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 quantum-field-...

learn more… | top users | synonyms (4)

1
vote
2answers
24 views

Meaning of “Metrically defined” matrix

I got stuck on understanding stuff from Methods of molecular quantum mechanics written by R. McWeeny. The content is: It must be stressed that the representation property embodied in $\mathcal{A} \...
13
votes
2answers
808 views

What are the odds of finding an electron a long way from an atom? As in metres away?

Pretty self-explanatory, heard that there is a chance that you might find an electron on the other side of the universe, just wanted to know.
4
votes
0answers
60 views
+50

Zeno effect in quantum gravity?

Given that "the quantum zeno effect can "freeze" the evolution of the system by measuring it frequently enough in its known initial state." Is there any equivalent/similar Zeno effect in quantum ...
0
votes
0answers
12 views

Quantum Entanglement Simulation

Suppose there is a digital "quantum entanglement simulator" with a pair generator and two spin detectors, and it Allows to create as many pairs as you want (100 pairs per single click) Allows to ...
-1
votes
1answer
37 views

What happens with QM particle at constant velocity? [on hold]

In QM particles have wavefunctions that disperse quite fast. The FWHM for gaussian wavepacket increases in time as $\Delta x(t)= \Delta x_0\sqrt{1+(t/\tau)^2} $, $\tau=2m(\Delta x_0)^2/\hbar$ What ...
1
vote
2answers
78 views

What is the program of quantum field theory? What is its derivation?

Paraphrasing Griffith's: For some particle of mass m constrained to the x-axis subject to some force $F(x,t)=-∂V/∂x$, the program of classical mechanics is to determine the particle's position at any ...
1
vote
1answer
76 views

How to use Heisenberg to explain atom cannot be at rest

In a Dutch national physics exam at the level just before entering university there was a question about a molecule HI: one atom hydrogen plus one atom iodine. Within the molecule, the hydrogen ...
3
votes
2answers
393 views

'The size of an atom' using Uncertainty Principle

Suppose we have a hydrogen atom, and measure the position of the electron; we must not be able to predict exactly where the electron will be, or the momentum spread will then turn out to be infinite. ...
0
votes
1answer
62 views

Approximation of an integral for time dependent quantum Hamiltonian

I want to approximate $\displaystyle\int_{t}^{t+h} dt' H(t')$ in its taylor series, $$H(t)h + \frac{1}{2}\frac{dH(t)}{dt} h^2 + \frac{1}{3!}\frac{d^2H(t)}{dt^2} h^3 + \frac{1}{4!}\frac{d^3H(t)}{dt^3} ...
5
votes
0answers
25 views

What's the intuition behind the Choi-Jamiolkowski isomorphism?

What is the intuition behind the Choi-Jamiolkowski isomorphism? It says that with every superoperator $\mathbb{E}$ we can associate a state given by a density matrix $$ J(\mathbb{E}) = (\mathbb{E} \...
0
votes
1answer
46 views

Quantum entanglement of spins

Suppose we have a pair of spins in the state $\left|\Psi\right>=\frac{1}{\sqrt2}(\left|\uparrow\downarrow\right>-\left|\downarrow\uparrow\right>)$. We know that the measurements of the spins ...
-2
votes
2answers
59 views

How do Rabi oscillations act on the relative phase of state kets

Suppose we have a system with a lower state $\left|{\downarrow}\right\rangle$ and and upper state $\left|{\uparrow}\right\rangle$, coupled by a pulsed electromagnetic field. I thought that from an ...
2
votes
1answer
31 views

Switching Parity in a Superconductor

I've got a pretty simple question that's been bothering me for a while. This is the issue. Suppose we have a superconductor with phase $\phi$, and we change this to $\phi+2\pi$. One would think that ...
2
votes
1answer
59 views

Proper definition of the Wannier function

I am studying the construction of the Wannier functions for 1D system with periodic boundary conditions and my reasoning seems to be a little bit different from what I can find in textbooks. First ...
5
votes
1answer
73 views

Energy Spectrum and Gauge Transformations

There is a question in SE about the fact that the Hamiltonian isn't invariant under the EM gauge transformations. I wanted to ask about its consequences here. I know that in general, the Hamiltonian ...
0
votes
1answer
129 views

Question regarding NaCl equilibrium separation

So I am tutoring someone later and one of the problems is from Eisberg/Resnick Ch 12. The potential energy $V$ of NaCl can be described emperically by $$V = \frac{-e^2}{4\pi\epsilon_0 R}+Ae^{-R/\...
0
votes
1answer
21 views

What is the maximum limit for concurrence for a multiparticle state derived from a symmetric hamiltonian?

What is the limit for the Entanglement of a multi particle state? Say I have N spins and a symmetric Hamiltonian. Is it 1 or more? Is there a limit at all? I thought it would be 1, since that value ...
3
votes
2answers
196 views

Rigorous Mathematical Proof of the Uncertainty Principle from First Principles [duplicate]

While looking at an intuitive explanation for the Heisenberg Uncertainty Principle (related question below), there was a mention of an axiomatic approach to establishing the uncertainty principle. ...
0
votes
0answers
19 views

Quantum Confinement in Quantum Dots

I have seen it referenced in several places that an electron will behave classically in a bulk material but as the dimension of the material in which the electron exists is limited below the electron'...
5
votes
1answer
68 views

Which charge to use in the Dirac quantization condition?

I have a follow-up question to Dirac magnetic monopoles and quark fractional electric charge quantization, regarding whether the "unit of electric charge" in the Dirac quantization condition should be ...
8
votes
3answers
2k views

What is a completely positive map *physically*?

I am sure this question is really stupid, but I could not refrain from asking it in this forum. This can be considered as a continuation of this question. ...
2
votes
1answer
141 views

Thermionic emission, delayed emission and predissociation

In molecular photodissociation, the thermionic emission, delayed emission and predissociation are the same? Otherwise, what is the difference between them? My question is not about the solids, but I ...
2
votes
1answer
62 views

Is the use of term Measurement appropriate in quantum physics

It may not sound like a physics question, but I think, it is. When we measure spin of an electron in a certain direction, we are actually, setting it up in that direction. That does not sound like ...
4
votes
2answers
633 views

Can 3 photons be combined to give a spin-0 projection?

Motivation: The neutral pion decays to 2 photons ($\pi^0\to\gamma\gamma$) most of the time. For the decay of the neutral to 3 photons ($\pi^0\to 3\gamma$) we have an upper limit on the branching ...
2
votes
2answers
67 views

Express state as eigenkets

This is very basic but I just suddenly got confused. Any state can be expressed as complete set of eigenkets with discrete eigenvalues: $$|P\rangle = \sum^n c_n |p_n\rangle$$ I understand the above. ...
2
votes
1answer
51 views

Relationship between position and momentum matrix elements

I am going through Faist's Quantum Cascade Lasers book. In the first section of the fourth chapter, he asserts that (in the context of a perturbative analysis of electronic transitions) the ...
0
votes
0answers
55 views

What is the expression for a Lorentz invariant wave function? [on hold]

I was recently thinking about Lorentz invariance description of a particle. In special relativity is based on the invariance of $$ \text{constant} = c^2t^2 - x^2 - y^2 - z^2$$ I was wondering if ...
3
votes
1answer
48 views

How to make a double slit electron interference as a home experiment? [on hold]

I want to see electron interference making my own experimental equipments. Do you think it is possible? Especially, double slit, how can I have it?
0
votes
0answers
34 views

Why can the y and z-components of spin be measured simultaneously? [duplicate]

I have a gut feeling that this is wrong. By the uncertainty principle where $x,y,z$ are the $x,y,z$ components of spin $$ \sigma_{y}\sigma_{z}\geq \frac{\hbar}{2}\langle x \rangle $$ and it can be ...
4
votes
2answers
64 views

If orbital shells are just probability functions, why are quantum numbers only ever integers? [on hold]

Quantum numbers are supposed to denote every individual orbital. But if orbital shells are probability functions, then orbitals can't be definite, solid things. So in that case, there can be variation ...
1
vote
0answers
35 views

Are all kinds of energy quantized? [duplicate]

I was reading this answer of David Z about meaning of "Quantum" that this question came in mind: If light is quantized (i.e. is a integer multiples of some unit $\hbar c/k$), are other kinds of ...
0
votes
2answers
204 views

Why only 1 component of angular momentum?

Griffiths says that you can have only 1 well defined component of the angular momentum because of the uncertainty principle. From the uncertainty principle, we get that $$ \sigma_{L_x}\sigma_{L_y} \...
-2
votes
0answers
32 views

Coordinate determines the speed and thus coordinate [on hold]

I have a question about configuration space. You say that coordinate and momentum have one-to-one correspondence in Quantum Mechanics. Imagine that coordinate of a particle is known absolutely in ...
-3
votes
0answers
67 views

Why isn't CPU made of cesium 133? [on hold]

If this atom can move there and back 9 bilion times per second, then why are we using so poor CPUs?
9
votes
1answer
634 views

Confusion in a trick in solving an energy eigenfunction

Given a non-relativistic energy eigenfunction for a central potential $\left|\Phi \right>$ In solving relativistic hydrogen atom, one of the terms is $$ \left<\Phi\middle|\frac{e^2}{r}\middle|\...
0
votes
1answer
49 views

What is the process of a photon transforming into an electron and positron? [duplicate]

How did they come into existence from a photon? Is it really understood how the process works? Is there even a process or is it just something fundamental?
-6
votes
0answers
72 views

Why is gravity viewed as a force? [on hold]

Before I say my thoughts on this I wanted to state that most of my knowledge on physics and such is from college books, and not from actual instruction. From my understanding of forces, there must be ...
-2
votes
0answers
48 views

Does “Schroedinger's cat” need entity knowledge to work? [on hold]

I've never studied physics, but am interested in whether this paradox, from an interpretation of quantum mechanics, can still be stated if we don't know anything about unobservable entities. My ...
3
votes
1answer
110 views

On a measurement level, is quantum mechanics a deterministic theory or a probability theory?

Quantum mechanics is a non-commutative probability theory. As such, it fundamentally behaves differently from classical probability theories. This manifests itself most pronouncedly in the uncertainty ...
15
votes
4answers
508 views

Is it possible to reconstruct the Hamiltonian from knowledge of its ground state wave function?

Is it possible to "construct" the Hamiltonian of a system if its ground state wave function (or functional) is known? I understand one should not expect this to be generically true since the ...
0
votes
1answer
53 views

When should parameters be replaced while changing basis? [on hold]

I have the following equality: $$a\lvert 00\rangle + b\lvert 11\rangle = a\lvert ++\rangle + b\lvert --\rangle$$ for any $a,b \in C$ that satisfy the normalization condition. For $a = \frac{1}{\...
6
votes
4answers
9k views

Does a magnetic field arise from a moving charge or from its spin, or both?

I learned that a moving charge creates a magnetic field perpendicular to its direction of motion. I also learned that charged particles like electrons have spin and they also create a magnetic field ...
0
votes
2answers
3k views

Landau Levels and Magnetic length

I am looking into Landau Levels and keep coming across a magnetic length defined as follows for a 2D system: $l_B=\sqrt{\frac{\hbar c}{e B}}$ I have seen numerous sources say this is: $l_B\approx ...
8
votes
3answers
227 views

Are there any true discontinuities in physics?

When we first learn physics, it's often presented very 'discontinuously'. For example, pop quantum likes to talk about objects being "either" particles or waves, leading to a lot of confused questions ...
1
vote
1answer
55 views

Trouble understanding Nielsen & Chuang exercise

I am probably just stuck on something very simple, but I'm having trouble understanding a premise of Exercise 10.40 in Nielsen & Chuang. The full details of the exercise are not important for my ...
2
votes
2answers
77 views

Two particles system

Source: this video For a system with two particles (09:30), why is its wave function a product of each particle's wave function? E.g. $$\psi(x_1,x_2)=\psi_a(x_1)\psi_b(x_2)$$ For indistinguishable ...
2
votes
0answers
36 views

When we say particle in a box has quantized energy, is that kinetic or total energy?

In quantum mechanics, it is usually said that energy of the bound (constrained) systems such as particle in a box (infinite potential well) is quantized. It confuses me exactly what type of energy is ...
1
vote
1answer
137 views

Lindblad equation derivation

I'm reading A simple derivation of the Lindblad equation. It introduces a Hamiltonian for a system consisting of a principal system $S$, a heat bath $B$ and an interaction term: $\hat{H}=\hat{H}_S+\...
14
votes
4answers
1k views

Are there more entangled states or non-entangled ones?

I'm trying to understand entanglement in terms of scarcity and abundance. Given an arbitrary vector $v$ representing a pure quantum state of, say, dimension 4, i.e. $v \in \mathcal{H}^{\otimes 4}$, ...
1
vote
2answers
47 views

Why is the energy expressed in an electron orbital change of state electromagnetic (photon)? [on hold]

As I understand it, Schrodinger's wave equation predicts the allowable energy states an electron can have under the electromagnetic forces of a given nucleus (and I assume other 'orbital' electrons). ...