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 ...

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Finding Hamiltonian to imitate this physics

I have the following assignment problem I need help with A two state quantum system has basis states |+> and |->. These basis states are not eigenstates of the Hamiltonian of the system. At t = ...
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9 views

Singular density matrices

Consider the quantum states in an $n$-dimensional Hilbert space. They can be represented as $n\times n$ matrices with trace $1$. Or in generalized Bloch representation, they can be written as ...
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22 views

2x2 Matrices that are not valid quantum states

Given a 2-dimensional Hilbert space, quantum states can be expressed as $2\times 2$ density matrices. In terms of the Pauli matrices, or Bloch representation, they can be written as \begin{equation} ...
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Why the thermofield double is correspondent to 'maximally entangled' eternal black holes?

For a TFD state $\sum_ne^{-\beta E_n/2}|E_n>_L|E_n>_R/\sqrt{Z(\beta)}$arXiv:hep-th/0106112, the reduced density matrix is $\rho_L=\sum_n e^{-\beta E_n}|E_n>_L<E_n|/Z(\beta)$, obviously not ...
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38 views

A problem about the harmonic oscillator in Quantum mechanics

When I learned quantum mechanics by reading Griffith's book called Introduction to quantum mechanics, I was confused by his description. In Page 53 of the 2ed edition book, after got the recursion ...
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27 views

Using open system dynamics to define a quantum state

Background The density matrix of a closed quantum system with Hilbert space $\mathscr H$ evolves according to the von Neumann equation \begin{align*} i\hbar\dot\rho=[H,\rho]. \end{align*} Given a ...
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17 views

Quantum teleportation of atomic states that involve energy

With quantum computing the concern is with information, and when quantum teleportation appears, the states being transferred are often spin states. However, there is an energy difference, even if ...
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1answer
20 views

Will a charge attract or repel bar magnet?

I read in many answers that magnetic fields are basically electrostatic fields arose due to motion of charges usually. It gave me a question, Let there is a charge of 1C in vicinity, and a bar ...
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17 views

Gravitational imaging

As the most recent discovery in physics suggested the gravitational waves are now scientific fact.My question is: If that's the case so is it possible in principal to make perfect 3 d imaging of an ...
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1answer
41 views

Can photons undergo quantum tunneling?

Most leptons can do this so I'm curious to see if photons or any bosons at all can.
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30 views

Observer inside Schrodinger's box

Let's place a human observer with Schrodinger's cat inside the box. After some time, the inside observer finds out that the cat is dead. For an outside observer however, the state is a superposition ...
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2answers
33 views

microstates and internal energy

Consider a system having an internal energy U. The internal energy U is a macrostate parameter but has many microstates. What is the difference between 2 microstates for a given internal energy U?
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1answer
26 views

Kronig Penney Model [on hold]

Why can't we solve the two differential equations obtained $$\dfrac{d^2\phi}{dx^2}+\dfrac{2mE}{h^2}\phi=0$$ and $$\dfrac{d^2\phi}{dx^2}+\dfrac{2m(E-V)}{h^2}\phi=0$$ .... By the same method by which ...
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32 views

Tensor product distributive property?

Is it true that for vectors $a$, $b$, $c$, $d$ we have $$|a\rangle \otimes |b\rangle \langle c| \otimes \langle d|= |a \rangle \langle c| \otimes |b \rangle \langle d|?$$ So does this kind of ...
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1answer
23 views

Direct vs. indirect CP violation: theoretical foundations

I know very little about the difference between direct and indirect CP violation. I've been studying QFT from Peskin and Schroeder's "An Introduction to QFT", and they don't seem to address this ...
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14 views

Resonant Scattering and the Breit-Wigner Formula

Most of what I ask relates to the derivation found here: https://farside.ph.utexas.edu/teaching/qm/lectures/node91.html The basic idea is that we first consider zero energy s wave scattering from a ...
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27 views

Can something made of atoms be subatomic? [on hold]

Let's say there is an object made of atoms, this object is subatomic in scale. Can such a thing exists?
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30 views

Has Jaynes' argument for quantum mechanics as a possible theory of inference been debunked?

To my understanding, there is currently no scientific consensus on which interpretation of quantum physics is the correct one, if any. The most famous one, perhaps for historical reasons, is the ...
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53 views

Why quantum field theory is never used in engineering? [on hold]

Serious question: Why quantum field theory is never used in engineering? I know quantum mechanics plays a role in things such as lasers but I think laser engineers will only use tables and diagrams ...
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53 views

Identical particles and topological phases

I've been wondering about the argument that is always put forward for why the only possible identical particles in three dimensions or more are bosons or fermions. My question is related to a very ...
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1answer
61 views

Sakurai Exercise 2.17 (Harmonic Oscillator, Ladder operators) [on hold]

I the field of the harmonic oscillator and ladder operators I am trying to solve exercise 2.17 from Sakurai and want to proof the following relation $$ \langle x^{2n} \rangle = (2n - 1)!! \langle ...
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0answers
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Compton Scattering problem verification [on hold]

A photon emergre in an angle $\theta=\frac{\pi}{4}$ in a Compton scattering the kinetic energy of the photon is $K_i=2K_f$. I must find $K_i$ in $MeV$. $\lambda_f -\lambda_i= \lambda_c$. Then: ...
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1answer
58 views

What is the Quantum Mechanical Operator for Electric Potential?

I understand that charge and electric potential are conjugate observables in QM. See https://en.wikipedia.org/wiki/Conjugate_variables The quantum mechanical operator for charge, q, is simply equal ...
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23 views

Compton Effect, why does electromagnetic radiation lose less energy when hitting particles with greater mass?

One of the problems in Rae's Quantum mechanics book, at the end of the first section, relates to x-ray's scattering off either an electron or a carbon atom. Using the equation given to you in the book ...
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12 views

Writing Breit-Pauli spin-spin-coupling Hamiltonian as a sum of irreducible spin tensor operators

The spin-spin interaction part of the Breit-Pauli Hamiltonian for two electrons is essentially (omitting constants and the contact term) \begin{equation} \frac{3(\mathbf S_1 \cdot \mathbf r)(\mathbf ...
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1answer
56 views

Is all energy quantised?

I'm wondering if all energy is quantized? Can a particle with kinetic energy have any value of kinetic energy or is kinetic energy also quantized? My reason for asking this is that if a particle is ...
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42 views

Can two different objects or system of molecules have different temperatures, but having same internal kinetic energy?

If I take an extreme case, where a body has only an internal potential energy with zero internal kinetic energy, does this body have a temperature? Another question related to it: if two objects A and ...
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1answer
18 views

How do we acquire c-coefficients (“weights”, “probabilities”), when dealing with superposition of stationary states?

I searched quite a lot textbooks to answer this question, but I couldn't find anything satisfactory. This could be more general question, but it will be easier to just restrain to particle (say ...
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2answers
40 views

Confusion over basis sets in quantum mechanics

In a sense, I feel like I understand every step in the following (fairly typical) calculation, but I don't understand the conclusion. I consider a vector space spanned by two basis vectors ...
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37 views

The question is related to quantum mechanics [on hold]

The energy gap in germanium is 0.72 eV. Find the maximum wavelength of electromagnetic radiation which can cause transition from the valance band to the conduction band.
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1answer
45 views

flux quantization in superconducting ring

I am trying to understand SQUID microscopy from the ground up, so I am starting with flux quantization in superconducting rings. I found a nice presentation that covers some of the details, but I am ...
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1answer
28 views

Basic question about valence/conduction bands

In most introductions to solid state physics that I've read, they explain the origin, basically, of the band gap as follows: By doing a superposition of the standing waves inside a BZ, one can find ...
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0answers
54 views

Why the quantization of things is “forbidden” by classical physics?

I would like to know why we say that what gave the idea of Quantum Physics was the fact that things appeared to be quantized Why is it impossible to quantize things in classical physics? Why ...
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What is the mass defect of stable nucleus? [on hold]

What is mass defect of stable nucleus? A-positive B-zero C-negative D-less than unstable nucleus
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29 views

Converting a Bell state to a less entangled state by LOCC [on hold]

How can I convert a maximally entangled state, $ \frac {1} {\sqrt2} (|00\rangle +|11\rangle) $, to a less entangled state like, $$ cos(\theta)|u_0 v_0\rangle + sin(\theta)|u_1 v_1\rangle $$, by using ...
3
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2answers
91 views

Successor to Copenhagen Interpretation as Orthodox Interpreation of Quantum Mechanics

First, I read the questions FAQ for this and I hope this does not violate the rules. I am not asking for personal opinion, but for observations of hard evidence of trends on this subject. When I ...
3
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1answer
88 views

Can a particle have an imaginary mass?

Now, I'm not very involved in the physical sciences, beyond some high-school classes - though I did listen to some college lectures for physics. I was reading "Do Tachyons Exist?" by John Baez, which ...
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1answer
28 views

Degenerate perturbation theory

I don't understand the part about turning off the perturbation. What is meant by or what is he referring to when he says "upper" and "lower" states. Why must the "upper" state reduce to a ...
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1answer
96 views

Which non-Copenhagen interpretations of Quantum Mechanics have actually produced something useful? [on hold]

The question has most of it in the name. "Useful" may be interpreted as: measurably different from Copenhagen interpretation and true potentially measurably different and potentially true (i.e. not ...
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35 views

What is the correct setting for Kirchhoff's Law of Thermal Emission? [on hold]

For example, is the setting free space? Is it a surface? etc. So, what is the correct setting of Kirchhoff's Law of Thermal Emission?
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22 views

Why does the interference patterns in delayed choice quantum eraser cancel out? Why/how does it have a phase shift?

Talking about this experiment https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser#The_experiment_of_Kim_et_al._.282000.29 I am assuming that anyone able to answer my question is already ...
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21 views

Multiple contemporary measures of a photon's state

From quantum mechanics, when you polarize a photon using computational base and you try to measure its state using a diagonal base, you'll get with the 50% of probability $|0\rangle_{{} + {}}$ or ...
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1answer
49 views

Expectation value position of sine wave in infinite square well

Given a neutron (mass$\approx$939 MeV/c$^2$) in an infinite square well of size $a$, the value of the expectation value for position should be in the range $[0-a]$. I know that the general form of the ...
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1answer
53 views

$[a,a^{\dagger}]=1$ or $[a,a^{\dagger}]=I$

$[a,a^{\dagger}]=1$ or $[a,a^{\dagger}]=I$? On every book i read $[a,a^{\dagger}]=1$. But doesn't $a$ have infinite dimension? So why does the commutator go to a number which is one dimensional?
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1answer
81 views

Why is particle superposition still part of quantum mechanics? [duplicate]

After reading an article on Schrodinger's Cat, it seems that if we take the environment as an observer, that superposition cannot occur because all atomic and subatomic entities would be observed all ...
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0answers
52 views

Does an expectation value over only part of a wavefunction have physical meaning?

Does the expression: $$\langle p\rangle_{x=a, b} =\frac{\int_a^b \Psi(x)^*\,\hat{p}\,\Psi(x)dx}{\int_a^b |\Psi(x)|^2dx} $$ have any physical meaning when $\int_a^b ...
1
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1answer
38 views

Fetter & Walecka's derivation of second quantised potential term in many-particle TDSE

For the potential term in the Hamiltonian, I understand that we go through the same process as with the kinetic energy term. On the RHS of the TDSE, we get something like ...
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3answers
36 views

How does commutation between the hamiltonian and angular momentum operator (squared) imply conservation of Angular momentum?

So we are looking at central potentials in QM; The lecturer poses the question, when is $\textbf{L}$ conserved? He then considers the commutator of $\hat{H}$ and $\hat{L^2}$. We have; ...
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25 views

Momentum Operator [on hold]

Hello Community, I'm writing a paper on quantum mechanics and have a question about following formulas and the derivation of them. It's in german but I was told that I have an error somewhere in the ...
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0answers
27 views

Classical Quantum or Relativistic? [on hold]

An ensemble contains free electrons at 10^3 electrons per m^3 at 10^7 K. What can this ensemble be treated as: a Classical Quantum or Relativistic gas or in some overlapping domain?