# quantum

1. ### Five State Quantum System, need help understanding the question

I’ve never worked with a quantum system with more that two states 1, -1, and I’ve just gotten this homework problem. I'm not sure what it means. Does this mean it has five states? Why are there two 0’s and two 1’s?
2. ### Quantum Video lecture suggestion for Sakurai quantum mechanics Textbook

hello! I've been trying to read through Sakurai's Modern quantum mechanics textbook ( My goal is to finish the first 3 chapters and understand the Dirac formulation of QM specifically) but I find myself stumbling at many places. Are there any video lectures on the internet that follows this text...
3. ### I Using the Variational Method to get higher sates

In a typical quantum course we learn how to approximate the ground state of a particular Hamiltonian by making an educated guess at an ansatz with a tunable parameter then calculating the expectation energy for the ansatz. The result will depend on the tunable parameter if done correctly. Then...
4. ### A Expectation Value of a Stabilizer

Given that operator $S_M$, which consists entirely of $Y$ and $Z$ Pauli operators, is a stabilizer of some graph state $G$ i.e. the eigenvalue equation is given as $S_MG = G$ (eigenvalue $1$). In the paper 'Graph States as a Resource for Quantum Metrology' (page 3) it states that...
5. ### I Why can't there be a Universal Interpretation of Quantum Mechanics?

Why can't there be a Universal Interpretation of Quantum Mechanics? If you unite Copenhagen and Many Worlds than all other interpretations will fall under the umbrella of a Universal Interpretation of Quantum Mechanics. The main problem with interpretations seems to be the role of the observer...
6. ### The key to unifying the theories of the very large and the very small

Summary: The key to unifying the theories of the very large and the very small is asking if the object in question is physical Uncollapsed(stateless | unphysical) Quantum Waves + State(Matter Field or wave collapse) = Physical Matter Is a Matter Field state change the same thing as wave...
7. ### A Overlap of nth QHO excited state and momentum-shifted QHO ground state

$\newcommand{\ket}{|#1\rangle}$ $\newcommand{\bra}{\langle#1|}$ I have a momentum-shifting operator $e^{i\Delta p x/\hbar}$ acting on the ground state $\ket{0}$ of the QHO, and I want to compute the overlap of this state with the n$^{th}$ excited QHO state $\ket{n}$. Given...
8. ### Solutions to schrodinger equation with potential V(x)=V(-x)

C is just the constant by $\psi''$ My initial attempt was to write out the schrodinger equation in the case that x>0 and x<0, so that $$\frac {\psi'' (x)} {\psi (x)} = C(E-V(x))$$ and $$\frac {\psi'' (-x)} {\psi (-x)} = C(E-V(-x))$$ And since V(-x) = V(x) I equated them and...
9. ### I Gravitons and the Basic Forces

W and Z bosons, gluons, and photons are all gauge bosons that have been found. Since the graviton can be connnected to atoms and mass in certain ways, I think that CERN may eventually prove the graviton(if real) through smashing atoms that, supposedly, have a graviton orbiting them. Could...
10. ### Electric field of light emitted by atom, given the intensity

For t < 0 , all I can think of is a qualatative " the field is zero because the intensitity is 0 when the burst of light hasn't been emitted yet " For t >= 0 , I've tried squaring the given E and that lets me say the amplitudes are proportional (with a cos^2 term in the mix) But I feel like...
11. ### B Can entangled particle pairs be measured at the same time?

Has there been an experiment where 2 particles that are entangled are measured at the same time? If so what was the result? Can any observer occupy the same frame of reference down to an electron? Don't we all exist at different times based on our frame of reference so none of us can share the...
12. ### I Notion of a "clock" in Quantum Mechanics

Suppose the unitary operator $e^{-\frac{i}{\hbar}\hat{H}t}$ acts on $|\psi (0) \rangle$, does it make sense for one to think of the time-evolved state as some sort of time-keeping device? If not, why? If so, is such a notion useful? Thanks in advance!
13. ### Infinitesimal Perturbation in a potential well

If I calculate $<\psi^0|\epsilon|\psi^0>$ and $<\psi^0|-\epsilon|\psi^0>$ separately and then add, the correction seems to be 0 since $\epsilon$ is a constant perturbation term. SO how should I approach this? And how the Δ is relevant in this calculation?
14. ### Quantum Dirac's Quantum Mechanics

Hello, I remembered once hearing of a must-have quantum mechanics book by Paul Dirac. I don't remember if it was his Principles of QM or Lectures on QM. Based on the table of contents, I believe it was the Principles of QM book; however, looking at both I was thinking about getting his Lectures...
15. ### I Quantum Causal Modelling: Mathematics

I have been trying to read through the following paper: https://iopscience.iop.org/article/10.1088/1367-2630/18/6/063032/pdf, but am stuck at the parts from page 6 onwards. What is the "approach/formalism" used in the following sections of the paper, and are there any gentle introductions...
16. ### B Cat's point of view

Hi, In the Schrodinger's Cat thought experiment, we say that the cat is in a superposition of states of being either dead or alive. But isn't that opinion biased from being outside of the system? From the cat's point of view, it is either dead or alive but never both. The same argument can be...
17. ### B What’s more “important": GR or QM?

Let’s say to the average Astronomer conducting research; generalizing the research to “astronomy” ; which would be more useful for the “average” Astronomer on a day to day basis: Quantum Mechanics or General Relativity? Obviously most are, but which so more? Which would be referred to more?
18. ### Eigenstates of a free electron in a uniform magnetic field

I started with the first of the relevant equations, replacing the p with the operator -iħ∇ and expanding the squared term to yield: H = (-ħ^2 / 2m)∇^2 + (iqħ/m)A·∇ + (q^2 / 2m)A^2 + qV But since A = (1/2)B x r (iqħ/m)A·∇ = (iqħ / 2m)(r x ∇)·B = -(q / 2m)L·B = -(qB_0 / 2m)L_z and A^2 =...
19. ### B Is spacetime a quantum error correcting code?

This is a fascinating discussion. I know some people don't want to debate this or they can't debate it but the truth doesn't care about your feelings. This isn't speculative, it's backed by Scientific research. First paper. Is Spacetime an Error Correcting Code. Published in the Journal of High...
20. ### B Proietti 2019 experiment: Doubt in measuring A1(B1)

Hi everyone! Sorry for the bad english! Paper: https://arxiv.org/abs/1902.05080 I guess I understood the experiment until the moment Alice and Bob chooses to measure A0(B0) or A1(B1). I guess it's kind of straightforward that without a bell state measurement with photon alfa(beta) and photon...
21. ### A Quantum statistical canonical formalism to find ground state at T

For my own understanding, I am trying to computationally solve a simple spinless fermionic Hamiltonian in Quantum Canonical Ensemble formalism . The Hamiltonian is written in the second quantization as $$H = \sum_{i=1}^L c_{i+1}^\dagger c_i + h.c.$$ In the canonical formalism, the density...
22. ### I How do irrational numbers give incommensurate potential periods?

I am trying to understand Aubry-Andre model. It has the following form $$H=∑_n c^†_nc_{n+1}+H.C.+V∑_n cos(2πβn)c^†_nc_n$$ This reference (at the 3rd page) says that if $\beta$ is irrational (rational) then the period of potential is quasi-periodic incommensurate (periodic commensurate) with...
23. ### Quantum Entanglement textbook/notes

Hi everyone, Could anyone recommend a good QM textbook (undergrad-ish level) or some lecture notes that treat entanglement from the ground-up? Most of the stuff I have seen online on entanglement seem to fly pretty quickly into information-theory or abstract group-theory type stuff, which I am...
24. ### I Continuous variable entanglement

Hi all, I have learnt the very basics of entanglement (discrete, 2 particle systems) and was hoping that someone can recommend introductory (undergrad-level) material for continuous-variable, 2 particle entanglement. Stuff I have found online so far (like this...

27. ### A The most boring quantum quench (looking for a textbook/paper)

Hi, usually, when we talk about quantum quench dynamics we assume situation when Hamiltonian of a system has a sudden change from $H_0$ to $H_1$. System was initially in the ground state (or more generally - eigenstate) of $H_0$. The interesting dynamics appears when the commutator...
28. ### I Uncertainty of momentum and position

I was reading a book which had some comments on EPR paper (Einstein, Podolsky, Rosen - 1935) like following: In Newton's physics, when two identical billiard balls hit each other head-on, bouncing off in opposite direction, knowing one ball's position and speed will also indicate other ball's...
29. ### What happens to a light photon when it hits glass?

Homework Statement Using quantum mechanics, what happens when a photon of light hits glass? Homework Equations Momentum= Mass x velocity The Attempt at a Solution I am not sure of my answer but suffice to say the following: Assuming the collision between the photon particle and those in the...
30. ### I Energy in Everett's Many-worlds Interpretation

Hi, Is anybody able to explain how energy is "distributed" in the many-worlds interpretation. I'm using scare quotes as I think this may be the wrong line of thought. It's tempting to imagine energy being distributed amongst subsequent branches as the wave function evolves but I'm not certain...