# Tagged Questions

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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### What is the name of basis states of bulk k.p Hamiltonian?

A k.p Hamiltonian for a bulk material can be represented by 8x8 matrix in basis of $|S\uparrow\rangle$, $|S\downarrow\rangle$, $|X\uparrow\rangle$, $|Y\uparrow\rangle$, $|Z\uparrow\rangle$, ...
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### About de Broglie relations, what exactly is $E$? Its energy of what?

Well, you may know de Broglie relations, here they are: $$E = h\nu, \quad\quad p = \frac{h}{\lambda}$$ My question is simple: What exactly is $E$? Is it the total energy? Maybe only kinetic energy? ...
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### Global and relative phases of kets in QM

In one of the questions I'm trying to solve it is asked to, first, compute probabilities for the respective results of the Stern-Gerlach measurements performed on each state $\lvert\psi_1\rangle$, ...
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### Probability of measuring a pure qubit state after some unitary rotation [closed]

Suppose I have the prepared state $$|+\rangle = \frac{|0\rangle + |1\rangle}{\sqrt{2}}$$ and the unitary $Z_{\pi/2}$ which rotates a state in the Bloch sphere by $+\pi/2$ about the $z$-axis. As I ...
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### If we had a “perfectly efficient” computer and all the energy in the Milky-way available, what number could it count to?

The idea for this question comes from an example in cryptography, where supposedly 256-bit symmetric keys will be enough for all time to come (brute-forcing a 256-bit key is sort-of equivalent to ...
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### Does anyone know how/where I could view the Double slit experiment in person? [closed]

I want to see the double slit experiment in person! The one where the observer effects the wave/particle state of an electron. Where would I be able to view this experiment? Is it on display in any ...
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### Is there any atom which is dia-electric?

Take an atom. Suppose we impose some magnetic field on it. For some atoms, the energy increases---this is a phenomenon of diamagnetism. The question is, how about an electric field? Can the energy ...
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### Are electron densities and electrostatic forces possibly responsible for gravity? [closed]

The idea I had was that whilst electrons usually orbit within the Bohr radius of atoms there is a chance of them appearing significantly further away than that, meaning that if all the electrons in ...
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### Question on doing the integral for Fermi golden rule

Today in the lecture, my professor did something which confused me As an example, we consider the photoelectric effect, in which an electron bound in a Coulomb potential is ionized after ...
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### Parity operators and spin

Consider the following excerpt from Weinberg's Lectures on Quantum Mechanics: I follow everything up until the last statement in the excerpt. In fact, from other things I've read, it seems that one ...
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### Atom Particles Relationships

I am an agriculture student, and we study tons of chemistry, and despite I took the exams I still have a good doubt on atoms. Through my studies I would say electrons are very tiny containers of ...
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### Why can one suppose $\alpha^i$ and $\beta$ matrices in the derivation of the Dirac Equation?

On the derivation of the Dirac Equation one usually supposes that it is possible to write $$E = \mathbf{\alpha}\cdot \mathbf{p} + \beta m.$$ One then deduces that in order to have $E^2 = p^2+m^2$ it ...
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### Local Phase Transformation of the Dirac equation

The Dirac Equation ("free Dirac") is a relativistic Equation of Motion (EoM) for a free ($V=0$) Spin $1/2$ particle (like an electron). The free Dirac equation is invariant under global phase ...
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### Fermi energy of electron gas with electrostatic interaction

I have been given the following exam question and am unsure how I would go about solving it: Consider the case of a one-dimensional metal, consisting of a chain of $N$ positive charges $+q$ ...