What is Quantum: Definition and 999 Discussions

In physics, a quantum (plural quanta) is the minimum amount of any physical entity (physical property) involved in an interaction. The fundamental notion that a physical property can be "quantized" is referred to as "the hypothesis of quantization". This means that the magnitude of the physical property can take on only discrete values consisting of integer multiples of one quantum.
For example, a photon is a single quantum of light (or of any other form of electromagnetic radiation). Similarly, the energy of an electron bound within an atom is quantized and can exist only in certain discrete values. (Atoms and matter in general are stable because electrons can exist only at discrete energy levels within an atom.) Quantization is one of the foundations of the much broader physics of quantum mechanics. Quantization of energy and its influence on how energy and matter interact (quantum electrodynamics) is part of the fundamental framework for understanding and describing nature.

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  1. forever_physicist

    I Quantum operators and trasformation under rotations

    Good morning! I have a problem in understanding the steps from vectors to operators. Imagine you are given a vectorial observable. In classical mechanics, after rotating the system it transform with a rotation matrix R. If we go to quantum mechanics, this observable becomes an operator that is...
  2. M

    What are the prerequisites to study quantum field theory?

    Summary:: What are the prerequisites to study quantum field theory? What are the prerequisites to study quantum field theory?
  3. micklat

    I Explaining atoms and bonding using entropy

    I am a biology undergraduate interested in abiogenesis. The entropic explanation for the origin of life is that life is allowed to exist because it increases universal entropy. I am curious about how far we can take this theory down. How can you explain the emergence of atoms and atomic...
  4. L

    A Quantum computers and Annular Josephson junctions

    Are annular Josephson junctions qubits in some quantum computer right now? https://www.nature.com/articles/425133aI found this article from 2003. What is the progress right now? Also are vortices and fluxons same thing?
  5. Haorong Wu

    Courses I am a graduate student in quantum optics. Should I learn QFT?

    I am particularly interesting in QFT, and I am going to be a graduate student in quantum optics and quantum information this autumn. Strangely, I find that there is no courses for QFT. After all, I though QFT are about quantum and field, and quantum optics are about quantum and field, too...
  6. AndreasC

    Quantum Hilbert spaces and quantum operators being infinite dimensional matrices

    I just realized quantum operators X and P aren't actually just generalizations of matrices in infinite dimensions that you can naively play with as if they're usual matrices. Then I learned that the space of quantum states is not actually a Hilbert space but a "rigged" Hilbert space. It all...
  7. bob012345

    I Is Quantum Mechanics Infinitely More Complex than Classical Mechanics?

    Please critique this text. It came from a research article* I found but I'm only interested if the sentence is 100% accurate or not and not in the specifics of the article itself. Are they suggesting Hilbert space is always infinite? Thanks. Quantum mechanics is infinitely more complicated than...
  8. mktsgm

    Quantum of cells directly infected by a virus?

    In any viral infection, we know that an invading virus enters the cell and damages them (cytolytic/cytopathic). This may be a starting point for inflammation down the road. Though initial inflammation may be beneficial, a longer than necessary inflammatory process proves more damaging than...
  9. Y

    A Quantum finance equation explanation please

    Dear all, Dr. Raymond S. T. Lee in his book on Quantum Finance (page 112), normalizes quantum price return QPR(n) using the following scaling: Normalized QPR(n)=1+0.21*sigma*QPR(n). I don't know of any way of explaining this equation. sigma is the standard deviation of the wave function...
  10. G Delta

    I Quantum Realism and Bell Locality

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  11. A. Neumaier

    I Steven Weinberg on the interpretation of quantum mechanics

    I think Weinberg is quite clear about this: On p.87 of the second edition of his quantum mechanics book, he says, and on p.88: After having discussed decoherence, he says on p.92: For the instrumentalist approach (apparently your view of the matter), he states on p.92f this drawback: Then...
  12. B

    A Quantum Period is Compression Robust

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  13. G

    I How does Bohmian mechanics explain quantum bomb testing?

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  14. SamRoss

    I Seeking better explanation of some quantum stats formulae

    In "Introduction to Quantum Mechanics", Griffiths derives the following formulae for counting the number of configurations for N particles. Distinguishable particles... $$ N!\prod_{n=1}^\infty \frac {d^{N_n}_n} {N_n !} $$ Fermions... $$ \prod_{n=1}^\infty \frac {d_n!} {N_n!(d_n-N_n)!}$$...
  15. PGaccount

    I Partition function of quantum mechanics

    In quantum mechanics, we have the partition function Z[j] = e-W[j] = ∫ eiS+ jiOi. The propagator between two points 1 and 2 can be calculated as ## \frac{\delta}{\delta j_1}\frac{\delta}{\delta j_2} Z = \langle O_1 O_2 \rangle## The S in the path integral has been replaced by S → S + jiOi...
  16. P

    I Help! I Don't Understand Quantum Darwinism

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  17. victor01

    I Clebsch–Gordan coefficients: An Identity

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  18. K

    I Quantum Field Operators for Bosons

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  19. troglodyte

    I Struggling with one step to show quantum operator equality

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  20. AndreasC

    I Exploring Foundations of Quantum Mechanics & QFT

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  21. E

    Extended Essay IB - Quantum Physics Oriented

    Summary:: - IB Extended Essay - Physics - Quantum Mechanics and Electricity/ Electrical Components I've been asked to pick a topic for my IB Extended Essay. Basically the extended essay is a piece of independent research done during the course of IB. It's meant to be 4000 words and you have to...
  22. T

    A Evaluating Matrix Spin Dependent Term in Dirac Quadratic Equation

    I derive the quadratic form of Dirac equation as follows $$\lbrace[i\not \partial-e\not A]^2-m^2\rbrace\psi=\lbrace\left( i\partial-e A\right)^2 + \frac{1}{2i} \sigma^{\mu\nu}F_{\mu \nu}-m^2\rbrace\psi=0$$ And I need to find the form of the spin dependent term to get the final expression $$g...
  23. shintashi

    B Is there a Relativistic Quantum Chemistry Table?

    Wiki said "Arnold Sommerfeld calculated that, for a 1s orbital electron of a hydrogen atom with an orbiting radius of 0.0529 nm, α ≈ 1/137. That is to say, the fine-structure constant shows the electron traveling at nearly 1/137 the speed of light.[9] One can extend this to a larger element with...
  24. K

    Quantum High-level book on scattering in quantum mechanics

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  25. Replusz

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    Hey everyone, This is more of a motivational thread, and of course if anyone wants to join in, please do! Any comments are welcome. It's also fine if no one comments. Maybe don't remove the thread though please. I hope this might be useful later on for others as motivation. So the challenge is...
  26. Z

    I Could Quantum Physics explain the singularity in a Black Hole?

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  27. nomadreid

    I Quantum logic based on closed Hilbert space subspaces

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  28. Jarvis323

    I Modern Assesment of Grete Hermann's Philosophy on Quantum Mechanics

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  29. G

    A Quantum Mechanic/wave function/junction condition

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  30. kellys

    Help with this quantum circuit textbook problem about controlled gates

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  31. Adwit

    A Quantum Field Theory: 3-4 Equation Steps Explained

    I understand how do 3 no. equation come from 1 & 2 no. equation. But I am struggling to understand how do 4 no. equation come from 3 no. equation. Will anyone do the steps between 3 no. equation and 4 no. equation, please ?
  32. E

    Bohr-Sommerfeld, WKB, and current-biased Josephson junction

    I've already found the turning points, in the case of the left turning point, the local minimum of the potential, ##\delta_{min}=1.11977## when evaluating for an arbitrary value of current ##J=0.9I_C##. The left turning point is therefore ##\delta_r=2.48243##. I know the Bohr-Sommerfeld...
  33. E

    I Quantum Computing: How are quantum properties exploited?

    To elaborate a little on what I think I do understand / accept: 1. I don't think I have a problem accepting the "weirdness" of quantum concepts. So, for example, I am willing to accept the concept that a quantum system can "exist" in a large number of different states simultaneously. 2. I...
  34. A

    Quantum physics and the theory of evolution

    How did you find PF?: Searching a way to share own thoughts I was wondering if we could gather all the laws of the universe and try to link anything to everything. I'm thinking of the possibility of finding links between quantum physics and the theory of evolution. It reminds me of the...
  35. E

    Quantum motion of a charged particle in a magnetic field

    Once I know the Hamiltonian, I know to take the determinant ##\left| \vec H-\lambda \vec I \right| = 0 ## and solve for ##\lambda## which are the eigenvalues/eigenenergies. My problem is, I'm unsure how to formulate the Hamiltonian. Is my potential ##U(r)## my scalar field ##\phi##? I've seen...
  36. cemtu

    Quantum Mechanics Hydrogen Atom Expectation Value Problem

    I can not solve this problem: However, I have a similar problem with proper solution: Can you please guide me to solve my question? I am not being able to relate Y R (from first question) and U (from second question), and solve the question at the top above...
  37. cemtu

    Quantum Mechanics hydrogen atom eigenfunction problem

    This is a general property of eigenvectors of Hermitian operators. State functions are a particular class of vector, and it is easiest to work in the general formalism (I am hoping to show how ket notation makes qm easier, not just do standard bookwork at this level). Suppose O is a Hermitian...
  38. N

    I Is it possible to explain Quantum tunneling with the HUP?

    I'm a student in South Korea(It is my first English question ever). I found descriptions of quantum tunneling explained by the uncertainty principle in Korea. There are two kinds of descriptions to explain quantum tunneling; position-momentum and time-energy uncertainty principle. First...
  39. The black vegetable

    I Vertex function, quantum action

    I am looking at Srednicki ch 64 , how does equation 64.1 follow from 64.3 as stated. Explicitly in QED how does ## u_{s'}(p')V^{u}(p',p)u_{s}(p)=e\bar{u'}(F_{1}(q^{2})\gamma ^{u}-\frac{i}{m}F_{2}(q^{2})S^{uv}q_{v})u ## follow from the quantum action ## \Gamma =\int d^{4}x(eF_{1}\bar{\varphi...
  40. A

    A Quantum vacuum fluctuation in BEC

    Hi, I have some problems with visualization (I'm trying to understand Jeff Steinhauer's experiment, but my questions are general). Why the quantum vacuum fluctuations are guaranteed by the underlying pointlike atoms composing a BEC? And if vacuum fluctuations generate excitations (i.e...
  41. J

    A Why is the quantum Fisher information useful in quantum metrology?

    I'm getting interested in quantum-enhanced metrology and have come across the quantum Fisher information (QFI) as a measure of how much a quantum state ##|\Psi(\theta)\rangle## changes with respect to some variable, for example, the phase accumulated during an interferometer, ##\theta##. This is...
  42. B

    Finding energy eigenvalues with perturbation

    I know the basis I should use is |m_1,m_2> and that each m can be 1,0,-1 but how do I get the eigenvalues from this?
  43. J

    Schools Quantum Computing Phd: UNM vs LSU vs WashU

    Hello I am trying to choose a school and am interested in quantum computing theory in the physics department. I haven't decided whether or not I want to stay in academia so I would like the option to go into industry maybe at Google, IBM or a startup. Ideally I'd like the reputation/research to...
  44. A

    A Superluminal origin of Quantum Mechanics

    A relativistic origin of QM is proposed in https://iopscience.iop.org/article/10.1088/1367-2630/ab76f7 It is proposed that lorentz transformation that include superluminal observers (whether those observers exist or not) explain the indeterministic behavior of QM. Not only that, it also would...
  45. J

    B Quantum computing -- How is the relevant solution selected?

    During a quantum computation, all possible solutions exist in a state of superposition. How is the relevant one selected?
  46. L

    A Phonons as a quasiparticles in a quantum LHO

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  47. E

    S-wave phase shift for quantum mechanical scattering

    a.) The potential is a delta function, so ##V \left( r \right) = \frac {\hbar^2} {2\mu} \gamma \delta \left(r-a \right)##, therefore ##V \left( r \right) = \frac {\hbar^2} {2\mu} \gamma ## at ##r=a##, and ##V \left( r \right) = 0## otherwise. I've tried a few different approaches: 1.) In...
  48. U

    I Quantum Mechanics Particle in a Box

    I need help .I did not A) E < V0 for T =? (passing coefficient ) B) E = V0 for T = ? C ) E > V0 for T =? A
  49. L

    A Potential step at a Barrier in Quantum mechanics

    In quantum mechanics in books authors discuss only cases ##E<V_0## and ##E>V_0##, where ##E## is energy of the particle and ##V_0## is height of the barrier. Why not ##E=V_0##? In that case for ##x<0## \psi_1(x)=Ae^{ikx}+Be^{-ikx} and for ##x\geq 0## \psi_2(x)=Cx+D and then from...
  50. CrosisBH

    Computing the wave function of a square potential

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