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. N

    B U.Delft Quantum Results - Is the explanation parsimonious?

    Thx !
  2. Someone_physics

    A Conditional time evolution entropy and the experimenter?

    Question --- So I've done a calculation which seems to suggest if I combine the system of a measuring apparatus to say an experimenter who "reacts" to the outcome of the the measurement versus one who does not. Then the change in entropy in both these situations is bounded by: $$ \Delta S_R...
  3. J

    I General Relativistic Quantum Theory?

    Is it possible to have some kind of General Relativistic Quantum Theory without passing through the stage of Quantum Field Theory (where Quantum Theory is married to Special Relativity)? Einstein attached primary significance to the concept of general covariance as shown in this letter in 1954...
  4. S

    I Quantum Computing "Not" Operation - Mathematics Steps For Deriving It

    Hello, I was watching a video lecture from MIT 8.04 (Allan Adams)– lecture #24 (around the 38 minute mark give or take) The topic is quantum computing, Dr. Adams is deriving / explaining how to get various computing operations. For the “NOT” operation he explains that the operator $$ U_{Not}...
  5. E

    Quantum mechanics - several constant potentials

    What I tried to do was using the fact that the wave function should be continuous. Asin(kb)=Be^{-\alpha b} The derivative also should be continuous: kAcos(kb)=-\alpha Be^{-\alpha b} And the probability to find the particle in total should be 1: \int_0^b A^2sin^2(kx) dx + \int_b^{\infty}...
  6. AryaKimiaghalam

    A Interface between Biological Physics and Quantum Physics

    Are there areas of studies which could be characterized as an interface of biological physics and quantum physics? Does such an interface even exist?
  7. J

    A Quantum tomography: Where does the magic happen?

    My question is: How does this happen? Less measurements than 4^n-1 means that literally we don't have enough information to label the state. How can the neural network overcome this lack of information?
  8. T

    A What happens after a theory of quantum gravity is found?

    Hello there. What will physicists do after a theory of quantum gravity is found?Will they ask, if it is found ,more questions about it and try to develop it?What other questions will they make probably?Thank you.
  9. Someone_physics

    I Realizing a Quantum Mechanical Watch-Stop

    Question --- I can show for a position eigenstate ## | x \rangle ## if it evolves in time ##U(\Delta t) | x\rangle ## (where ##U## is the unitary operator). Then one can bound the time elapsed by finding the probability amplitude ##| \langle x | U^\dagger(\Delta t)| x + \Delta x \rangle |^2 ##...
  10. I

    B Black Hole singularities in Quantum Gravity

    If singularities don't exist in QG then what prevents particles from just collapsing falling further until they collapse into a singularity? Is there a repulsive force in QG ? Is time infinitely stretched near a singularities? What else could be happening?
  11. S

    A Question on many world interpretation of Quantum Theory

    Every second the universe branches into 5000 universes and each of those 5000 universes branches into 5000 more after one more second. Now, consider an 80 year old person, he has lived close to 80*365*24*60*60 seconds, which is 2.5 Billion seconds. So, in his life time, universe has branched...
  12. J

    A Restricted Boltzmann machine for Quantum state tomography

    I'm struggling with my Final Degree Project. I would like to perform a quantum simulation and perform quantum tomography for a single-qubit using a resrticted Boltzmann machine. In order to do so I'm trying to follow the recipe in the paper "Neural Network quantum state tomography, Giacomo...
  13. A

    I Quantum Foundations: outlook on this research field?

    Recently, it has come to my attention a field called Quantum Foundations. This is exactly what brought me into Physics, even though back then I didn't know it was a research area. In my Physics classes, I got disappointed and unmotivated at the "Shut up and calculate!" attitude of my Physics...
  14. Lutz-F

    B Theory of Loop Quantum Gravity

    From the german Version of Carlo Rovellis book "La realtà non è come ci appare. La struttura elementare delle cose" I have learned about the theory of Loop Quantum Gravity that space and time arise through the interactions of gravitational quanta, the space quanta have discrete volume spectra...
  15. Muthumanimaran

    I Exploring the Physics of LASER: Classical vs Quantum Mechanics

    My question is the physics behind the LASER such as stimulated emission can be only explained by quantum mechanics only. We can represent LASER as coherent state in quantum mechanics only. Then how can we say LASER can be thought of a classical light source?
  16. fluidistic

    Quantum Schwinger's Quantum Mechanics: Symbolism of Atomic Measurements

    I had never heard of Schwinger's Quantum Mechanics: Symbolism of Atomic Measurements until very recently. I wonder what you people think about that QM textbook. Is it a good introduction to QM? A reference? Or, possibly an outdated and bad book? At first glance, it seems a masterpiece to me...
  17. steve1763

    A Exploring Free and Interaction Terms of L in Quantum Field Theory

    With free part L=-½(∂Φ)^2 -½m^2 Φ^2 and interaction term L=½gΦ^2Any help would be appreciated, thank you.
  18. L

    I Equivalence of these quantum circuits

    In the attached image, there are two quantum circuits that are equivalent. I am trying to understand how. Let's call the top qubit ##q_1## and the bottom one ##q_2##, and the outputs ##q_1'## and ##q_2'##. From what I understand, the C-NOT gate doesn't affect the control qubit. Because Hadamard...
  19. H

    A Ads/cft correspondence and quantum / classical correspondance

    Hi Pf I do not know if it is only an analogy but i consider these two correspondences: I) the transformation of a quantum spin into a statistical problem by a Wick rotation. we start with a qubit evolving in time between 0 and t from S0 to St. we use a exp(iHt) operator to describe its...
  20. cutecat

    Quantum Do you have a quantum physics book? Can I borrow it?

    Homework Statement:: I need a quantum physics book because I am learning it Relevant Equations:: Quantum physics I can borrow a pdf one
  21. vintagelover007

    Quantum Uncertainty Questions (HUP)

    a) Consider a dust grain of mass 0.1µg whose position is known to a distance of 1µm. How precisely can its velocity be known? b) A company claims to have invented a device that can measure the momentum of objects inside it with extreme accuracy. The device fits within a matchbox, and the...
  22. T

    I Quantum numbers for energy levels

    Hello Can some one explain how you work out the combinations of quantum numbers for infinite wells in higher dimensions? For example if i have an energy level $$E_4$$ In a 2D well, then for quantum numbers does this mean the combinations allowed must be: $$4^2 + 1^2$$ $$1^2 + 4^2$$ So then...
  23. iVenky

    I Are quantum computers required to be cold to reduce Brownian motion?

    I understand that based on what I have read online quantum computers are required to be close to absolute zero because it introduces less error. Is it because brownian motion due to thermal agitation of molecules reduces with temperature?
  24. J

    B Is a Quantum Entanglement Camera the Key to Seeing Into the Future of Earth?

    I do not have the education to grasp the math of quantum mechanics but I am very interested in it and understand some of the concepts. I often find myself pondering those concepts. One thing I began to wonder about was the possibility of using quantum entanglement to observe distant objects so...
  25. T

    Quantum Mechanics determining the normalized constant of a particle

    In my book it has the following example, A particle confined to the surface of a sphere is in the state $$\Psi(\theta, \phi)= \Bigg\{^{N(\frac{\pi^2}{4}-\theta^2), \ 0 < \theta < \frac{\pi}{2}}_{0, \ \frac{\pi}{2} < \theta < \pi}$$ and they determined the normalization constant for ##N##...
  26. B

    I Interpretation of quantum theories through the interpretation of classical ones

    Perhaps the reason is that the principle of least action is elevated to a divine principle that does not require materialistic interpretation. If so, then the solution to the "secrets" of quantum theories lies in the solution of the materialistic essence of the physical magnitude of action.
  27. P

    Layman asks about Quantum "interaction"

    I'm a college grad, but not in science or physics. I'm useless on the math. However, I have a solid layman's understanding of double slit experiment as well as the delayed choice quantum eraser . I also have a layman's understanding of quantum physics via reading some mainstream science...
  28. koroshii

    Finding a unitary operator for quantum non-locality.

    My trouble might be from how I interpret the problem. Alice and Bob are entangled. After Alice makes the measurement both of their states should collapse to one of these states with a certain probability. (Unless my understand of how entanglement is wrong.) The way I am understand the question...
  29. patric44

    A Would it matter which inner product I choose in quantum mechanics?

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  30. I

    I Quantum Mechanics Wave Function in 3D

    I was wondering if it's possible to plot a wave function that is a function of 3 coordinates, such as (x, y, z). The text my class uses calls this Quantum Mechanics in 3 dimensions, but wouldn't this technically by four dimensions?
  31. MichPod

    I Quantum vs Classical multiphoton interference on double slit

    For a setup analogous to double-slit experiment, do I understand it correctly that if we have two photons, one going only through slit A and another going only through slit B simultaneously (or vice versa as they have a symmetrical wave function), then there will be no interference pattern...
  32. D

    B 2D-cross-double-slit experiments for understanding quantum world

    How did you find PF?: gmail The regular double-slit experiment is the "the basic Mystery" in quantum (Feynman). now I have done several cross-double-slit experiments with different configurations, My question is: how photons “sense”: (1) which slit they pass through; (2) what photons they...
  33. danielhaish

    I Why quantum wave function collapse is not consider a signal?

    I read about the non-communication theorem and I understand why when changing one practical will not change the other . But suppose that there is two observers that doing the double slit experiment, but using it with two entanglement practicals. observer one should send signal of yes or...
  34. .Scott

    The Fast Fourier Transform is described in the Quantum Domain

    In August, "Quantum Information Processing" published an article describing a full FFT in the quantum domain - a so-called QFFT, not to be confused with the simpler QFT. According to the publication:
  35. Q

    I What is the current perspective on quantum interpretation?

    Back in the day, there were a few Quantum Interpretation polls on here, as of late I have not seen any. I love that we now have a sub-forum for Foundations discussions. I figured it would be interesting to see how the participants of PhysicsForums feel about the different interpretations these...
  36. Vanilla Gorilla

    B Could Quantum Fluctuations exist W/out the presence of Spacetime?

    I want to know whether Quantum Fluctuations could exist without the presence of Spacetime. Would it be possible, in the event of a Big Rip scenario, and if Spacetime really would get ripped apart, that quantum fluctuations could still occur? And if Spacetime is ripped apart, does that mean the...
  37. TechieDork

    Courses I think I won't get an A in Quantum Mechanics I

    This is by far the hardest undergraduate class I have ever take. The majority of class got less than 40% on the midterm. Unfortunately, I was sick during the exam hours too ,so it's hard for me to concentrate and think clearly Thank god,the professor uses the norm-referenced grading and My...
  38. Quantum23

    A What is gravity in the superfluid vacuum model of quantum gravity?

    It has been hypothesized by quantum field theory that the vacuum is not empty due to the energy time uncertainity relationship. Instead it is filled with a sea of virtual particles popping in and out of existence and renormalized to the observed value we see today in experiments like the casimir...
  39. E

    Griffiths Quantum Mechanics 2nd edition Chapter 8 equation is confusing

    In Griffiths Quantum Mechanics 2nd edition, in Chapter 8 he calculates the following integral on page 323 and he gets I disagree with this result, I think the integral should be since Maybe somebody can explain why I am wrong? Also, from equation 8.24 to 8.25, he makes the assumption that...
  40. johnconner

    I Copenhagen interpretation & Delayed-choice quantum eraser

    For explaining Delayed-choice quantum eraser experiment, what if I change the Copenhagen interpretation a little. When the experiment is concluded, we observe one detector (the first detector with the shortest path) has detected the pattern long before the wavefunction is collapsed (before the...
  41. Haorong Wu

    Quantum Anyone tried "Problem Book in Quantum Field Theory" by Radovanovic?

    It is a wonderful book for learning QFT. Interesting problems with detailed solutions. I have tried the problems from chapter 1 to chapter 7. In most chapters, I could at least solve some part of the problems. But I got stuck in chapter 4, the Dirac equation. I could not solve any of the...
  42. L

    Quantum double slits moving past a quantum particle?

    Open question: Can we be sure that a quantum model double slit doesn't create a carrier interference pattern around the slits, in the fields, already there? Relativity is such a big deal in physics but so many models only consider seemingly weird quantum particles going through normal classical...
  43. chocopanda

    Quantum Mechanics: creation and annihilation operators

    Hello everyone, I'm new here and I'm struggling with the mathematical formalities in quantum mechanics. $$\langle n+1|b^\dagger bb^\dagger + \frac 12 |n \rangle = \langle n+1|b^\dagger bb^\dagger |n \rangle + \langle n+1| \frac 12 |n \rangle $$ $$ = \langle n+1|b^\dagger b \sqrt{n+1} |n+1...
  44. jjson775

    Quantum numbers for p-mesons substituted for electrons

    n = 3, l = 2, me = -2, -1, 0, 1, 2. ms = -1 or 1. The correct answer is that ms can also be 0. Why?
  45. Mayan Fung

    Perturbation from a quantum harmonic oscillator potential

    For the off-diagonal term, it is obvious that (p^2+q^2) returns 0 in the integration (##<m|p^2+q^2|n> = E<m|n> = 0##). However, (pq+qp) seems to give a complicated expression because of the complicated wavefunctions of a quantum harmonic oscillator. I wonder whether there is a good method to...
  46. Jamister

    Quantum Best book for quantum foundations

    I'm looking for a book on foundation of quantum mechanics. I started reading the book "do we really understand quantum mechanics" of franck laloë , and it seems to be very chatty and long. Do you have any recommendations? Thank you
  47. dontknow

    A Statistical physics : Irreversibility

    I was reading mehran kardar (books and lectures) it says the concept of irreversibility comes from an assumption (in which we increase the length scale by interaction disctance between two particles). So My question is the concept of irreversibility is still valid in the case of 1 particle...
  48. S

    Physics Solid State Physics or Quantum Electronics/Optics

    For which field is there more demand in the industry? And is knowledge of quantum electronics/optics useless without having a phd, as I see most job offers ask for a doctorate degree. Are the skills and knowledge from quantum electronics transferable to engineering positions or is what you...
  49. Vanilla Gorilla

    I Would Black Holes Be Coherent Under Quantum Mechanics?

    I just want to know if Black Holes would be quantumly coherent.
  50. Jamister

    Programs Searching for universities to do my PhD on foundations of quantum mechanics

    I want to do my Phd on foundations of quantum mechanics, but I don't find researchers in the U.S.A that work on that. Is there a good way to search other than to go to each university and go over the PI's? Thanks
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