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. Salmone

    I Inner products with spherical harmonics in quantum mechanics

    Let ##|l,m\rangle## be a simultaneous eigenstate of operators ##L^2## and ##L_z## and we want to calculate ##\langle l,m|cos(\theta)|l,m'\rangle## where ##\theta## is the angle ##[0,\pi]##. It is true that in general ##\langle l,m|cos(\theta)|l,m'\rangle=0## ##(1)## for the same ##l## even if...
  2. microatx

    B Does the quantum vacuum have information?

    Hey everyone, My question is simple. Has quantum vacuum information ? And can we measure it with Shannon Entropy and the other ways. By the way I just started to learn english so ı have any grammer mistake please tell me. I know this is out of subject but thank you.
  3. atyy

    A Calmet & Hsu: Quantum hair and black hole information

    https://www.bbc.com/news/science-environment-60708711 Scientists claim hairy black holes explain Hawking paradox https://arxiv.org/abs/2112.05171 Quantum Hair and Black Hole Information Xavier Calmet, Stephen D.H. Hsu It has been shown that the quantum state of the graviton field outside a...
  4. guyvsdcsniper

    Finding the quantum number using Morse Potential

    This problem had me take the taylor series of the Morse Potential, until I got the first non zero term. My result was U(x)=Aα2(x-x0)2. I know to find the quantum number I can use En=(n+1/2)ℏω and I know I can relate that to the potential energy of a harmonic oscillator, 1/2kx2. So if this...
  5. H

    A Exploring Quantum Measurements and Unitary Operators

    Hi Pfs I read this answer in https://quantumcomputing.stackexchange.com/questions/136/if-all-quantum-gates-must-be-unitary-what-about-measurement Quantum measurements are special cases of quantum channels (CPTP cards). Stinespring dilation states that any quantum channel is realized by...
  6. GiuliettaL

    I with my quantum magnetism paper

    (I have to write a 1500 word essay briefly explaining quantum magnetism. But i am having a hard time structuring my essay as I need to select what is crucial and what is not since 1500 words is not a lot. is there anyone with any input for me? Ideas, recommendations, sources anything is...
  7. C

    A Absorption and emission spectrum in quantum optics

    The emission spectrum or resonance fluorescence for a quantum dot, atom or defect center are discussed in many quantum optics textbook, for example see "Quantum Optics" by Marlan O. Scully and M. Suhail Zubairy Chapter 10 , "Quantum Optics" by D. F. Walls and Gerard J. Milburn Chapter 10 and...
  8. thedubdude

    B Special Relativity violation via Quantum Mechanics?

    We know that both momentum and position can not be known precisely simultaneously. The more precisely momentum is known means position is more uncertain. In fact, as I understand quantum mechanics, position probability never extends to 0% anywhere in the universe (except at infinity) for any...
  9. F

    I How to define expectation value in relativistic quantum mechanics?

    In non relativistic quantum mechanics, the expectation value of an operator ##\hat{O}## in state ##\psi## is defined as $$<\psi |\hat{O}|\psi>=\int\psi^* \hat{O} \psi dx$$. Since the scalar product in relativistic quantum has been altered into $$|\psi|^2=i\int\left(\psi^*\frac{\partial...
  10. jedishrfu

    So You Wanna Program a Quantum Computah Just Twist

    https://spectrum.ieee.org/quantum-programming-language-twist?share_id=6917385
  11. Arquimedes

    I What is the physical meaning of concurence (quantum information)?

    Hello, I am currently studying about entanglement on spin-1/2 chains and I was able to find some information about the mathematical point of view of concurrence but I can't understand the physical meaning of it . Can somebody help me, please?
  12. C

    I Building Quantum GR out of quantum rulers and quantum clocks

    Hi, has anyone tried to build "quantum GR", using the expectation value of |Psi(x)> as a "quantum ruler" and |Psi(t)> as a "quantum clock" to build up the idea of a "quantum metric"?
  13. A

    I Plasma-like Description for Elementary and Composite Quantum Particles

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  14. A

    Quantum Need book recommendations for Quantum Chem/Nanomaterials

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  15. jedishrfu

    How Effective is Credit Card Information Theft?

    https://www.nature.com/articles/d41586-022-00339-5
  16. A

    I Why Are There So Few Quantum Time Machine Models?

    As of today, there are plenty of time machine mathematical models based on general relativity theory (warp drives, wormholes), but few ones based on quantum physics. However, back in 2010, Seth Lloyd wrote: "quantum mechanics supports a variety of counter-intuitive phenomena which might allow...
  17. Lynch101

    I Quantum Ball and Cup - Thought Experiment

    I'm sure most will be familiar with the well-known ball and cup trick. The dynamics of the game itself are unimportant, we just need to have the image of 5 cups with a single ball being revealed when the relevant cup is lifted. The Set-up Imagine a machine which has a conveyor belt coming out...
  18. entropy1

    I Quantum Mechanics without time?

    Is there a view in quantummechanics, of quantummechanics, without time as a concept?
  19. M

    I Informational Interpretation of quantum mechanics?

    I heard something today about the "informational interpretation" of quantum mechanics and a phrase used was "it from bit." Is there actually such a thing? What does it mean, and how is it distinguished from other interpretations like MWI or Copenhagen?
  20. W

    Does Quantum Entanglement Define Our Existence?

    hello: I don’t know where to post this and I think this is as good a place as any here. question: does quantum entanglement explain how we are? I am having trouble Even trying to express the question above. But, I just viewed PBS space-time series 6 episode eight: “how do quantum states...
  21. Haorong Wu

    A About Ruling Out Real-Valued Standard Formalism of Quantum Theory

    https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.040403 In QM, I was taught that the imaginary unit ##i## in wave functions is merely a mathematical tool. It has no physical meaning. We can always take the real part of the complex wave functions. Therefore, there should be some...
  22. K

    I Definition of magnetic moment in quantum mechanics

    * The general formula for the magnetic moment of a charge configuration is defined as ##\vec{\mu} = \frac{1}{2} \int \vec{r} \times \vec{J} \,d^3r##* For an electron it's said that the correct equation relating it's spin and magnetic moment is is ##\vec{\mu} =g\frac{q}{2m}\vec{S}## * It's...
  23. phoenix-anna

    I Applications of the proposed global quantum network of clocks

    Physicists have proposed linking a global network of cesium clocks in a phase-coherent entangled state, for example in the article A Quantum Network of Clocks (arXiv:131045v1). My audience would like to know how better synchronization or more accurate timekeeping would lead to advances in our...
  24. D

    I Frequency of EM waves in classical and quantum physics

    in classical physics, when a charged particle oscillates, it emits an electromagnetic wave, and the frequency of the wave depends on the frequency with which the particle oscillates. But in quantum physics, when an excited atom emits a photon, the energy of the photon depends on the magnitude of...
  25. L

    B Quantum Entanglement information transmission idea to knock down....

    I work in IT and am a layman in the quantum world. I have obviously misunderstood something in my amateur reading of quantum, but if someone could explain my mistake in the above scenario it might be very insightful for me! Forgive me if the terminology is not correct - or if indeed lay folks'...
  26. hyksos

    A Call for experiment : Delayed Choice and Quantum Zeno Effect

    Under several historical experiments, measurement back-action has exhibited the ability the suppress a system's transitions to other states, especially when measurements are taken at a high frequency in time. This phenomena has become known as the Quantum Zeno Effect. In short, a quantum...
  27. Ravi Mohan

    I am revisiting the mathematical formulation of quantum mechanics with the dimensional (MLT) perspective

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  28. A. Neumaier

    Insights Quantum Physics via Quantum Tomography: A New Approach to Quantum Mechanics

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  29. M

    I Is quantum gravity still an immensely popular field?

    I was just wondering how much work is being done in the field of quantum gravity nowdays. Is there still a huge volume of research published on the topic? Are we closer to a "solution" nowdays than we were a few years ago? And also, what exactly would constitute a solution to such problem?
  30. benagastov

    Finding all possible energy states faster without using a calculator

    I tried to find states in direct method using ##\frac{E}{E_0}=\:nx^2+ny^2+nz^2## and ##100\:<nx^2+ny^2+nz^2\:<\:136## But it was too long, found it using phi approximation there are around 300 energy states, and Python find around 271 states using direct method but I need manual or recursive...
  31. Somaiyah

    Help with deciding electives: Differential Geometry or Quantum Info

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  32. M

    Help on Learning Quantum Mechanics (Undergraduate)

    Summary:: I am in the highest level Quantum class at my university- technically considered a grad class. I am an undergrad and need advice on just how to learn it. What study tips? Good Youtubers? Physical simulations? Anything that helped you in quantum mechanics. Hello! I am an undergrad...
  33. Vectronix

    Quantum Modern Quantum Mechanics 3rd Ed: J. J. Sakurai & Jim Napolitano Review

    Is Modern Quantum Mechanics, 3rd Edition, by J. J. Sakurai and Jim Napolitano a good book to learn quantum mechanics from?
  34. P

    A Clarifying Fradkin's Terminology on Quantum Numbers of Gauge Groups

    Hi, I'd like to clarify the following terminology (Fradkin, Quantum Field Theory an integrated approach) "carry the quantum numbers of the representation of the gauge group": Does the author basically mean that the wilson loop is a charged operator, in a sense that it transforms non-trivially...
  35. A

    A Loop Quantum gravity or String theory?

    I understand that string theory has almost no testable predictions, however loop quantum gravity is an enticing candidate for only quantum gravity and it doesn't explain much of symmetry, constants, mixing angles etc in Standard model. There is obviously not enough evidence to create a full...
  36. M

    I Schrödinger's astronaut as a quantum computer?

    Consider the following scenario: a space station is put into orbit, which is absurdly well shielded from all sorts of radiation, a.k.a. "a box". You cannot make any effective observation of what the astronaut inside is up to. (This postulate may be implausible, but in the age of "weakly...
  37. Question69

    B Quantum Decoherence: What Happens to Other Outcomes?

    What happens to the other possible outcomes after decoherence has been finalized? Where do they go?
  38. C

    B Tardigrade is first multicellular organism to be quantum entangled

    https://www.newscientist.com/article/2302337-tardigrade-is-first-multicellular-organism-to-be-quantum-entangled/ A tardigrade has been quantum entangled with a superconducting qubit – and lived to tell the tale. It is the first time a multicellular organism has been placed in this strange...
  39. S

    I A thought experiment concerning determinism in quantum mechanics

    According to the uncertainty principle, when we measure a micro-object with a measuring device, we cannot predict what value the device will show. But if we knew exactly the wave function of this device, together with the wave function of the micro-object, could we exactly predict the result of...
  40. schrodingers_cat

    Quantum Good resources for a beginner getting into Quantum Physics?

    Summary:: I’m a beginner trying to get into Quantum Physics, and want some good resources (e.g. books) to get me started. Hi everyone! I’m a beginner trying to get into Quantum Physics, and want some good resources (e.g. books) to get me started. I have a very basic knowledge of physics (10th...
  41. Santiago24

    Quantum Introductory quantum mechanics textbook for self-study

    Hi! I want to self study some of quantum mechanics so i need introductory textbook. I've taken courses on linear algebra that covers all "Linear algebra done right" by Sheldon Axler, multivariable calculus, two courses on general physics and the basics of differentials equations. I really like...
  42. M

    The Quantum Uncertainty of a Particle

    Summary:: I'm trying to understand the meaning behind the answer of 2.34... but I haven't taken a quantum class yet so I'm utterly lost. So I took this physics class that's teaching us python and for our final project our teacher assigned random problems to work out, well I got stuck with a...
  43. M

    I How are good quantum numbers related to perturbation theory?

    Hello folks, I am currently studying from Griffiths' Introduction to Quantum Mechanics and I've got a doubt about good quantum numbers that the text has been unable to solve. As I understand it, good quantum numbers are the eigenvalues of the eigenvectors of an operator O that remain...
  44. H

    Potential energy, Wave function, Quantum physics

    I am totally confused about the task. Any help will be nice. Thank you so much
  45. curiouschuck

    I Gravity & Time Dilation: Quantum Level Questions

    If gravity is the affect of time dilation because of the distance from the bottom of an object to the top then wouldn’t that mean that gravity doesn’t exist at the quantum level?
  46. J

    A Measurement in QFT: Mapping Fields to Theory's Math Formalism

    How do we map experimental measurements of quantum fields, such as those seen in accelerators, to the theory's mathematical formalism? When we see images of particle tracks produced in accelerators such as the LHC, I think it's safe to say a measurement (or series of measurements) has been...
  47. A

    A Quantum computing with superconductors

    I have some questions to this Ansatz and I would really appreciate it if I can get some insight: 1. What is the current status of gate fidelity and what are the current struggles to improve it? 2. Same question but with coherence times. 3. What are the current issues with scalability? 4. Could...
  48. K

    Quantum Finding the Perfect Self-Study Book for Intro Stats & Quantum Mechanics

    Can you please suggest a good introductory statistical and quantum mechanics book which can be self studied. My math background : I've done multivariate calculus, vector calculus, linear algebra ,some complex analysis all at the usual undergraduate level. The books I've self studied thus far...
  49. tworitdash

    A Applications of weak measurement of quantum mechanics in other domains

    This is a surface level question and I don't want to go into detail. Imagine an algorithm which when used with a sensor output gives the statistical moments of a variable in nature (for example mean and standard deviation of a variable). The sensor measures this once in a while (like once in a...
  50. J

    Discretizing a 1D quantum harmonic oscillator, finding eigenvalues

    ##x## can be discretized as ##x \rightarrow x_k ## such that ##x_{k + 1} = x_k + dx## with a positive integer ##k##. Throughout we may assume that ##dx## is finite, albeit tiny. By applying the Taylor expansion of the wavefunction ##\psi_n(x_{k+1})## and ##\psi_n(x_{k-1})##, we can quickly...
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