Quantum Definition and 999 Threads

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. F

    B Quantum entanglement phenomenon

    Hi there, Question from a biologist with very poor background in physics, but willing to understand quantum physics. I think quantum entanglement shocks everyone, even if it has been proven right. I would love to know if there is any hypothesis or crazy theory out there to explain why or how...
  2. learn.steadfast

    I Interpretations of diffusion in Quantum Mechanics.

    Recently, I've been told I was wrong concerning the nature of stationary states and diffusion being related. Even though I pointed out to the people involved that I was merely paraphrasing Max Born, who was apparently quoting the same idea as Linus Pauling. No one has been able to tell me...
  3. J

    I What is the basic scheme of quantum field theories?

    In classical mechanics I would say: a particle can have any initial position and velocities, Newton laws give you the evolution of the particle: F=ma , is the basic equation, if you know the forces acting on the particle by solving this equation you get the future values for velocity and...
  4. K

    Quantum The main source books for quantum mechanics?

    I would like to know which books are the main sources for the Quantum mechanics 1/2 courses, and the professors use them most during their courses? Thanks for you reply in advance.
  5. D

    Quantum Quantum mechanics Zettili 3rd Edition available yet?

    Hi. I really like this book (2nd edition) and was thinking of buying it. I have seen mention of a new edition , the 3rd edition but it seems to be unavailable to buy. Anyone know if the 3rd edition is out yet or soon ? Thanks
  6. Marisa5

    Quantum Textbooks on qubits/trapped ions/BECs for quantum computing

    Hello, I was wondering if any of you know about good books on various qubit approaches for making quantum computers. There are a lot of road maps on the internet covering the pros and cons of each approach (for example: https://qt.eu/app/uploads/2018/04/QT-Roadmap-2016.pdf ) but I was hoping...
  7. Rodia

    B How are outer products quantum states?

    In my textbook, quantum states are infinite dimensional vectors. But I was watching a lecture on QM and the professor referred to ##|v> <u|## as itself being a quantum state. Also I saw online people saying the same thing. Are tensor products just things that tell you whether or not the two...
  8. B

    B Quantum eraser and time travel

    i don't understand, how does quantum eraser not imply time travel? i firmly don't believe in retrocausality, but it really does seem to imply it . can someone please explain in layman terms? in here paul davies says observers now can "constrain" the past, does this mean our perception of...
  9. A

    I PROOF: Quantum Fidelity with pure states

    Hi, I'm currently working on showing the relation of quantum fidelity: The quantum “fidelity” between two pure states ρ1 = |ψ1⟩⟨ψ1| and ρ2 = |ψ2⟩⟨ψ2| is given by |⟨ψ1|ψ2⟩|^2. Show that this quantity may be written as Tr(ρ1ρ2). I've been following the wikipedia page on fidelity but can't...
  10. J

    B Help with math in a quantum circuit

    Hi! Sorry for the bad English! I'm trying to get a better understanding in quantum mechanics by doing the math of quantum circuits and I'm stuck! So, please, if someone can, help me! =) I'm trying to the math of this circuit: q0 ... l 0 ) .... x ... (I) q1 ...
  11. B

    Quantum state of entangled photons

    Homework Statement Suppose two polarization-entangled photons A and B in the following Bell state: \begin{equation} \Phi=\frac{1}{\sqrt{2}}\bigl(\left|H_{A},H_{B}\right\rangle + \left| V_{A},V_{B}\right\rangle\bigr) \end{equation} 1. What is the state if the photon A passes through a...
  12. W

    I Assumptions made in stationary-state scattering

    Hi all, I recently started learning about quantum scattering in school and came across a few things I find confusing. Thanks in advance for any assistance! 1. Plane wave approximation to incident waves. In past QM courses, I kept reading that plane waves were not "physical" since they do not...
  13. Auto-Didact

    Quantum Entangled Living Bacteria

    "Schrödinger's Bacterium" Could Be a Quantum Biology Milestone I can't believe I'm only seeing this article now. Achieving quantum mechanical effects with large systems, especially complicated ones such as bacteria - let alone one in vivo - has been a longstanding goal in experimental QM. To...
  14. 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 ±...
  15. 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...
  16. 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...
  17. jedishrfu

    B NOVA Show on Einstein and Quantum Entanglement

    NOVA presents a show on Einstein and Quantum Entanglement:
  18. 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
  19. 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?
  20. 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...
  21. 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 =...
  22. 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...
  23. B

    I Info traveling faster than light, and quantum entanglement

    OK, going to ask a question that I sort of know is going to be shot down but at the moment I can't make sense of this. If I send a machine/robot with a particle that is quantum entangled with another particle that is left on earth. When one particle is blue the other is red. The machine also...
  24. 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...
  25. 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...
  26. 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]...
  27. B

    A Quantum mechanical integral equation problem

    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, what are the <x> and <P> ?
  28. 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 -...
  29. 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 /...
  30. 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...
  31. F

    Engineering What Is the Correct Probability Calculation for a Quantum Circuit Measurement?

    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...
  32. 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?
  33. 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...
  34. 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.
  35. 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...
  36. 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...
  37. 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
  38. 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...
  39. Christopher Rourk

    A Quantum Dot Solids: Electron Minibands & Potential

    Do electron minibands in quantum dot solids have a potential?
  40. 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...
  41. Q

    B Neutral pion quark composition help

    Hi guys, Merry Christmas to you all! I wanted to know whether a neutral pion can be made up of a strange quark and an anti-strange quark. I know that the kaon is the only strange meson and all variations contain an s quark but wouldn't the strangeness be zero in an s quark/anti-s quark pair as...
  42. P

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

    Hi all! Happy New year! I will select my supervisor soon but i want to involve in the field that uses QM or GR frequently I love physics and i want to apply physics to explain astrophysical phenomenon. I know that Compact objects and Astrochemistry use such physics but i wonder to know more...
  43. 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...
  44. 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##...
  45. 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...
  46. 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...
  47. 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...
  48. 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...
  49. 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...
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