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 ...

learn more… | top users | synonyms (4)

-1
votes
1answer
51 views

Expectation value [on hold]

Which of the following equations is correct? why?
0
votes
1answer
22 views

Under what conditions does a beam splitter entangle two input photons?

There is a dispute on PhysicsForums related to what are the conditions necessary for two photons to be entangled by a beam splitter. Lots of references given by the forum users but they never arrive ...
1
vote
3answers
97 views

Why is $np$ always equal to $n_i^2$?

For you guys who studied semiconductor physics must be familiar with the equation: $$np=n_i^2$$ I can understand why this is true for the intrinsic case (the broken bonds would always provide ...
1
vote
1answer
39 views

Show the Berry phase is invariant under $U(1)$ unitary transform [on hold]

Recall that $$\gamma_n = \oint A_n(R) \cdot dR = \oint \langle\psi_n(R)|i\nabla_R|\psi_n(R) \rangle \cdot dR.$$ Under the $U(1)$ transform, $$\psi_n \to \psi'_n \equiv e^{i\xi_n(R)}\psi_n,$$ where ...
0
votes
0answers
23 views

Production of entangled photons by spontaneous down conversion [on hold]

I have found that the conditional uncertainty product of position and momentum of any one photon produced in the spontaneous parametric down conversion process using a gaussian pump beam decreases ...
2
votes
0answers
26 views

Kitaev chaing, time reversel symmetry, particle hole symmetry

I was wondering if the Kitaev chain has time reversal symmetry. I think it probably doesn't because by staking Kitaev chains it is possible to create a so called Chern insulator with propagating ...
1
vote
1answer
42 views

Decoupling coupled differential equations in dynamically coupled two state system

Consider the following dynamically coupled two state hamiltonian, $$H=-B\sigma_z-V(t)\sigma_x$$.Taking the eigenfunctions of $\sigma_z$ ($|+> and |- >$) as basis vectors, we have the wave ...
0
votes
0answers
38 views

How many observations over an atom can be made?

In order to determine the orbitals of any unknown atom , is it possible to make and record direct observations of electrons around the atom without disturbing the atom ? How many observations can one ...
0
votes
0answers
16 views

What is Loewdin downfolding method?

I am a student in solid state physics. I wonder if somebody could explain the mathematical background of downfolding method that is often used by Ole K. Anderson. Which restriction (conditions) to ...
0
votes
0answers
32 views

Solving Schrodinger's Equation in Bunimovich Stadium Boundary Condition [closed]

I need to solve, as mentioned, Schrodinger's equation in a Bunimovich stadium-shaped infinite potential well with Dirichlet BC Numerically (this isn't possible analytically). In order to do so, I need ...
4
votes
2answers
115 views

Does QM unequivocally violate the law of bivalence?

I had heard that QM violates the law of bivalence. Does anyone claim that?
0
votes
2answers
88 views

Unitary operators evolving the set of Pauli matrices

Consider the Heisenberg picture of Quantum Mechanics. For a two state system we have the Pauli matrices evolving according to the relation $$\sigma_i(t)=U^+\sigma_i(0)U$$ where $U=e^{-iHt/\hbar}$ and ...
0
votes
1answer
62 views

Quantum Mechanics: how exactly does “delta function normalization” work for eigenfunctions in 1-d free space case?

The definition of "delta function normalization" says a basis of eigenfunctions of a particle in free space are orthonormal when ...
0
votes
2answers
44 views

Order of operators and numbers inside a bracket

I had an argument with my professor. Let $H$ be an operator (e.g. hamiltonian). Let capital $X$ denote the position operator. Let $f$ and $g$ be functions of $X$ that do NOT commute with $H$. Now ...
0
votes
1answer
32 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 ...
1
vote
0answers
47 views

Single particle diffraction: how is this possible?

The intensity distribution of diffraction patterns are typically explained by looking at points of constructive and destructive interference of the diffracted waves on the detector. These diffracted ...
2
votes
1answer
64 views

Diagonalisation: Schmidt vs eigenvalue - when to use which?

In physics we encounter diagonalisation of matrices or operators in a variety of areas. But there are different kinds, the main two being Schmidt decomposition and eigenvalue diagonalisation. The two ...
0
votes
1answer
22 views

Is linear polarization of entangle photons in 2-particle decay always correlated?

In Aspect's paper "Bell's Theorem: The naive..." and in an 2002 AJP article by Dehlinger and Mitchell "Entangled photon apparatus..." the photons are described to be in the $|xx\rangle+|yy\rangle$ ...
0
votes
0answers
23 views

Relative motion in particle measurements

I was thinking about measurement of particles at almost-zero energies/temperatures and the movement associated with it. Compared to an observer next to the particle who sees the particle moving at ...
0
votes
1answer
18 views

Energy (voltage) correction on energy level between metallic electrodes with dielectric and accounting for work function difference

My goal is to understand how to correct for the field drop and the work function difference when performing electrical measurements on a certain energy level of a sandwiched system. The situation is ...
1
vote
0answers
47 views

I want to know about the quantization of mass [duplicate]

Is mass quantized? If yes, then why do we see all amounts of mass? Well , can it be said quantized just for smaller masses?
-5
votes
1answer
39 views

CNOT Gate for quantum systems [on hold]

A you know the CNOT gate is 4 by 4 matrix, is there any way to show it by a 2*2 matrix? if yes what will be the elements?
0
votes
0answers
33 views

Does elementary particle decay simply swap mass for speed?

I'm looking at different decays of elementary particles. And I am wondering about the masses (in energy) not matching up. For example, W and Z bosons are far more massive than the particles that decay ...
0
votes
1answer
57 views

Exact solution of Qubit Decoherence using Transfer Matrix

I'm going through a particular paper on decoherence: Exact Solution of Qubit Decoherence models by a transfer matrix method I'm having trouble understanding a particular step in the mathematics ...
0
votes
3answers
96 views

Is this true about low-light/one photon at-a-time double-slit interference?

I've consistently noticed in pictures of double-slit interference when very low-light or one photon at-a-time is used, that there's lots of "stray" photons detected in the areas of destructive ...
0
votes
2answers
58 views

How to recognize a Complete Set of Commuting Operators (CSCO)

A question about 'completeness'. These two operators are commuting, but I want to know more about their completeness. How do you know if {H}, {B}, {H,B} and/or {$H^2$,B} are forming (a) Complete ...
1
vote
0answers
26 views

Thermal de Broglie wavelength - definition

The thermal de Broglie wavelenght is often defined by the formula $\lambda=\frac{h}{\sqrt{2\pi mkT}}$ but equally frequently is it defined as de Broglie wavelength for a free ideal gas of massive ...
3
votes
1answer
65 views

Limits used to find non-rel limit of the Klein-Gordon equation

I just have a question regarding assessing the non-relativistic limit of the Klein-Gordon equation. In the book I'm following (Quantum Mechanics by Bransden & Joachain) they use the limits (Chpt. ...
0
votes
0answers
16 views

Separation of center of mass and relative frame for 2 particles in Haldane model in coulomb impurity

In the following paper http://journals.aps.org/prb/pdf/10.1103/PhysRevB.81.045428 the author says that the center of mass and relative motion cannot be decoupled. But the Hamiltonian can be separated ...
2
votes
1answer
47 views

Using symmetry to determine a hydrogen electron's decay route from $|300\rangle$ to $|100\rangle$

Lets say we have an electron in state $|nlm\rangle = |300\rangle$ of the hydrogen atom. By selection rules, we know that it can only decay to ground state in 3 ways, namely through the $|21m\rangle$ ...
0
votes
0answers
11 views

Counting the possible states for an electron configuration

How do you find the terms and energy levels for the electron configuration $(n_1p)(n_2 p)(n_3 s)$ in the case of LS coupling, where $n_1, n_2$ and $n_3$ are different? How do you find the number of ...
1
vote
1answer
59 views

Cooper pairing from repulsive potential

Suppose the Hamiltonian of a many-electron system consists of a potential which is repulsive : $\langle k_1, k_2 |\hat V |k_1',k_2' \rangle > 0$ where $k_1, k_2, \cdots$ are possible momenta that ...
0
votes
1answer
29 views

Tensor products of Hilberts spaces: definition of outer products and commutators

Suppose one has two single-particle Hilbert spaces $\mathcal{H}_{A}$ and $\mathcal{H}_{B}$ and consider the tensor product of these such that $\mathcal{H}_{A}\otimes\mathcal{H}_{B}$ is a two-particle ...
0
votes
0answers
13 views

How is the lifetime of a symmetric and antisymmetric state determined by its constituents

In the context of quantum information, there is the concept of so called symmetric and antisymmetric states, bright and dark, or superradiant and subradiant depending on the source you are using. The ...
0
votes
1answer
27 views

How is conversion inside heat engines of heat (random motion) to work (organized motion) explained in quantum physics?

When heat engines convert heat into work, they change the random motion of millions of particles into motion in a single direction. What is the phenomenon responsible for this alignment of ...
-1
votes
0answers
29 views

Electron travel speed in spinning object [closed]

My question is in the hypothetical circumstance that you could spin a disk or something close to the speed of light how would that effect the travel of electrons through the material. Picture a circle ...
4
votes
0answers
53 views

Elegant method to show $[L^2,[L^2,\vec{r}\,]\,] = 2\hbar^2\{L^2, \vec{r}\}.$ [duplicate]

Show that $[L^2,[L^2,\vec{r}\,]\,] = 2\hbar^2\{L^2, \vec{r}\},$ where $\vec{r} = x\, {\hat x} + y\, {\hat y} + z\, {\hat z}.$ "Edit: $\{A,B\} = AB + BA$ is the anti-commutator." I am able to solve ...
4
votes
2answers
93 views

A doubt regarding Black Hole Complementarity

A friend was explaining Black Hole Complementarity to me, and at one point he said that to get a (horrendously) mixed quantum state, i.e. a thermal density matrix without a heat bath, one takes a ...
0
votes
0answers
36 views

Uncertainty in orientation of angular momentum

To calculate the uncertainty it looks like I'm going to find an expression for the root mean square of either $J_x$ or $J_y$, or the $J$ in the x/y plane? But I'm not sure if that's what it means by ...
3
votes
0answers
20 views

Gaussian Minimizes Uncertainty - Statement Qualification [duplicate]

On the last page of this paper, the following statements are made (I'll jump right to around the point of interest): Example: Consider $A=p_x$, $B=x$. Then $$\langle A\rangle=\langle ...
0
votes
0answers
15 views

How do you state the quantized radial acceleration of an orbiting electron? [closed]

Assuming that each electron in the orbit around the atomic nucleus possesses the radial acceleration, is it feasible to write down the statement that the radial acceleration is quantized?
0
votes
1answer
71 views

Backing out of interactions: Does physics account for such a thing?

Does physics account for interactions between light and matter ever being "not completed" or backed out of? Here's what led me to the question. In learning about interference in light, I ended up ...
4
votes
1answer
62 views

A seemingly paradox for Eigenstate Thermalization Hypothesis (ETH)

ETH states that for a system, all of its eigenstates thermalize. To be more specific, consider an energy eigenstate of the full system $H|n\rangle=E_n|n\rangle$. If the full system is in this ...
0
votes
0answers
13 views

Semiconductor physics resonant tunneling diode

What happened if the barrier in resonant tunneling diode is of unequal width?
1
vote
0answers
51 views

Quantum mechanics - “God does not play dice” - does he? Or might he not? [duplicate]

I'm a mechanical engineer by training, so please forgive ignorance in my question. Heisenberg's uncertainty principle states (to my understanding) that one cannot measure both position and momentum ...
6
votes
0answers
35 views

How is Mössbauer spectroscopy so resistant to thermal motion?

Mössbauer spectroscopy detects tiny shifts (on the order of $\mu$eV or meV) in a gamma ray's energy due to the chemical environment of the nucleus. The scan consists of moving a source of excited ...
0
votes
1answer
42 views

If we can't clone quantum states, then how does stimulated emission work? [duplicate]

So we know we cannot fully copy a quantum state. But doesn't stimulated emission does just that? Say, a photon in a particular qubit state $|\psi\rangle = \alpha |0\rangle + \beta |1\rangle$ passes ...
3
votes
2answers
78 views

What is the qualitative difference between quantum superpostion and mixed states? [duplicate]

As I understand it, if one has a complete knowledge of the state of a quantum system (insofar as one knows the statistical distributions of all the observables associated with the state) then one can ...
4
votes
2answers
81 views

What does $(\delta\vec{r}\cdot\nabla)^2$ mean in the derivation of the Lamb shift, and how do you find its expectation?

The Wikipedia page on the Lamb shift includes the following first steps: $$\Delta V = V\bigl(\vec{r}+\delta \vec{r}\bigr)-V(\vec{r})=\delta \vec{r} \cdot \nabla V (\vec{r}) + \frac{1}{2} \bigl(\delta ...
0
votes
0answers
36 views

Collective angular momentum , Dicke states and indistinguishable particles

During course of quantum mechanics we dealt with addition of angular momenta. If we have two particles with spin $j_1$ and $j_2$ we can introduce total spin operator: $$\mathbf{J} = \mathbf{j}^{(1)} ...