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)

0
votes
0answers
24 views

Why is there zero point energy at absolute zero temperature?

If we define the absolute zero as a temperature which there is no entropy in it so why should we have zero point energy?
1
vote
3answers
109 views

What is the meaning of “ Ψ is not a measurable quantity in itself”?

I want to know that why the wavefunction Ψ as a complex quantity (i.e $A+iB$ form) in quantum mechanics and somewhere I have studied that Ψ is not a measurable quantity in itself that's why we ...
0
votes
1answer
16 views

How can I calculate the partial trace for a combined state of a pair of two-level atoms to get a reduced state?

Let's say I have a combined state of a pair of two-level atoms, $A$ and $B$, given by the density matrix: $$ \rho = \frac{1}{2}\mid g_A, g_B \rangle \langle g_A, g_B\mid + \frac{1}{2} \mid g_A, e_B ...
0
votes
2answers
37 views

Levi-Civita symbol and transpose conjugate

When we take the dagger of an expression which contains a Levi-Civita symbol, do we need to transpose the Levi-Civita symbol? E.g., do we have: $$(\textbf{a}\times ...
1
vote
2answers
79 views

Questions about the formalism of Quantum Mechanics

I have to do a presentation on this. I'm not expected to do something really detailed, but I'm not understanding the mathematical formalism. I would like to receive general answers to these questions: ...
0
votes
1answer
14 views

What would be the Slater's determinant representation for an excited state?

Setup Introducing this spinorbital notation: \begin{align} \Psi_1=\chi_{(r1)}\alpha_{(\omega1)} = 1 \\ \Psi_1=\chi_{(r1)}\beta_{(\omega1)} = \bar{1} \end{align} and the Slater's determinant, for ...
0
votes
0answers
11 views

What is the connection between Bragg's condition with reduced EK diagram?

In my course notes the professor mentioned that there was some relationship between the Bragg's condition and the first Bernoulli zone of the reduced EK diagram. Specifically, the boundary before ...
1
vote
2answers
67 views

What state does the particle in a box occupy?

My textbook derives the equations for the different energy states $E_n$ of the particle in a box. But my professor in class said this example was a good one because it spoke about the "superposition ...
2
votes
0answers
50 views

Quantum Mechanics and Economics… What

I was reading this paper: http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2002698&download=yes The author has the model presented here: ...
0
votes
0answers
32 views

what is a clock state?

What is a clock state in atomic physics ? I read this term here http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2708678/ and tried to find a reference to explain the same but have been unable to find this ...
2
votes
2answers
121 views

Changing of spin for electron?

Can we change the spin of electron by applying magnetic field from $\uparrow$ to $\downarrow$ configuration?
3
votes
1answer
55 views

Considering $\langle \underline{q} \mid \underline{p} \rangle=\frac{1}{(2\pi\hbar)^{n/2}}e^{i\underline{q}\cdot\underline{p}/\hbar}$ [duplicate]

I have been given the following complete systems of eigenvectors $$\mathbf{Q}\mid\mathbf{q} \rangle=\mathbf{q}\mid\mathbf{q} \rangle, \quad \mathbf{P}\mid\mathbf{p} \rangle=\mathbf{p}\mid\mathbf{p} ...
1
vote
1answer
32 views

About shift operators

The question is this: Does $$L_+ L_- Y_{lm} $$ ,where $Y_{lm}$ is a spherical harmonic function, equals to zero. If so, why? The two operators above are defined as $$L_+ ={L_x + iL_y } $$ $$L_-={L_x ...
2
votes
3answers
61 views

How do I operate on a spin state with a sigma operator?

For any arbitrary spin state $|s\rangle$. How do I operate on it with the Pauli spin matrix, $\hat{\sigma_z}$? Does this have something to do with a Bloch sphere?
0
votes
2answers
50 views

Derivation of plane wave from inner product of position ket and momentum ket

In textbooks it seems to be taken for granted that $$\langle \mathbf{r}|\mathbf{k}\rangle ~=~ \frac{1}{\sqrt{\Omega}}\exp(i\mathbf{k}\cdot\mathbf{r}).$$ I'm sure it's obvious but is there a ...
-2
votes
1answer
95 views

What happens in a universe with only two electrons? [on hold]

What happens in a universe with only two electrons? Do they stay as waves or do they collapse into particles?
3
votes
1answer
105 views

Spin drift velocity?

I am currently reading this Phys Rev paper by H C Torrey. In this paper, he derives the Bloch equations with an additional diffusion term. He says that the current density is given by $$\mathbf ...
1
vote
0answers
46 views

Deriving effective model without integrating out degrees of freedom in path integral formalism?

In path integral formalism of quantum field theory (particle physics or condensed matter), one can in principle integrate out part of the degrees of freedom so as to attain an effective model ...
1
vote
3answers
217 views

How does De Broglie–Bohm theory or pilot wave theory explain the results of the Stern–Gerlach experiment?

The Copehagen interpretation of QM explains the Stern–Gerlach experiment by asserting that a particle is in a superposition of states and doesn't have a definite spin until measured. However, the de ...
7
votes
1answer
454 views

Have they really photographed light behaving both as a particle and a wave?

I just came across this article where they are claiming that they have photographed light behaving both as a wave and a particle! The paper has been published in Nature Communications and I read the ...
3
votes
1answer
224 views

Do Bell inequality violations appear instantly when the source is turned on, or do they increase over time?

This experimental Question is a result of reading a particular article on Bell violations. I addressed the e-mail below to the corresponding authors —because who knows, they might reply— but it is not ...
14
votes
8answers
2k views

What is an observer in quantum mechanics?

My question is not about (pseudo) philosophical debate; it concerns mathematical operations and experimental facts. What is an observer? What are the conditions required to be qualified of observer, ...
0
votes
1answer
87 views

Commutator and Hamiltonian [on hold]

Assume that $[\hat{A},\hat{H}]_-=0$ and $[\hat{B},\hat{H}]_-=0$ but we know that $[\hat{A},\hat{B}]_-\neq 0$. Then there exists degenerate stationary states of $H$. How to prove it?
3
votes
1answer
187 views

First order coherence through double slit

The state $$|\Psi \rangle = |0\rangle + \sum_j \int d\omega f_j(\omega)\hat{a}^\dagger_j (\omega) |0\rangle $$ is coming from a far field and incident on a double slit setup. Here j is the index of ...
0
votes
1answer
20 views

Monoatomic fluids and free space around atoms

In monoatomic fluids the atoms can move quite freely around each other. Is there any thermodynamic/statistical mechanic equation how much free space there is between the atoms? This has to be ...
6
votes
1answer
1k views

Trace of an operator matrix (Quantum computation and quantum information)

I'm reading the book Quantum computation and quantum information by Mike & Ike and I'm stuck at 2.60/2.61. There, the author says that, given the operator $A|ψ⟩⟨ψ|$, its trace is: $${\rm ...
0
votes
0answers
28 views

Angular momentum wavefunctions with respect to different axes

I've been learning about quantum angular momentum, and I have a question about the relationship between quantum mechanical angular momentum wavefunctions with respect to different axes. I know that ...
17
votes
3answers
3k views

Nature of gravity: gravitons, curvature of space-time or both?

General relativity tells us that what we perceive as gravity is curvature of space-time. On the other hand (as I understand it) gravity can be understood as a force between objects which are ...
0
votes
1answer
20 views

Can a particle accelerator really be constructed which can actually change the properties of a material (create a new kind of atom)? [on hold]

Can a particle accelerator really be constructed which can actually change the properties of a material (create a new kind of atom)? If not, what are some ways to create or synthesize a new atomic ...
0
votes
0answers
15 views

For an entangled state consisting of systems A and B, if A is measured when does the wavefunction at B collapse? [duplicate]

If there are two systems A and B, with an entangled state consisting of $$\mid \psi \rangle = \frac{1}{\sqrt{2}} (\mid \uparrow _A \rangle \,\, \otimes \mid \uparrow _{B} \rangle \,+ \mid \downarrow ...
3
votes
1answer
60 views

Can reduced density matrices of sub systems of an entangled composite system be different?

In a 4-dimensional hilbert space, only 4 entangled states( normalized ) are possible ( if I am not wrong ), the bell basis. In each of the state in bell basis the reduced density matrix is ...
0
votes
2answers
42 views

Understanding notation regarding particles states and wavefunctions

In the development in my notes of second quantisation I have a problem in understanding notation. We start by considering a basis $\psi_i(\mathbf{r})$ for the Hilbert space of single particle ...
2
votes
3answers
86 views

Physical meaning of quantum interpretations

Do interpretations of quantum mechanics have physical meaning? An argument for no would be the fact that no matter the interpretation, one gets the same measurements. They also do not follow logical ...
2
votes
2answers
97 views

Half-integer spin and infinitesimal rotations

On p. 692 of 'Quantum Mechanics' by Cohen-Tannoudji, he states that: Every finite rotation can be decomposed into an infinite number of infinitesimal rotations, since the angle of rotation can ...
2
votes
1answer
56 views

Quantum mechanics and Classical limit(s)

I have tried to make sense of this and i am not sure i get it. What i gather from this page about the classical limit is: You need coherent states something like $\hbar \to 0$ is not really enaugh. ...
0
votes
0answers
86 views

Schrödinger Equation [closed]

A stronger Adam replies, "...No Eve Honey! We instead eat the snake!" We can only now do away with Voodoo Physics, if we go back to square one. If an object travels in a circle around a fixed point ...
1
vote
2answers
96 views

Quantum Harmonic Oscillator and the Classical Limit

We can solve for the stationary states of a quantum harmonic oscillator denoted by $|n\rangle$ with energy eigenvalues $(n+\frac{1}2)\hbar\omega$. However if our system is in a stationary state, the ...
3
votes
0answers
23 views

Is there a physical interpretation of Neumann boundary conditions for the free Schrodinger equation on a domain?

Let $\Omega$ be a domain in $\mathbb{R}^n$. Consider the time-independent free Schrodinger equation $\Delta \psi = E\psi$. Solutions subject to Dirichlet boundary conditions can be physically ...
9
votes
1answer
175 views

Do Franck-Condon oscillations have natural lineshapes?

I recently found a paper (for the curious, this one) that talks about observing the motion of a nuclear wavepacket in H2O, as initiated by tunnel ionization. This wavepacket should be thought of as a ...
0
votes
1answer
54 views

Can I say that physical entities do not exist and everything is observed “as if” they exist? [on hold]

Maybe it is a bit philosophical question. If I have a model of the universe with all the laws in a computer program. Can I say that the electrons that are modelled in the program exist ? For me it is ...
1
vote
0answers
41 views

Quantum mechanics question in derivation of Heisenberg-Euler Lagrangian in Schwartz “QFT” notes

In http://isites.harvard.edu/fs/docs/icb.topic1246957.files/IV-9-EffectiveActions.pdf (Page 20) Schwartz derives the Heisenberg-Euler Lagrangian using Schwinger's proper time method. To do so, he ...
0
votes
0answers
20 views

Measurement of two qubits in a tensor product space

I understand that if we have two qubits, say $\Psi \in \mathcal{H}_1 \bigotimes \mathcal{H}_2$ where Alice has the first qubit, and she makes a measurement and ends up with the state $\phi \in ...
0
votes
1answer
72 views

Time evolution of two orthogonal states in Time Dependent Perturbation Theory

Given the two orthogonal states for $H_0$ , $|n(t)>_I, |m(t)>_I$, in the interaction picture, we want to find the probability of transforming from one to the other after time t, aka: $ \ (1) \ ...
1
vote
0answers
12 views

Classical limit and generalized coherent states

In quantum optics coherent states introduced by Glauber have a localized probability distribution in classical phase-space with maximum following classical equations of motions. This is not a ...
-2
votes
0answers
24 views

Time dependent and Time Independent Schrödinger Equation [on hold]

What is the meaning of Schrödinger's Time dependent and Time Independent Equation?
2
votes
3answers
222 views

Normalization problem with hydrogen wavefunction

Suppose you have a mix of states made up of the Hydrogen $\lvert nlm \rangle$ states where one of the coefficients is unknown. For example: $$ \lvert \psi\rangle=A\lvert 100\rangle + ...
0
votes
0answers
24 views

Physical significance of Cayley Transform

In the book on Quantum Mechanics by Capri (in Chapter 6), its said that an operator $A$ is self adjoint if the operator, $U$ given by $$ U = (A - i I)(A + i I)^{-1} = -(I+iA)(I-iA)^{-1} = -\text ...
2
votes
1answer
63 views

Methods to distinguish between pure/mixed states and entangled/separable states

I'm a little confused about how we can distinguish between pure/mixed states and entangled/separable states and I would really appreciate some help! I understand a density operator $\rho$ represents ...
2
votes
1answer
61 views

WKB Quantization Condition - negative?

In deriving the quantization condition for a bound state in a potential with "no verticle walls" we start with the WKB connection formulas to find the wavefunction in the interior of the well ...
0
votes
0answers
19 views

Unitarity of Symmetrisation operators

How can i proove that the symmetrisation operators $S_{\pm}$ are unitary? Defining $S_{\pm}$ on the $N$ Particle Fock Space by it's effect on $|\Psi \rangle=| \Psi_1 \rangle \otimes... \otimes| ...