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. T

    I Quantum Astronomy - well sort-of

    It is a proposed method for relaying the light from multiple telescope, long baseline, interferometers. Some proposed hardware has passed the initial proof-of-concept stage. https://www.scientificamerican.com/article/quantum-astronomy-could-create-telescopes-hundreds-of-kilometers-wide/...
  2. Mayan Fung

    I How is Coulomb's Law compatible with quantum physics?

    In classical physics, we treat an electron as a point charge with a Coulomb potential ## V = \frac{q}{4\pi\epsilon_o r}##. However, in quantum mechanics, we treat it as an electron cloud. In this situation, how shall we describe the Coulomb potential? Shall we treat the electron as a charge...
  3. J

    A 1D transverse-field Ising model - classical vs quantum differences?

    The 1D transverse field Ising model $$ H(\sigma)=-J\sum_{i\in \mathbb{Z}} \sigma^x_i \sigma^x_{i+1} -h \sum_{i \in \mathbb{Z}} \sigma^z_i$$ is usually solved in quantum way, but we can also solve it classically - e.g. parametrize angles of spins ##\sigma^x_i = \cos(\alpha_i)...
  4. K

    I How Does Spin-Orbit Coupling Affect Quantum Number Validity?

    Hello! Assume I have a system containing intrinsic spin and orbital angular momentum and before coupling the two, ##|J,J_z,S,S_z,L,L_z>## is a good basis (i.e. all these quantum numbers are good), with ##J=L+S## and ##J_z = S_z + L_z##. If I add a term of the form ##S\cdot L##, ##L_z## and...
  5. K

    B Question about quantum entanglement

    Dear PF Forum, I've been having this question for a long time. I want to know how or what is measurement. Supposed there are two observers. Alice and Bob, They are separated 10 light minutes away. They are in the same frame of reference, meaning that their distance is the same all the time...
  6. Erik Ayer

    I Understanding Coherence vs. Entanglement in Quantum Eraser

    As I understand it, either coherence or entanglement can be in an experiment or some of each, varying continuously between the two. Because of this, coincidence detection is needed to pick out interference patterns among all the data. Coherence would refer to quantum waves of photons taking...
  7. The Baron

    I Could Light Absorption Be Linked to Quantum Tunneling?

    Hi, I have just now started learning about quantum mechanics, and I have an Idea, which i am 99 percent sure is wrong, but i wanted to post it just in case. Okay, so I researched it a little and it turns out that the probability for a single particle to experience quantum tunneling is 0.1%...
  8. B

    B Sun powered by quantum tunneling - question

    I recently found out that for nuclear fusion to occur the temperature needs to be around 100 Million Kelvin and that the hottest part of the sun is only at 27 Million Kelvin. And that the reason nuclear fusion is occurring in the sun is because of quantum tunneling—allowing a very small fraction...
  9. MathematicalPhysicist

    A Quantum Chaos: Expert Discussion and Q&A

    I might have a few questions about this topic in the future. So I am just asking if there are any experts on this topic here? Thanks in advance! BTW I asked also in physics stackexchange why they don't have a tag of QC, but you know how that site is...
  10. M

    I Can a quantum system with internal memory be in an energy eigenstate?

    If a system is in an eigenstate of the hamiltonian operator, the state of the system varies with time only with a "j exp(w t)" phase factor. So, the system is in a "stationary state": no variation with time of observable properties. But the system could in theory (for what I understand) be...
  11. J

    I Simulation of non-Hermitian quantum mechanics

    I noticed the research on NHQM in the following news release. New physics rules tested on quantum computer Published: 19.2.2021 Information for relevant paper is provided as follows. Quantum simulation of parity–time symmetry breaking with a superconducting quantum processor Shruti Dogra...
  12. nomadreid

    I "Filming" a quantum measurement

    The link is https://www.su.se/english/research/scientists-film-a-quantum-measurement-1.487234 How do they arrive at this distribution over time? It does not appear to be saying that this is the prediction (in it were, then why the "filming"?), and measurement of a single atom's superposition is...
  13. E

    Other Quantum Physics Career With Bachelor's Degree?

    Hello! I wanted to work first before I go to grad school for Ph.D in physics (especially in quantum). I wanted to work in a field close to quantum physics, but I was wondering if there were such jobs for those with bachelor's degree in physics?
  14. A

    A Green's function for tunneling electrons between quantum dots

    Good afternoon! I am writing with such a problem, I hope to find someone who could help me. I'm almost desperate! So, there is such a thing as the Braess paradox, this is a classic paradox for roads and power grids, and there is also such an article...
  15. Erik Ayer

    I Question on circular polarization and the quantum eraser

    Looking at the wikipedia page for the (original) quantum eraser (https://en.wikipedia.org/wiki/Quantum_eraser_experiment), quarter-wave plates in front of the slits change photons from being linearly polarized to being circularly polarized. I think this is the case when there is no polarizer in...
  16. J

    I Quantum Tunneling in an STM: ELI5 Requested

    In an STM, you image the surface topography by tunnelling electrons from the metallic tip to the conductive surface, while measuring the current. I have worked with these instruments before and I never understood why does one need a quantum explanation for this. Wouldn't the electron jump to...
  17. ILoveParticlePhysics

    I Calc QED Charge Particle Repulsion/Attraction

    Maybe this will help.
  18. S

    B Quantum info and its indestructability

    What is information? IIRC it has something to do with bits. What is quantum information? Can either be destroyed? Why (not)?
  19. A

    I The sum of S and L quantum numbers

    I am reading the textbook Magnetism and Magnetic Materials by Coey and I am confused about how they grouped the terms and how they ended up getting the sums of L and S. My confusion lies in the two red boxes. Also, how is D even considered here when we have up to $2p_1$? And why would the spin...
  20. Fikremariam

    Studying Where shall I start? (16 y/o wanting to become quantum physicist)

    I am a 16 years old boy who have adream to become quantum physicist but IAM also aboy who doesn't know the beginningof the road so I thought maybe some comments will be helpful so tell me where shall I start?
  21. Sophrosyne

    A philosophy of quantum mechanics question

    There is an interpretation of quantum mechanics out there, and I was not sure if physicists take this seriously, or if it's one of those woo-woo popular misunderstandings of quantum mechanics. So I am posing it to our esteemed physicists here. It says that there can be all sorts of universes...
  22. lelouch_v1

    I How to work in the |F,m_F> states in hyperfine structure

    Suppose that we have two atoms with one proton one electron each, and these electrons interact with each other. The states for the electrons are the singlet(S=0) and the triplet states(S=1). My question is if i have to keep the nuclear spin of the protons parallel when i write the states, for...
  23. lelouch_v1

    A Eigenvalues of Hyperfine Hamiltonian

    I was reading a paper on Radical-Pair mechanism (2 atoms with 1 valence electron each) and the author used the hyperfine hamiltonian $$H_{B}=-B(s_{D_z}+s_{A_z})+As_{D_x}I_x+As_{D_y}I_y+as_{D_z}I_z$$ and found the following eigenvalues: a/4 (doubly degenerate) , a/4±B , (-a-2B±2√(A^2+B^2)) ...
  24. A

    Quantum Resources for learning Quantum Field Theory

    hello :) i would very much like study some quantum field theorie, but have not previously study any regular quantum mechanic (i am not so interest in regular quantum mechanic, but more the relativistic theories). so i ask, this is possible or not? to what extent knowledge of regular quantum...
  25. J

    I Origin of Probabilities in Quantum Mechanics?

    The non-normalized wavefunction of a general qubit is given by: $$|\psi\rangle=A|0\rangle+B|1\rangle.$$ The complex amplitudes ##A## and ##B## can be represented by two arrows in the complex plane: Now the wavefunction can be multiplied by any complex number ##R## without changing the...
  26. Tuca

    I Solving Schrödinger's equation for a hydrogen atom with Euler's method

    Hi, first-time poster here I'm a student at HS-level in DK, who has decided to write my annual large scale assignment on Schrödinger's equation. My teacher has only given us a brief introduction to the equation and has tasked us to solve it numerically with Euler's method for the hydrogen atom...
  27. A

    A How Would MOND Solve the Dark Matter Problem and Affect Quantum Gravity?

    Assuming that the dark matter problem is finally solved by some kind of MOND, how that would change the scene for quantum gravity?, how that would change the proposed/expected properties of the Graviton (massless spin 2 particle)?
  28. S

    B What is actually spinning in quantum spin?

    Some texts say quantum spin is analogous to the spin of a planet in that it gives a particle angular momentum and a magnetic moment. However, as subatomic particles are tiny, the surfaces of charged particles would have to be moving faster than the speed of light in order to produce the measured...
  29. P

    Explaining Quantum Correlation by Comparing It To Road Traffic

    I know quantum correlation means that the particles are entangled and so the state of each cannot be determined independently of the other. However I'm not sure how it applies to this particular scenario - If there are more cars on the M25 I suppose we could say technically there are less on...
  30. L

    B Does Quantum Eraser Prove That Intention Does Not Create Physical Reality?

    If you perform a double slit experiment but never look at the results could we say that the experiment never actually happened? The beginning of the experiment would show intention to find results but we believe intention does not create physical reality. Quantum Eraser could be preventing us...
  31. AuntyMatter

    A Shakespearean Guide to Quantum Mechanics

    When we think of the fathers of quantum mechanics we tend to think of Max Planck, Albert Einstein, Niels Bohr, Louis de Broglie, Max Born, Paul Dirac, Werner Heisenberg, Wolfgang Pauli, and Erwin Schrödinger. However I think I am in solid ground in suggesting that William Shakespeare was way...
  32. D

    I Shankar Quantum Mechanics, Chapter 5, Page 160-161

    For the case of general potential V(x), what does it mean when he says that there are always one more constraint than free parameters? At each interval, ψ and ψ' must be continuous, so that is 2 constraints at each interval, and I understand that there are 2 parameters of the wavefunction in...
  33. CallMeDirac

    B Exploring the Fabric of Our Universe: The Strength of Quantum Strings

    Do quantum strings have a specific strength, as in how much does it take to break the fabric of our universe if even possible? What would happen if it is broken or has happened?
  34. Danny Boy

    A Entanglement Witnesses close to GHZ states

    Consider page 2 of Toth's paper Entanglement detection in the stabilizer formalism (2005) . To detect entanglement close to GHZ states, they construct entanglement witnesses of the form $$\mathcal{W} := c_0 I - \tilde{S}_{k}^{(GHZ_N)} - \tilde{S}_{l}^{(GHZ_N)},$$ where...
  35. M

    I Quantum Entanglement with Public Variables

    For a thought experiment, if you defined: An electron's "state" can be in 16 states measured as increments of 45 "degrees" starting at 22.5 (modulus 360) An entangled electron (e') simply gets aligned 180 degrees away from (e) A "measurement" M(n) at n degrees simply adds n degrees "Spin" is...
  36. Demystifier

    A Criteria for a good quantum interpretation

    Matt Leifer gives criteria that any good quantum interpretation should satisfy. http://mattleifer.info/2006/06/28/professional-jealousy/?fbclid=IwAR2wl5kEzcfbn6doA53UMty42JP5dADhA5SDxRjaTMNPkq1i9Ag3WPCwB5Q
  37. Y

    Help with the matrix representation of <-|+|->. Does "+"=|+>?

    Trying to use <+|+>=1=<-|-> and <-|+>=0 to prove each iteration of the equation, so I have 6 different versions to prove. But the part I'm currently stuck on is understanding how to simplify any given version. I've written out [S_x,S_y]=S_xS_y\psi-S_yS_x\psi and expanded it in terms of the...
  38. ProtonNeutron

    A What are the recent advancements in quantum gravity?

    I am currently doing my masters in theoretical physics and I would like for my phd to be something quantum gravity oriented. However it seems hard to find what's "hot" in the field those past few years and I only know the basics of these topics (though I believe this can be improved). What are...
  39. J

    I Quantum mechanics supports solipsism?

    There is a lot of information on the Internet that quantum physics supports solipsism and that physicists believe in solipsism. I only trust this forum and the people who are here, so I want to ask you: 1. Is it true that quantum physics says solipsism is true? If this is true, then only one...
  40. J

    I Quantum Tunneling in the Sun and Conservation of Energy

    Hi, In my textbook it says that if you consider the electrostatic repulsive barrier that protons in the Sun need to overcome in order to get into the range of the strong nuclear force to fuse together then it fails to fully account for the measured power output of the Sun. It says that the...
  41. sergiokapone

    I What does motion mean in quantum mechanics?

    Consider the Schrödinger equation for a free particle: \begin{equation} -\frac{\hbar^2}{2m} \partial_i^2\psi = i\hbar\partial_t \psi. \end{equation} Let us be interested in the motion of a free particle in quantum mechanics. We say ok, we have a solution to the Schrödinger equation for a...
  42. C

    I Quantum stuff is neither particles nor waves

    When I asked my professors about quantum interpretations like whether it is particles or waves in the double slit experiment. They always say it is just quantum stuff which is neither particles nor waves, so one must not describe them at such. So why do we have so many interpretations. Why not...
  43. F

    A First Course in Graduate Quantum Mechanics Resources

    What are some resources that you would suggest for a first course in graduate quantum mechanics? This includes textbooks, online courses such as MIT OCW(includes homework/exams), and online lecture notes?
  44. Someone_physics

    A Thought experiment in relativistic quantum mechanics?

    Background --- Consider the following thought experiment in the setting of relativistic quantum mechanics (not QFT). I have a particle in superposition of the position basis: H | \psi \rangle = E | \psi \rangle Now I suddenly turn on an interaction potential H_{int} localized at r_o =...
  45. S

    I Paper: To catch and reverse a quantum jump in flight

    This video www.youtube.com/watch?v=j5HyMNNSGqQ and the paper it refers to https://www.nature.com/articles/s41586-019-1287-z (available also at : http://qulab.eng.yale.edu/documents/papers/zlatko_paper.pdf ) ... talks about how a quantum transition can be monitored and manipulated in real time...
  46. A

    A Are all quantum computers feedforward networks?

    Hi! I am trying to understand how advanced quantum computers have in fact become as well as how advanced they can in principle become. To probe these issues, I am asking the group whether or not quantum computers are restricted to feedforward processing, both currently and in principle? Here is...
  47. T

    B Problems on the making of a quantum gravity theory

    Hello. What are the problems specifically or mathematically or physically that physicists find difficulty in solving to make a theory of quantum gravity? Thank you.
  48. Ali Beladi

    I What does this tricky quantum mechanics equation mean?

    I'm a current high school student and I’m aspiring to become a biochemist. I’m at the moment writing an article about adaptive mutations but there is a lot of tricky quantum mechanics in it which I simply don't get. I have asked everyone and got no answer until someone recommended to ask it in a...
  49. Francis

    A Quantum CSP Algorithm for Aircraft Recovery Problem

    The Aircraft Recovery Problem: consists of finding solutions for a disrupted aircraft flight schedule such that a set of constraints is satisfied, namely continuity, turn-round time, scheduled maintenance, and airport departure/arrival capacity. My approach consists of finding the domains for...
  50. Decimal

    I Does the Wigner-Eckart theorem require good quantum numbers?

    I have a question related to the following passage in the quantum mechanical scattering textbook by Taylor, Here Taylor makes the choice to use a basis of total angular momentum eigenvectors instead of using the simple tensor product given in the first equation above (6.47). I understand that...
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