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. J

    Help with quantum computing notation

    Hi everyone! Sorry for the bad English! I'm trying to read the "entanglement between photons that never coexisted " from 2012. Avaliable at: https://arxiv.org/abs/1209.4191 And there's this equation: ## |φ± \rangle = \frac 1{√2}(|HaHb ± |VaVb\rangle) ## ## |ψ± \rangle = \frac 1{√2}(|HaVb ±...
  2. W

    Quantum Mechanics and Electrodynamics/Electrostatics

    Hi all, I have a question relating to the title above. The uncertainty relation tells us that an electron that is localised (in terms of its PDF) is space has a large uncertainty in momentum space. However in classical electrostatics/dynamics we seem to make attempts to do things like...
  3. Robert Friz

    B Are Quantum Entities in Macro Sized Objects Entangled?

    Hi. This is my first posting on the Physics Forum so please forgive any issues as a result. I am a (reasonably educated) lay person with a strong physics interest with extensive readings -- so please be patient with my questions. :-> My questions and interest in these issues are sincere. I...
  4. jedishrfu

    B NOVA Show on Einstein and Quantum Entanglement

    NOVA presents a show on Einstein and Quantum Entanglement:
  5. S

    A Dipole moment of an isolated quantum system

    How to prove the dipole moment of an isolated quantum system in isotropic space is identically equal to zero, unless there exists an accidental degeneracy. Thanks in advance
  6. GANTI_RAVITEJA

    I One-Dimensional System: Boundary Condition Applicability

    In one dimensional system the boundary condition that the derivative of the wave function Ψ(x) should be continuous at every point is applicable whenever?
  7. RealKiller69

    I Quantum Oscillator in 1D: How Can a Real Particle Have an Imaginary Velocity?

    I have got a simple qstion. We have a particle in 1d oscillator with E0( fundamental level).We know that phi~ e^-x^2 for any x, so We can measure a position and get a value x=a, such that V(a)>E0 . In this case T<0 so the velocity of the particle is imaginary, how is this even possible?, (a real...
  8. i_hate_math

    Quantum Gases - Mixing of 3He and 4He

    Homework Statement Consider a solution in which 99% of the atoms are 4He and 1% are 3He. Assuming that the 3He atoms behave as an ideal gas of spin-1/2 particles determine the Fermi energy of the 3He atoms. You may assume that one mole of 4He occupies a volume of 28 cm3.Homework Equations EF =...
  9. A

    The effect of macroscopic quantum phenomena on human evolution

    In a discussion between Sam Harris and Brian Greene, at this point, Brian stated that even if we return the brain and all the environment to its previous state, we "WON'T MAKE THE SAME NOISES"; I know that for example, indeterminacy in determining the precise time of decay of an atom (and the...
  10. B

    I Info traveling faster than light, and quantum entanglement

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  11. Ackbach

    MHB Interesting Result in Quantum Error Correcting Codes versus Relativistic Gravity

    Thought you all might be interested in this article. It could just be hype, but there also might be something deeper to it. You hear of so many schemes of tying together quantum mechanics with a relativistic understanding of gravity, that it's easy to respond to yet another announcement like...
  12. jgarrel

    Quantum Field Theory-Mass spectrum of Lagrangian

    Homework Statement We are given the Lagrangian density: $$ \mathcal{L}=\partial^\mu \phi ^* \partial_\mu \phi - m\phi^* \phi +\sum_{\alpha =1} ^2 ( \overline{\psi}^\alpha (i\gamma^\mu \partial_\mu -m)\psi^\alpha -g\overline{\psi}^\alpha\psi^\alpha \phi^* \phi) $$ , where ##\phi## is a complex...
  13. B

    Quantum mechanical integral equation problem

    Homework Statement The question is; for a qunatum mechanical particle, Ψ(x) = [1/(a1/2.π1/4)].[e-(x-xo)2/2a].[eip0x/h] in here, x0, p0 and h are constants, so, Homework Equations what are the <x>; expetation value, and <P>;expectation value of momentum ? The Attempt at a Solution , [/B]...
  14. B

    A Quantum mechanical integral equation problem

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  15. Cerenkov

    B The Spectral Gap and Quantum Vacuum Fluctuations

    Hello. Recently Scientific American magazine carried an article about the work of the authors of this paper. https://arxiv.org/abs/1502.04573 The Undecidability of the Spectral Gap. The SciAm article is linked to here -...
  16. A

    I Math: classical to quantum locality

    Does anybody know if there is work being done on quantum locality vs nonlocality. Specifically, approaching the Planck scales there is no nonlocality. That is all points in space and time are local. This would also provide an explanation for Einstein Minkowsky's space time. Any material /...
  17. H

    A Quantum fields and the harmonic oscillator

    When defining quantum fields as a sum of creation and annihilation operators for each momenta, we do it in analogy with the simple example of the harmonic oscillator in quantum mechanics. But why do we assume that the coefficients in the expansion can be interpreted in the same way as in the...
  18. F

    Engineering Evaluate the quantum circuit

    Homework Statement input state is |1>(tensorproduct)(2|0>+|1>). Homework Equations Find the probability of the outcomes being 0 and 1 measuring the second qubit. And then what the outcome states of each are. The Attempt at a Solution I have attempted this problem and for the probability of...
  19. P

    I Van der Waals force in quantum physics

    According to QFT, are there hydrogen bonds or Van der Waals force? Or this an outdated concept of classical physics?
  20. W

    I Reading materials on quantum foundations

    Hi all, I have been trying to read up on quantum foundations after being first introduced to it on this Perimeter Institute page: https://www.perimeterinstitute.ca/research/research-areas/quantum-foundations/more-quantum-foundations However, I have had difficulty finding notes and papers on...
  21. Q

    I Quantum State: Electron, Proton, and Neutron Explained

    Can anyone tell me, What quantum state really is? Is it applicable for all sub atomic particles? Then, Can anyone explain how two electron are never in same quantum state. And Does proton or neutron follows the same law as electron for obtaining unique quantum state.
  22. L

    Quantum - Two State Problem in different bases

    Homework Statement [/B] (Working through a problem from a practice set for which I have a solution available, but still don't understand. I get the same answer as they do for part a, but get lost in part b, I think. Relevant portions below) Consider a two-state quantum system. In the...
  23. Christopher Rourk

    A Is the Fenna-Matthews-Olson complex a quantum dot?

    The FMO complex has a size that is within the typical size range for quantum dots, and absorbs photon energy at what appears to be an effective bandgap between 2-3 eV. While various techniques have been used to investigate the behavior of the FMO complex, such as femto photography or...
  24. Dhammika

    I What is the common & connection of quantum entanglement and entropy?

    Many people talking about there are similarities and common positions in quantum entaglement and superposition with entropy. I need to know about this phenomenon
  25. F

    A How to measure the first qubit in two qubit system? QC

    I was wondering how to measure the first or even the second qubit in a quantum computing system after for example a Hadamard Gate is applied to the system of these qubits: A|00>+B|01>+C|10>+D|11>? A mathematical and intuitive explanation would be nice, I am a undergraduate sophomore student...
  26. Christopher Rourk

    A Quantum Dot Solids: Electron Minibands & Potential

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  27. microsansfil

    I Does quantum entanglement depend on the chosen basis?

    Hi, In this presentation about quantum optics it is mentioned that the same quantum state |Ψ> has different expressions in different mode bases : factorized state or entangled state. This presentation is related to this video : In some way entanglement isn't intrinsic. It depend on the...
  28. Q

    B Neutral pion quark composition help

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  29. P

    A Is there any astro-field that uses QM or GR frequently?

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  30. Y

    I Schmidt decomposition and entropy of the W state

    Hello, The state | W \rangle = \frac { 1 } { \sqrt { 3 } } ( | 001 \rangle + | 010 \rangle + | 100 \rangle ) is entangled. The Schmidt decomposition is : What would the Schmidt decomposition be for | W \rangle ? I am also intersted in writing the reduced density matrix but I need the basis...
  31. Another

    Question commutation in quantum mechanics

    Homework Statement Show that ##[L_{x}^2,L_{y}^2]=[L_{y}^2,L_{z}^2]=[L_{z}^2,L_{x}^2]## Homework Equations ##L^2 = L_{x}^2+L_{y}^2+L_{z}^2## ##L_x = yp_z-zp_y## ##L_y = zp_x-xp_z## ##L_z = xp_y-yp_x## ##[x_i,p_j]=iħδ_{ij}## ##[L_x,L_y]=iħL_z## ##[L_y,L_z]=iħL_x## ##[L_z,L_x]=iħL_y##...
  32. gimak

    Programs Working at the center for Computational Quantum Chemistry

    Hello, I'd like to work in the above named center at the University of Georgia in Athens, Georgia under Dr. Henry Schafer. However, he's a professor in the chemistry department. I'm guessing if I go in the university as a physics graduate student I can't have him as a Ph.D advisor. However, is...
  33. M

    I Raising the ladder operators to a power

    Hi! I am working on homework and came across this problem: <n|X5|n> I know X = ((ħ/(2mω))1/2 (a + a+)) And if I raise X to the 5th, its becomes X5 = ((ħ/(2mω))5/2 (a + a+)5) What I'm wondering is, is there anyway to be able to solve this without going through all of the iterations the...
  34. kroni

    A Simulating Quantum Loop Theory: Can S-Knots Be Represented Numerically?

    Hello, I am contacting you because I would like to know if there is a way to simulate quantum loop theory. Indeed, the S-Knots are much more complex objects than graphs because between the points there is a curve that can be knotted. S-Knots are graph embeddings in 3D and I do not see how such...
  35. A

    Quantum energy of a particle in a 2 dimensional space

    Homework Statement [/B]Homework Equations Doing this problem like e.g setting the determinant of potential matrix and the ω2*kinetic matrix equal to 0 ,det(V-ω2T)=0,I got the frequency of the normal modes of vibration to be 2ω0 and ω0 where ω0 is the natural frequency, But sir how to treat...
  36. A

    Quantum energy of a particle in 2 dim space

    Homework Statement Homework Equations Doing this problem like e.g setting the determinant of potential matrix and the ω2*kinetic matrix equal to 0 ,det(V-ω2T)=0,I got the frequency of the normal modes of vibration to be 2ω0 and ω0 where ω0 is the natural frequency, But sir how to treat this...
  37. M

    Quantum Teleportation Homework: Deriving EPR Pair & Measuring Spin 1/2 Particles

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  38. Konte

    I Resolvent formalism in quantum mechanics

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  39. Edge5

    I Solution of Quantum differential equation

    (I think I couldn't add the image) you can see my answer in link https://pasteboard.co/HPKZ6KD.jpg (Please first see my answer in the link) But in answer it is φ= Asin(kx) + Bcos(kx) I know that euler formula is eix = cosx +isinx But I can't get this answer can you help me?
  40. I

    I Particle in a box, boundary co-ordinate change

    If you have a particle in a 1-d box with a finite potential when ##0 < x < L ## and an infinite potential outside this region, then the normalised wavefunction used to describe said particle is ## \psi (x) = \sqrt{\frac{2}{L}}\sin(\frac{n\pi x}{L})##. However, if you had say instead a finite...
  41. Zeynel

    I Transistors and Quantum Physics

    My question is: What is the contribution of Quantum Physics to the discovery, of the transistor? In Adam Becker's book What is real? I read that, "the discovery of quantum physics in the early twentieth century led directly to the [discovery] of silicon transistors..." He implies that, the...
  42. Bilal Rajab Abbasi

    I Introduction to Quantum Mechanics

    Hi! I am Bilal Rajab. I have a question regarding Quantum Physics. From where can we learn about Quantum Physics and what is its relation to Classical Physics? Why is there not one single Physics...? Thanks Regards, Bilal Rajab Abbasi
  43. S

    I Griffiths' Intro to Quantum Mechanics - primed variables

    Hello,I have been going through Griffiths’ Intro To Quantum Mechanics in an attempt to self-teach myself some quantum mechanics. I am currently in section 3.4 (Generalized Statistical Interpretation) I am seeing a “step” occur more than once. When I first came across it, while I didn’t...
  44. Morbidly_Green

    Expressing the density matrix in matrix form

    Homework Statement Given the above lambda system, is it wrong to say that the density matrix is of the form ## \rho = c_1|1> + c_2|2> + c_3|3> ## ? Hence when written in matrix form (basis of ##|i>##), ## \rho ## is a diagonal matrix who's elements are the ##c_i##s?
  45. A

    I Does a Quantum Field Creation Operator Create Particles at a Given Location?

    Hi, It appears that the definition of a quantum field creation operator is given by $$\Psi^{\dagger}(\mathbf r) = \sum\limits_{\mathbf k} e^{-i\mathbf k\cdot \mathbf r} a^{\dagger}_{\mathbf k}.$$ But then if we examine how this operator acts on the vacuum state, we get $$\Psi^{\dagger}(\mathbf...
  46. nomadreid

    I Can the quantum eraser experiment produce two different interference patterns?

    In the explanation of the quantum eraser, Wikipedia https://en.wikipedia.org/wiki/Quantum_eraser_experiment states "...the 'which-path' information is 'erased,' whereupon the interference pattern is recovered. (Rather than removing or reversing any changes introduced into the photon or its path...
  47. W

    I Is quantum mechanics formulated from 1st principles?

    I was surprised recently to learn that one of the reasons both Newton and Einstein were so revolutionary was that they performed a neat mathematical trick. For Newton, it was the mathematical derivation of Kepler's laws from Newton's laws of gravitation and motion. For Einstein, it was the...
  48. J

    I Quantum Suicide: Exploring Copenhagen & Many Worlds Interpretations

    Is it possible to modify the original quantum suicide experiment in a way in which rather than dying you are put to sleep for someone to distinguish between many worlds and Copenhagen interpretations?
  49. M

    Quantum Zeno Effect and Evolution Operator Properties

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  50. ParticleMan

    I Open Source Quantum Mechanics Visualization Software

    Are there any folks here who use open source alternatives to Matlab and Mathmatica for visualization of quantum mechanics? Also, are you aware of any introductory quantum material for that tool? If not, do you think such a thing would be worthwhile for visual leaners? Why or why not? I have...
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