about scattering matrix theory of non-free particles

Hi,I have a problem for scattering matrix theory. Currently, the book I've read is about collision between free particles. What if collision between non-free particles? For example, in lattice, only ...
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0answers
37 views

Does a gapped, local and non-frustration-free 1D Hamiltonian have an exact MPS groundstate?

(Fact 1) It is known that any arbitrary matrix product state (MPS) is a unique groundstate of a gapped, local, and frustration-free parent Hamiltonian (Perez-Garcia et al. 2007). It is by Fact 1, that ...
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36 views

Projecting out interactions with high-energy states

I have a single-particle Hamiltonian with a discrete energy spectrum $E_{n,k}$ with two degrees of freedom, $n=0,1,2,3...\infty$ and $k$ which has only a few possible values. $E_{n,k_1}$ and $E_{n,k_2}...
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1answer
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Why are massive particles clocks? Or are they not?

I recently came across a public lecture "Dark matter decay?" by Sir Roger Penrose. In his lecture he states that the two equations $E=mc^2$ and $E=h\nu$ can be combined to form a formula for the ...
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Interpretation of the single particle spectral function in a spin liquid

Experimenter here (fair warning). I've had a question nagging at me for some time revolving around this paper by Tang et al. I don't have a good intuition for what the single electron spectral ...
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1answer
68 views

Is the normalization of the wave-function preserved due to…?

Is the preservation of the inner product the same thing as the vector length of the wave-function staying constant with it's rotation through some R2 plane (ie it's evolution through time), that is ...
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0answers
30 views

Is it possible to determine the nuclear spin with the hyperfine structure experimentally?

I was wondering if there are experiments that allow to determine the nuclear spin with the hyperfinestructure, since it interacts with the magnetic moment of the nucleus there must be a correlation. ...
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0answers
37 views

What is the characteristic of a property?

Background: The following two observations are , in my understanding, pretty much accepted in quantum theory: Location is a property which is not preexisting but is established by measurement. It is ...
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2answers
262 views

What is the cause for the fine structure?

What is the cause for the fine structure? I know that it describes the splitting of the spectral lines of atoms but I don't understand its cause, any help is appreciated
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1answer
64 views

Is it possible to isolate a lab to the extent that the Wigner's friend experiment becomes possible?

It appears to me that experiments such as " Wigner's friend" or "Schrödinger's cat" cannot be done because the contents of the lab or box is always known for an outside observer in the form of ...
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0answers
22 views

Beam splitters: fiber vs. free space

In quantum optics, are fiber beam splitters described by the same operators as free space beam splitters? For example, a free-space non-polarizing beam splitter introduces a phase shift for the ...
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0answers
32 views

Can you lock multiple magnets to a single superconducting puck?

We've all seen the videos of a superconducting puck floating above a track of magnets. Could another magnet be locked above the puck? I ask this with the intention of attaching vacuum insulator to the ...
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1answer
45 views

Wave with 3 different wavelengths

Can a 3D wave have 3 different wavelengths ? Is there any example of such a wave ? I can't imagine such a wave because then 3 different frequencies must exist.
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1answer
49 views

Angular momentum operator in different bases

The Eigenvectors of $L_3$ (for spin 1) are $\left| m \right>$ with $m=1,0,-1$. One can compute the matrix $D_i=\begin{pmatrix}\left< 1 \middle| L_i \middle| 1 \right> & \left< 1 \...
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0answers
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Kraus operators for two interacting harmonic oscillators: Problem with the calculation (Ex. 8.21 of Nielsen-Chuang)

I'm working with Exercise 8.21 of the Nielsen-Chuang book on quantum information. It illustrates the amplitude-damping quantum channel by the interaction between two harmonic oscillators (the first ...
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1answer
69 views

Why does the wave function of a non relativistic particle flatten out over time?

The Hamiltonian I used is the classical one with no potential energy: H=p^2/2m $$i \hbar \frac{\partial \psi}{\partial t} = -\frac{\hbar^2}{2m} \frac{\partial^2 \psi}{\partial x^2} $$ I want to gain ...
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0answers
31 views

5 qubit quantum error correction property [closed]

Having $$\eqalign{ |v_0⟩&=|00000⟩+|11000⟩+|01100⟩+|00110⟩+ {}\cr &\quad |00011⟩+|10001⟩−|10010⟩-|10100⟩− {}\cr &\quad |01001⟩−|01010⟩−|00101⟩−|11110⟩− {}\cr &\quad |...
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3answers
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Is wave function an analogue of probability amplitude or a ket vector from Dirac notation?

The way I was introduced to wavefunctions was in form of Dirac notation: $$\psi(x)=\langle x| \psi \rangle$$ i.e. the probability amplitude of going from state $\lvert \psi \rangle$ to state $\lvert ...
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1answer
42 views

Can momenta eigenstate written in term of $x$ be an eigenfunction of position?

Being non-commuatable operators, momentum and position cannot have simultaneous eigenfunctions. But in "Theoretical Minimum: QM" by Lenny Susskind and Artsy Friedman, in explaining Heisenberg's ...
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1answer
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Transformation acting only on one of two qubits

Suppose you have a non-polarizing beamsplitter (NPBS) with 2 outputs: A and B. You place a polarizing beamsplitter (PBS) at the output A, and want to follow the evolution of the quantum state after ...
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1answer
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Can the “external” field in DFT be that of an electron?

In my experience with (Kohn-Sham) density functional theory, the external potential is due to a massive particle like a nucleus or ion. I have never seen an electron be the source of the potential, ...
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1answer
83 views

Delayed Choice Quantum Eraser with one additional detector at the slits?

Please take a look at this experiment https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser#The_experiment_of_Kim_et_al._(1999) . You can also look at this youtube video https://www.youtube.com/...