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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Find unitary for given rotations on Bloch sphere

I want to characterize a unitary by given rotations on the Bloch sphere. I know, that when I send in the State $|\Psi\rangle =\begin{pmatrix}1\\0 \end{pmatrix}$, I get the state ...
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1answer
57 views

Measurements in quantum mechanics

Why does measurement change things? I read that measurement changes things because we have to bounce photons off an object to 'see' it and that changes its position, momentum etc... But on the other ...
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1answer
50 views

WKB Approximation on an linear + harmonic potential

I have a quick question: I have performed the WKB approximation to find the energies of bound states in symmetric potentials (Square, harmonic, ...). To do this I just find the "turning points" by ...
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2answers
57 views

Interference experiment and entanglement with apparatus

Consider a single photon in a Mach-Zehnder interferometer. Considering the photon only, the output state is the sum over both paths $$\vert 1 \rangle + \vert 2 \rangle=\vert \psi \rangle + ...
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55 views

Finding $\psi(0)$ using Schrodinger equation with potential $U(x) = q\delta(x)$

I am having some trouble answering the following question in my "Advanced Quantum Mechanics" course: Using the integral equation: $$\psi(x) = Ae^{ikx} + Be^{-ikx} - ...
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1answer
131 views

Obtaining the temperature from Bose-Einstein and Fermi-Dirac distribution

Lets say you are given a distribution function $f(p)$ and you want to define a temperature, $T_f$, for this distribution. (I assume $\mu = 0$.) It is then natural to define a temperature the ...
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46 views

Concerning The Oil Drop Experiment and Photon Absorption

Concerning The Oil Drop Experiment and Photon Absorption What would happen if you emit photons into the droplets, the molecule(s)? Is it possible for the photons to be absorbed by the droplet? I ...
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44 views

Gaining intuition over Hamiltonian for qubit systems

A typical Hamiltonian for a two state system with some driving field can be written as $$H=J(t)\sigma_z+h\sigma_x$$ This represents a qubit system driven along a single axis. On the other hand we ...
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77 views

Does electron spin? [duplicate]

Spin quantum number I was reading above article on spin quantum number. It says Uhlenbeck, Goudsmit, and Kronig introduced the idea of the self-rotation of the electron, which would naturally ...
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34 views

Heisenberg equation of motion for angular momentum operator

I am working on problem 5, from chapter 3 of Sakurai's Modern Quantum Mechanics. The problem has the following Hamiltonian: $$ H = \frac{1}{2}\left ...
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0answers
36 views

Show transpose not completely positivity using a bell state [closed]

In Nielsen&Chuang Box 8.2, an example is given to show that transpose map is not completely positive using bell state: "Suppose a qubit is part of a two qubits system initially in: ...
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52 views

What is the accuracy when firing an electron at a target?

Consider firing an electron at a target. Let the target be at a distance $d$ and the electron be travelling at a non-relativistic speed $v$. How can we estimate the maximum possible accuracy ...
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28 views

The electronic wave function

Is it correct to say that the wave description of the probability of finding the electron some where in space as specified by the wave function is concluded or supposed because, in the double slit ...
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42 views

Alternatives to scattering experiments

Scattering experiments have been a fruitful and efficient way to determine the particles that exist in nature and how they interact. What are some of the other experimental techniques used to ...
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57 views

What is the upper limit of objects behaving as a wave-particle?

Photons, electrons are quantum particles to which the wave-particle duality apply. protons are heavier quantum objects and posses quantum tunneling which is a wave character. What is the upper limit ...
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241 views

In quantum optics if you measure the energy of a thermal state does it collapse to a Fock state (Photon number state)?

As I understand it a thermal state is a state of light such that the light is in a superposition of many Fock states (Photon number states) with the probability of the light being found in a ...
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2answers
133 views

Can superpositions of baryons with different charge and strangeness exist?

I am trying to find out whether the following baryons can exist: $$ |X\rangle = \frac{|u u u\rangle + |d d d\rangle + |s s s\rangle}{\sqrt{3}} $$ $$ |Y\rangle = \frac{|u u u\rangle + |d d d\rangle - ...
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1answer
67 views

Is there anything comparable to many-body localization in classical physics?

I've only just started looking into many-body localization, so this question may come off as a little vague. But my understanding is that it relates to how some quantum systems do not thermalize, as ...
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53 views

Interpreting group velocity of free particle wave packet

I am trying to understand the concept of group velocity of a free particle wave packet: $$\Psi(x,t) = \frac{1}{\sqrt{2 \pi}}\int_{-\infty}^{\infty} \phi(k)e^{ikx}e^{-\frac{i \hbar k^2 t}{2m}}dk.$$ ...
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40 views

Invertibility of Dirac matrices

Suppose we have Dirac equation in the following form : $i\partial_t\psi = (-i\vec{\alpha}\cdot \nabla +m\beta)\psi$ and assume that the Klein-Gordon equation is satisfied, i.e. $\partial_t^2 \psi ...
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2answers
46 views

Proportion of photons passing through three polarizers

I've started recently reading book "Quantum Computing, A Gentle Introduction". After each chapter there are exercises for self study. For some of them there are answers, for some not. So far I've been ...
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70 views

Calculating amplitude for chain of polaroids [closed]

I've recently started reading the book "Quantum Computing, A Gentle Introduction". After each chapter there are exercises for self study. For some of them there are answers, for some not. So far I've ...
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0answers
33 views

Show that partial derivative with respect to time is anti-hermitian [closed]

I have a definition that $$<s_1,s_2> = \int_{-\infty}^{+\infty}s_1^*(t)s_2(t) dt$$ I need to show that $\partial_t$ operator which is just the partial derivative with respect to time is ...
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Same photon or different photon?

Consider a typical optical focusing system: A small light source, then a collimating lens, then a focussing lens, and then a detector (e.g. CCD). Assume that source intensity is so low that only one ...
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42 views

Double-slit intensity distribution for particles

For waves, the intensity pattern for the double-slit experiment is given by the following equation (as of David Morin's notes on waves) $$ I(x) = I(0) \frac{D}{\sqrt{x^{2} + D^{2}}}cos^{2} \left ( ...
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38 views

Electron velocity in hydrogen

I was speculating over the feasibility of myon catalysed fusion. Somehow I drifted away from that a little, and I was wondering if one can find a quantum-mechanical expression for the velocity of an ...
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2answers
44 views

Annihilation operator in harmonic oscillator

In Wikipedia's QHO page there is a moment when the following is stated: I don't know why "the ground state in the position representation is determined by $a|0\rangle=0$". I would say that the ...
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29 views

Antiunitary operators in the tenfold way

In the classification of free fermion systems in condensed matter, physicists usually divide the systems into ten symmetry classes, first discovered by Altland and Zirnbauer. In their classification, ...
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2answers
70 views

How does electron makes transition from one state to another?

i am not asking why it makes. i am curious about how it makes the transition. Does it go from one state to another via straight line or it makes smaller and smaller orbits till it reaches the next ...
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29 views

Can these two systems be equal? [closed]

Suppose that there are two differents systems, $A$ and $B$, both associated with the same potential $V(x)$ (take, for instance, an infinite well). Then would the eigenstates of $A$ be the same as the ...
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2answers
53 views

Transformation of $| JM\rangle$ under the group of rotations

I am following the Quantum Mechanics I, Galindo A., Pascual P. and in page 207 explaining the matrix representations of the Rotation Operators in the angular momentum it appears the next (obvious) ...
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1answer
51 views

What is the difference and/or similarity between multiple universes in inflation and in quantumphysics? [duplicate]

Some theories propose that there are more universes. In one of them it is said that there are parallel universes, due to inflations after the big bang that created them. But in the two-slit ...
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1answer
81 views

Transforming to a rotating frame in the $x$-basis

I was reading this paper on analytically Solvable driven time-dependent two level quantum systems. The Hamiltonian considered in the paper is the following: $$H=\sigma_z\cdot J(t)/2)+\sigma_x\cdot ...
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1answer
33 views

Deriving Rabi oscillations using the Heisenberg picture of QM

The semiclassical treatment of an simple two level atom in a resonant electromagnetic field is usually done in the Schrodinger/Interaction picture of QM, by assuming that the wavefunction of the atom ...
5
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1answer
77 views

Heisenberg's uncertainty principle derivation in a ring [duplicate]

The standard derivation But now suppose the space is a ring of length $L$, it seems the derivation could work out exactly the same and we get $$\Delta p \Delta x \geq \hbar/2.$$ But since $\Delta x$ ...
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2answers
78 views

Is the momentum of a particle both uncertain and, independently, also random?

Is momentum of a particle "random" because it is uncertain, or is it uncertain in addition to being random? Is the uncertainty principle and quantum randomness different names the same physical ...
3
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133 views

Are symmetries of a degenerate ground-state manifold always broken?

If a Hamiltonian has a global symmetry and a degenerate ground state, then in the thermodynamic limit, the ground states $| \psi \rangle$ that are eigenstates of the symmetry operator typically become ...
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25 views

Compton Effect Demonstration using MATLAB

I have recently been taught about Compton Effect and I learnt about the Intensity variation with the wavelength . Now I also know a little bit about MATLAB so I was wondering if I could create plot ...
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2answers
107 views

Why the incessant confusion between diffraction and interference? [closed]

Feynman said, "No one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a question of usage, and there is no specific, important physical ...
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2answers
25 views

Photoelectric effect and work function

In a photoelectric effect we remove electrons from a metal using high energy photons, the work function is the minimal energy required for this effect. My question is why doesnt the work fucntion ...
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1answer
49 views

quantum clone of orthogonal quantum states

I am a little bit confused about the no-cloning theorem for two orthogonal quantum states. In Nielson&Chuang page 24-25, it states that an unknown state $|\phi\rangle$ cannot be copied since ...
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1answer
12 views

Does frequency of mechanical vibration of material affect its blackbody radiation spectrum

Can we see the difference in emissivity for instance between the same object vibrating in two different frequencies ?
16
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1answer
1k views

Is the existence of a sole particle in an hypothetical infinite empty space explicitly forbidden by QM?

Suppose the universe is completely empty with one sole particle trapped in it. To simplify, I will only be looking at the one dimensional case. However, all arguments are applicable for three ...
2
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1answer
51 views

Why is the energy operator special?

Only the energy operator controls the time dependence of a quantum system, but not the others, why is that?
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36 views

The $T\rightarrow \infty $ limit in quantum field theory

I am new to quantum field theory. Prior to this, I have been using quantum mechanics for a few years. I am reading the book by A. Zee, ''quantum field theory in a nutshell'', 2nd Ed.. On page 18, ...
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23 views

Higher $L$ lower energy?

For multi-electron atoms, what is the physical reason behind the fact that a higher total orbital angular momentum, $L$ gives a smaller total energy, $E$?
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1answer
61 views

Particle on $S^1$ and $U(1)$-principal bundle

I have a question arisen from a simple QM problem: let consider a boson on $S^1$ minimally coupled with a constant gauge field $A$. Taking the stationary Schrödinger (S) or Klein-Gordon (KG) equation ...
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11 views

Does the dispertion medium affects Quantum Dots fluorescence efficiency?

I would like to use CdTe Quantum Dots to make a fluorescent coating on a transparent support. There's a link to the material here: ...
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1answer
39 views

Why is the energy shift due to a 'sagging' potential negative and independent of box size?

Consider a box of width $L$ and the composed of the following potential $$V(x)=\frac{V_0x(x-L)}{L^2}, x\in[0,L]$$ and $V(x)=\infty$ elsewhere. Using perturbation theory - with a square box as the ...
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1answer
47 views

Writing operator evolution as a quantum dynamical map

In the Heisenberg picture we have the evolution of the operator in time given by: $$A(t)=U^+A(0)U$$ I was looking into the theory of open quantum systems where we introduce the concept of a quantum ...