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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Vector space of $\mathbb{C}^4$ and its basis, the Pauli matrices

How do I write an arbitrary $2\times 2$ matrix as a linear combination of the three Pauli Matrices and the $2\times 2$ unit matrix? Any example for the same might help ?
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142 views

Phonon vectors and characteristic length interpretation

Basically I have a set of vectors of unit length, $\{\nu_i\}$, describing the movement of phonons (all orthogonal to each other), $\{\omega_i\}$. Lets say I only have two atoms, $m_1$ and $m_2$. In ...
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1answer
337 views

Maximum probability of success for distinguishing between two pure states with one measurement

Suppose you have the states such that $\langle \psi_1| \psi_2 \rangle = \cos(\alpha)$ and you have one measurement to distinguish between the two. It is claimed that the probability of success at ...
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4answers
5k views

What errors would one learn from Eliezer Yudkowsky's introduction to quantum physics?

Eliezer Yudkowsky wrote an introduction to quantum physics from a strictly realist standpoint. However, he has no qualifications in the subject and it is not his specialty. Does it paint an accurate ...
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724 views

Comparison of different ab-initio codes

One may find on the web a lot of different computational packages to perform "ab-initio" calculations of electron structure of the solids. Usually, the documentation is not quite transparent about the ...
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2answers
696 views

Free particle propagation amplitude calculation

I have a quick calculational question. In Peskin and Schroeder, Chapter 2, they want to look at the amplitude for a particle to propagate between two arbitrary points, $x$ and $x_0$, in an arbitrary ...
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1answer
1k views

Tight Binding Model in Graphene

I'm following a calculation done by a guy who's done it a bit different than what I've done before (used nearest neighbour vectors and a DFT instead of what I will show below), I'm not quite sure how ...
4
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3answers
182 views

How do we know that internal conversion creates no intermediate photon?

I've read, from several sources, that in internal conversion -- an excited electron transferring its energy to another electron which is then emitted -- no intermediate gamma radiation is produced. ...
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2answers
149 views

In $H_2^+$, what is the Hamiltonian of the movement of the electron?

An electron is orbiting two protons. With the Born-Oppenheimer approximation that the protons do not move, I'd write the Hamiltonian of the electron's movement as: $$ \mathbf{H} = ...
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1answer
726 views

Wave packets v.s. wave trains

Could someone please explain the difference between a wave packet and a wave train? I have rummaged around online but have not been able to find a definitive definition.
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4answers
5k views

Are quantum mechanics and quantum physics the same field?

What is the difference between quantum mechanics and quantum physics?
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2answers
221 views

Quantization of momentum in nanotubes

I'm reading about carbon nanotubes and how the momentum (lets call it $k_x$) is quantized along the circumferential direction and not along the cylindrical (call this $k_y$). I can follow the maths ...
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2answers
812 views

How to find that a molecule has zero spin?

I read that Ne has $S=0$. How can this be found, knowing the electron configuration? Electrons, protons and neutrons all have 1/2 spin. The Pauli exclusion principle implies that the even number of ...
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5answers
677 views

Computer game with quantum optics/ information

Is there a computer game using principles of quantum optics or quantum information? By game I don't mean just a simulation or an interactive course, but something that can be played in an enjoyable ...
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2answers
207 views

Physical Significance for Duality Formula for Entropy

I am studying quantum statistical mechanics from the mathematician's perspective. I don't quite understand what the duality formula for entropy is really saying (or why there is a "duality"). If $A$ ...
8
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3answers
4k views

What makes the difference between ionic and covalent bonds?

Backstory: When I learned about chemical reactions, there were two types of molecular bonds: Ionic, where an atom or compound molecule with a low valence number loses those valence electron(s) to one ...
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4answers
3k views

How can one describe electron motion around hydrogen atom?

I remember from introductory Quantum Mechanics, that hydrogen atom is one of those systems that we can solve without too much ( embarrassing ) approximations. After a number of postulates, QM ...
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3answers
296 views

Subjectivity of decoherence

I read that quantum decoherence is subjective, in the sense that two observers may not have the same "environment" and after each one has traced over those degrees of freedom they will end up with a ...
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0answers
77 views

shouldn't we add the oscillating terms into Bohr-Sommerfeld quantization formula

shouldn't be the quantization formula (in one dimension) equal to $ N_{smooth}(E)+N_{osc}(E) = \oint_{C}p.dq $ ?? where the Oscillating term is just the correction from Gutzwiller trace formula or a ...
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3answers
574 views

Which is the biggest object which can interfere with itself?

I've heard that scientist proof that viruses of the Tobacco mosaic virus could interfere with themselves. I'm referring to quantum interference-- the same as photons. Unfortunately, I couldn't find ...
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1answer
84 views

semiclassical exact expression (in one dimension only)

let be $ N(x)= \sum_{n} H(x-E_{n}) $ the eingenvalue 'staircase' function and let be a system so $ V(x)=V(-x)$ and $ V^{-1}(x)=\sqrt \pi \frac{d^{1/2}}{dx^{1/2}} N(x) $ then would it be true that ...
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3answers
275 views

What is the physical difference between states and unital completely positive maps?

Mathematically, completely positive maps on C*-algebras generalize positive linear functionals in that every positive linear functional on a C*-algebra $A$ is a completely positive map of $A$ into ...
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1answer
332 views

The eigenvalue of Schrodinger Equation

I'm a student majoring in Mathematics.But now I'm studying the KDV equation which uses Schrodinger Equation. My question is that in time-independent Schrodinger ...
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3answers
2k views

Why can't we know the speed, $\vec{v}(t)$, and position, $\vec{r}(t)$, of an electron (the two) at the same time $t$?

I've read something about this and I conclude that it happens because of the uncertainty principle. But I don't understand very well the meaning of that. I mean, it's very abstract that the speed, ...
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2answers
935 views

Basic Question - Green's Functions in Quantum Mechanics

I am trying to learn about Green's functions as part of my graduate studies and have a rather basic question about them: In my maths textbooks and a lot of places online, the basic Greens function G ...
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1answer
449 views

Change in Vapour/Liquid change point, at very low pressure

In a previous question, I was given an answer: "A quick Google suggests that the triple point of Hydrogen is 13.8K and the triple point of Neon is 24.6K, so neither can exist as liquids at ...
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1answer
1k views

Representations of Pauli matrices involving outer product of qubit states

Let $| 0 \rangle$ and $| 1 \rangle $ be the states of qubit. Let $\hat{\sigma_x}$, $\hat{\sigma_y}$, $\hat{\sigma_z}$ be Pauli matrices: $$ \hat{\sigma}_{x} = \left( \begin{array}{cc} 0 & 1 \\ ...
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3answers
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The Double Slit Experiment and the changing of electron behaviour

As you will all know, when one tries to detect which slit an electron has gone through with close up observation, it changes from behaving like a wave and producing an interference pattern to behaving ...
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1answer
196 views

Quantum Coin Flipping Protocol

$\newcommand{\ket}[1]{\left|#1\right>}$ I have the next protocol: $A$ tosses a fair coin $a\in \{0,1\}$, if $a=0$, $A$ sends to $B$ $\ket{\psi_0}=\ket0$, if $a=1$ $A$ sends to $B$, ...
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3answers
749 views

Solution to the quantum measurement problem?

What can be a scientific solution to the Q-measurement problem (other than many worlds idea)? Can it be somehow verified through experiment?
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1answer
140 views

Experiment to find structure of water

Who first determined the structure of water (two hydrogen atoms stuck to an oxygen atom at approx 105 degrees), and, more importantly, how was this done?
3
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1answer
438 views

How to interpret the derivative in the momentum operator in quantum mechanics?

Given a stationary 1-D wave function $\psi(x)$, how is the derivative in the momentum operator interpreted? $$ \int_{-\infty}^\infty \psi^*(x) \hat{p} \psi(x) dx = \int_{-\infty}^\infty \psi^*(x) ...
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2answers
478 views

When “unphysical” solutions are not actually unphysical

When solving problems in physics, one often finds, and ignores, "unphysical" solutions. For example, when solving for the velocity and time taken to fall a distance h (from rest) under earth gravity: ...
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2answers
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Proof that the One-Dimensional Simple Harmonic Oscillator is Non-Degenerate?

The standard treatment of the one-dimensional quantum simple harmonic oscillator (SHO) using the raising and lowering operators arrives at the countable basis of eigenstates $\{\vert n \rangle\}_{n = ...
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1answer
1k views

Time-reversal symmetry

For a quantum system with time-reversal symmetry, other than the absence of a magnetic field, can we infer anything else about the system?
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1answer
690 views

Helium plasma in space and its properties

It is said, that, "Most of the Helium in the Universe, is in a plasma state". Plasma's are now talked of, as the forth state of matter, but this does not seem to be a majority opinion. Plasma's are ...
3
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1answer
408 views

More on the Feynman Path Integral Formula in Brian Cox' Lecture and its Consequences

This is a continuation of this question about Brian Cox' lecture Night with the Stars. I know the main steps to get from $K(q",q',T)=\sum_{paths}Ae^{iS(q",q',T)/h}$ to $\Delta t ...
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2answers
118 views

Heuristics for definitions of open and closed quantum dynamics

I've been reading some of the literature on "open quantum systems" and it looks like the following physical interpretations are made: Reversible dynamics of a closed quantum system are represented ...
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4answers
399 views

Slit screen and wave-particle duality

In a double-slit experiment, interference patterns are shown when light passes through the slits and illuminate the screen. So the question is, if one shoots a single photon, does the screen show ...
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1answer
488 views

Heisenberg's principle in Quantum Cryptography

In quantum cryptography why do we need the Heisenberg uncertainty principle? Edit: I only know the statement of the Heisenberg uncertainity principle. As I know that if Eve tries to know the ...
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3answers
343 views

Does Helium just naturally display BEC properties at <1K, or does it become a BEC?

I am researching low temperature, near absolute zero, and in particular Bose Einstein Condensate. There is a lot of research information, but it is confusing, and not explained. Technically a BEC is ...
3
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1answer
231 views

Can spatial coherence be maintained in fiber optic cables over time?

I am doing research with a double slit experiment, using a beam splitter and 2 lengths of fiber optic cable, whose ends brought close together form the effective double slit. I notice that the ...
12
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3answers
990 views

Bell's theorem and why nonlocality is problematic

I generally hear it assumed that Bell's inequality implies violation of counterfactual definiteness, because locality is considered sacrosanct. I understand of course that measurable violations of ...
3
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2answers
377 views

Decoherence when no one is looking?

I understand that in the single-electron-at-a-time double slit experiment, if a detector is placed before the slit, the interference pattern vanishes. Suppose I left the detector on, but put a bag ...
4
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1answer
849 views

Is all angular momentum quantized?

Angular momentum is definitely quantized in elementary particles and electrons in atoms. Molecules also have characteristic rotation spectra. Is it true that all angular momentum is quantized, ...
3
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2answers
342 views

A question from Weinberg QFT text

In page 71 Weinberg's QFT, $$A\Psi^{\theta }_{a,b} ~=~(a\cos{(\theta )}-b\sin{(\theta )})\Psi^{\theta }_{a,b}.$$ He says that massless particles represented by $\Psi ^{\theta }_{a,b}$ are not ...
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1answer
2k views

Is the universe finite and discrete?

Is the universe finite, both in the sense of being a closed spacetime manifold, as viewed from the macro level, but also in the sense of being fully discrete and finite in all of its intricate quantum ...
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1answer
231 views

Does measurement, quantum in particular, always increase the total entropy?

Measurement of a quantum observable (in an appropriate, old-fashioned sense) necessarily involves coupling to a system with a macroscopically large number of degrees of freedom. Entanglement with this ...
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3answers
909 views

Does this Zeilinger group result provide experimental proof of backward-in-time causation?

Does this recent Zeilinger group delayed choice entanglement experiment imply backward-in-time influences? http://arxiv.org/abs/1203.4834 From the abstract: "This can also be viewed as “quantum ...
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4answers
919 views

Questions on wave-particle duality

Wave-particle duality states that a particle has both wave properties and particle properties when one is not observing it. 1) What is an observer? Need it be anything living or can other particles ...