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. PainterGuy

    B Quantum entanglement and hidden variables

    Hi, I have a basic understanding of quantum physics. I was reading a Wikipedia article on hidden variables, https://en.wikipedia.org/wiki/Hidden-variable_theory . The article says the following. I was confused about the words "local" and "nonlocal" in the quote above so I checked out another...
  2. Demystifier

    A Quantum analog of Boltzmann entropy?

    In classical statistical physics, entropy can be defined either as Boltzmann entropy or Gibbs entropy. In quantum statistical physics we have von Neumann entropy, which is a quantum analog of Gibbs entropy. Is there a quantum analog of Boltzmann entropy?
  3. H

    I Does an electron have a quantum phase frequency?

    Stationary solutions to the Schrödinger equation factor into a spatial part, e.g. atomic and molecular orbitals, and a temporal part that gives the phase rotation frequency. It is often assumed that adding a constant to the potential leaves the physics unchanged. And clearly, any "spectroscopic"...
  4. A

    I Repeated measurements on a quantum system interacting with other quantum systems

    In quantum mechanics if I repeat a measurement of the same observable in succession I get the same quantum state if it is not a degenerate state. If I make the system under consideration interact with another quantum system and meanwhile keep measuring it what happens? Does the system not...
  5. nomadreid

    I Quantum effects near 0 Kelvin

    In https://phys.org/news/2016-09-cold-black-holes.html it is stated that a supermassive black hole interior could be 10^-14 degrees Kelvin. Is there a limit, perhaps due to quantum effects, below which a temperature (in a black hole or elsewhere) can go? Or do the possibilities approach 0...
  6. V

    A How to derive Quantum Mechanics in curved physical space?

    I am following [this YouTube lecture by Schuller][1] where he finds the appropriate formalism for the quantum mechanics in the physical curved space. Everything makes sense to me but at the very end I see that we find the pull backed connection one-form on the base manifold. He says to the end...
  7. O

    I Limit of quantum mechanics as h -> 0

    Starting from the Heisenberg equation of motion, we have $$ih \frac{\partial p}{\partial t} = [p, H]$$ which simplifies to $$ih \frac{\partial p}{\partial t} = -ih\frac{\partial V}{\partial x}$$ but this just results in ## \frac{\partial p}{\partial t} = -ih\frac{\partial V}{\partial x}## and...
  8. C

    I Ways of measuring open quantum systems

    At the heart of the theory of open quantum systems is the idea that the measurement statistics of many-body systems can be expressed in terms of a reduced density matrix, obtained by tracing over degrees of freedom that are irrelevant to the system of interest. In general, given a pure state...
  9. A

    A Schrodinger equation in quantum field theory

    What is the Schrodinger equation in QFT? is it the nonrelativistic approximation of a Klein-Gordon scalar field? or Is there more? I have read that the Schrodinger equation describes a QFT in 0 dimensions. I accept every answer
  10. Greg Bernhardt

    Quick tour of Google's Quantum Computer

    I thought this was a fun and quick video of an engineer giving a tour of their quantum computer. Only about 30 seconds, but it looks well, quite beautiful! The engineer mentioned they achieved milestone 2 which has something to do with size. I also haven't heard of the field "Quantum Metrology"...
  11. K

    B Searching for Quantum Mechanics Lecture on Delayed Choice Experiment

    Hi everybody. Some years ago I came across a video on youtube where they talked about an experiment with small and large envelops, when the small ones were placed into the large ones and then it resulted in something interesting. It might have been an instance of delayed choice, but I am not...
  12. physics2023

    A What is meant when a phase is said to have "symmetry protected"?

    What is meant when a phase is said to have "symmetry protected"?
  13. Dr Wu

    Entangled by Quantum Supremacy

    The many online articles about the future security issues posed by quantum supremacy — at least those pitched at the interested layperson (like myself) — tend for quite understandable reasons to focus on the outcomes that QS will have on today’s digital-based security systems. . . not as they...
  14. J

    I Why is quantum mechanics not valid for large objects?

    It this correct explanation why quantum is not valid for large object? Why would small object want to hide their information?
  15. C

    A Multiparticle Relativistic Quantum Mechanics in an external potential

    It is often argued that Dirac Equation is not valid as relativistic quantum mechanics requires the creation of antiparticles. But, there are also some arguments that suggest otherwise. For example, I saw Arnold Neumaier's website on this that there are multiparticle relativistic quantum...
  16. Leureka

    I Quantum wavefunction is real?

    In this 2011 paper, Lundeen & colleagues used weak measurement to map both imaginary and real components of a wavefunction directly, without destroying the state. It says: “with weak measurements, it’s possible to learn something about the wavefunction without completely destroying it”. And...
  17. Addez123

    Confused about quantum tunneling through 200V not eV

    I have the equations to calculate transmission probability, my problem is that the barrier is given in Volts not electron volts. $$200V = e \cdot 200 eV = 3.2 \cdot 10^{-17} eV$$ I am not even sure if that's a correct conversion. But if it is then this "barrier" is extremly small and 99.999%...
  18. hilbert2

    A The eigenvalue power method for quantum problems

    The classical "power method" for solving one special eigenvalue of an operator works, in a finite-dimensional vector space, as follows: suppose an operator ##\hat{A}## can be written as an ##n\times n## matrix, and its unknown eigenvectors are (in Dirac bra-ket notation) ##\left|\psi_1...
  19. L

    B Quantum Connection vs Entanglement: What's the Difference?

    quantum connection vs. quantum entanglement Do they mean the same? Are there any differences? Is the second one more common/newer/more standard than the first one? Could someone please answer these questions or recommend something to read?
  20. CuriousLearner8

    A Eigenvalue Problem of Quantum Mechanics

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  21. Morbert

    I What is the relationship between events and outcomes in quantum theories?

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  22. ramana3797

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  23. J

    Nobody understands quantum physics?

    Listen just 1 minute, what does it mean when he said nobody understand quantum mechanics? This sound like comedy
  24. Vectronix

    B Quantum Physics and Atomic Physics

    I think Atomic physics gets into specific examples and experiments but I think I’m probably wrong. So, I have to ask, what is atomic physics and how is it different than quantum physics?
  25. S

    I Are the proton and electron quantum entangled in the hydrogen atom?

    The proton and electron are described by separate wavefunctions. When they come together in the hydrogen atom are they quantum entangled and have a joint wavefunction.
  26. TerranIV

    B Shouldn't quantum gravity be an interaction between mass and spacetime?

    Einstein showed (via general relativity) that spacetime is curved by mass, mass moves in relation to this curvature, and that gravitation arises as secondary effect. Why then are we looking for quantum gravity as some sort of mass<->mass interaction? Aren't the fundamental interactions better...
  27. S

    Quantum entanglement and the Schrödinger experiment

    Ok so just tell me this, if you flip a coin with your eyes closed and it lands on your hand and then you look at the coin and it is heads… there was a time in the duration of the coin being in the air at which the coin was in a state of neither heads or tales. But only once it hits your hand it...
  28. S

    I Intuition for why linear algebra is needed in quantum physics

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  29. ThiagoMNobrega

    A What is the biggest problem to be solved in Quantum Physics in 2023?

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  30. S

    I Query on use of prism in delayed choice quantum eraser experiment

    Hi. I’ve seen a number of schematic diagrams for the Delayed Choice Quantum Eraser experiment. Some include a prism to make the two idler photon paths diverge. However, the prism is shown refracting the ‘idler rays’ in a way which contravenes Snell’s law. E.g. see prism marked ‘PS’ (not the...
  31. LittleSchwinger

    Quantum What are some good modern Quantum Optics textbooks?

    What would people here say are good modern Quantum Optics textbooks?
  32. L

    B Quantum field theory and wave particle duality

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  33. yucheng

    A How are quantum optical experiments designed? ;)

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  34. VVS2000

    Physics Choice of specialization for Quantum Chromodynamics PhD?

    I am entering into the 2nd year of my masters and I plan to do my phd in quantum chromodynamics. So for my specialization, would it be helpful if I choose nuclear and particle physics or quantum field theory?
  35. L

    A Quantum Perspectivalism by D.Dieks

    Dennis Dieks is a respected Phycisist who works in Utrecht University.He has been developing his unique understanding of quantum mechanics namely Quantum Perspectivalism,close to Rovelli's RQM.But his works seem highly unpopular . https://link.springer.com/article/10.1007/s10701-019-00264-0 This...
  36. P

    I Exploring the Relationship between Quantum Numbers and Energy in Atomic Systems

    The quantum number n determines the energy, and for each n the allowed values for the angular momentum quantum number are -(n-1),...,(n-1). This doesn't seem resonable to me. Classically increasing the orbital angular momentum will result in an increase in the energy of the system. But why is it...
  37. Peter Morgan

    I The collapse of a quantum state as a joint probability construction

    The titular paper can be found here, https://doi.org/10.1088/1751-8121/ac6f2f, and on arXiv as https://arxiv.org/abs/2101.10931 (which is paginated differently, but the text and equation and section numbers are the same). Please see the abstract, but in part this 24 page paper argues that we...
  38. ramana3797

    Quantum Espresso make all error

    The system at my college died and I did a fresh install of ubuntu and tried to install quantum espresso after installing the required dependancies according to the instructions in the link "https://osuginoissp.wixsite.com/mysite/a-install-quantum-espresso" . But the make all command keeps...
  39. H

    A Is decoherence necessary to observe a quantum event?

    Hi Pfs, I wonder if in quantum mechanics the question "Did this event occur?" has a yes or no answer like in classical phyics or in our daily life. We are accustomed to consider linear superpositions of states like "dead" and "alive" but id "dead" is a state the death of the cat is an event. i...
  40. J

    I Quantum fluctuations & 'virtual particles'

    I've been reading about how language around virtual particle fluctuations is metaphorical. This is helpful: https://www.physicsforums.com/insights/vacuum-fluctuation-myth/ I'm just trying to understand a bit more from a layman's point of view. I found Matt Strassler's article 'Virtual...
  41. A

    I How azimutal quantum number got its range value

    it is known that azimutal quantum number takes the values from 0 to n-1. how did they find it ?
  42. A. Neumaier

    A Ensembles in quantum field theory

    Then please explain how the transition in conceptual language from a single quantum field (extending all over spacetime, or at least over the lab during a day) to an ensemble of particles can be justified from the QFT formalism.
  43. K

    Normalize function - quantum chemistry

    Normalize function f(r) = Nexp{-alpha*r} Where alpha is positive const and r is a vector I was just wondering if the fact that we have a vector value in our equation changes anything about the solution
  44. Paul Colby

    A Recent papers refuting real-valued quantum mechanics

    So, in a rare instance I actually read APS News, I came across “New Experiment Suggests Imaginary Numbers Must be Part of Real Quantum Physics.” In November 2022, Volume 31, Number 10. Since complex numbers are isomorphic to a real 2x2 matrix algebra, I was confused how such a claim can be...
  45. A

    I Some classical models of particles and quantum gauge theories

    My article has been published in Quantum Reports. Expanded abstract: There is currently no consensus on the interpretation of quantum theory, so this article may be of interest as it contains a review and new results on some relevant mathematical models emulating well-known quantum theories...
  46. H

    Allowed energy for a potential in quantum mechanics

    Hi, I'm working on a problem where I need to find the different energies allowed for a potential, and I found this link https://quantummechanics.ucsd.edu/ph130a/130_notes/node151.html, which is similar of what I'm doing. I'm using mathematica to find the values of E. However, I'm not sure how...
  47. Llukis

    I Discover Beginner-Friendly Books on Quantum Information and Computation | Q&A

    Hello to everyone, I would like to ask you to brief questions. The first one is whether you could recommend any pedagogical books on Quantum Information and Computation. I tried Nielsen and Chuang but I found it too dense for a beginner in the field. The second question is the following: to...
  48. S

    B Quantum mechanical one-liners?

    I got a one liner for QP, Electron behave like wave when no observer is there but when we put observer ELECTRON saw this and they told themself hey they are observing us lets change our behaviour, WE are particle now. That type of strange QP is. Am I right?
  49. S

    B Is there any practical proof that quantum entanglement really works?

    What i mean if we change state/spin at one end it will immediately effect the other. Can we see that live using two camera which may be 10 meter apart so that minium time delay. Is there any video proof exist such kind?
  50. Ashish Somwanshi

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