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. S

    I How to quantum mechanically describe the photon?

    This might sound stupid , but I am wondering how exactly could I describe the momentum eigenfunctions of photons? EDIT: to destroy ambiguity, I am searching for a quantum mechanic description of monochromatic light similar to how we represent it classically as: E-> = a->cos(wt+phi)
  2. H

    B Taking Quantum Computers into Space

    Can the low temperatures in Space be employed to cool a Quantum computer installed on an operational Space vessel doing away with the need of a super fridge or whatever they use here on Earth? Thank you.
  3. B

    B Delayed choice quantum communication - "Berlin riddle"

    At 12.00h a specific event is being expected in Berlin, whether the wall ist teared down, or not. The sender is in Berlin and wants to send the information about the event to the recipient on the moon. Both sides know about the possibly upcoming event. As the light takes 1 second to the moon...
  4. A

    I Arrow of time and its reversal on a IBM quantum computer

    I would like to know what the implications of this paper are https://arxiv.org/abs/1712.10057. They say " Here we show that, while in nature the complex conjugation needed for time reversal is exponentially improbable, one can design a quantum algorithm that includes complex conjugation and...
  5. stevendaryl

    I Wallis' Formula and Quantum Mechanics

    Does anybody know what the connection is between Wallis' formula for ##\pi## and quantum mechanics? There was an article about it: https://www.eurekalert.org/pub_releases/2015-11/aiop-ndo110915.php but like all articles for the lay public, all the details were left out. Wallis' formula is...
  6. entropy1

    I The (quantum) world existing AND non-existing

    Can we suppose that a quantum property (like spin, polarisation, velocity, position) becomes a potential, a probability, when we are going to measure it? Can we say that this property does not exist while remaining unmeasured, and that when measured takes on a value that depends on both the...
  7. Q

    I "Basic" question about Grover's Quantum Computing algorithm

    Hi everyone, I'm a computer scientist (not a physicist), so I will ask a computer scientist's question. In all the descriptions I found of Grover's algorithm, there is an element that is puzzling the computer scientist in me: it seems that you need to tell the Oracle about the position of the...
  8. L

    I Symmetries in quantum mechanics and the change of operators

    When we make a symmetrie transformation in a quantum system, the state ##|\psi \rangle## change to ## |\psi' \rangle = U|\psi \rangle##, where ##U## is a unitary or antiunitary operator, and the operator ##A## change to ##A'##. If we require that the expections values of operators don't change...
  9. K

    B Instant Quantum Communicator: Does Time Dilation Affect Voice?

    Let’s say I create two hypothetical quantum entanglement based network devices, A and B, which means communication between A and B is instantaneous. It works like a walkie talkie in that I talk into A and you can hear my voice in B, and vice versa.Now. I take device B with me on a spaceship...
  10. Q

    I Will quantum computation lower energy requirements?

    According to this article, written by D-Wave's CEO, the advent of quantum computers will "change the way the world uses energy". However, on a first reading, it sounds like an advertorial that pushes nonsense. I'm no energy scientist (unless you put high energy physics in that category for some...
  11. Erik Ayer

    I Can the quantum eraser work without coincidence?

    I understand that downconverted light is not coherent, so that there are essentially many overlapping interference patterns and that coincidence detection filters out one of those patterns. However, could the downconverted light be filtered such that interference can be detected without...
  12. Danny Boy

    A Concepts in a quantum synchronization setup

    In a https://jila.colorado.edu/thompson/sites/default/files/pdf/PhysRevLett.113.154101_0.pdf on quantum synchronization, they introduce the setup given in the attached 'Fig1.png'. I would like confirm a few concepts regarding this setup. The setup is described in the following way: The general...
  13. Quantum Physics

    I Newtonian Gravity Vs. Quantum Gravity

    I want to know the differences between Newtonian Gravity and Quantum Gravity
  14. cHaRLie Bi0NiC

    B Quantum entanglement mechanics

    I should clarify, I am a linux administrator by trade and I have no physics or scientific background outside of working at a medical institute, So if my ideas are off and seem wild and misguided, please forgive my ignorance, and while you are at it be grateful for a fresh perspective. (joke) So...
  15. lekh2003

    How Does Energy Transformation Occur in Quantum Orbital Mechanics?

    Homework Statement Homework Equations I've used: ##mv^2/r## = Centripetal ##q^2/r^2## = Force pulling the electron in A bunch of other ones which I really can't be bothered listing. The Attempt at a Solution I managed to get Part A of the question using pretty simple methods. I set ##mv^2/r...
  16. W

    Quantum Recommendations for Understanding Entanglement in Quantum Mechanics

    Hi everyone, Could anyone recommend a good QM textbook (undergrad-ish level) or some lecture notes that treat entanglement from the ground-up? Most of the stuff I have seen online on entanglement seem to fly pretty quickly into information-theory or abstract group-theory type stuff, which I am...
  17. W

    I Continuous variable entanglement

    Hi all, I have learned the very basics of entanglement (discrete, 2 particle systems) and was hoping that someone can recommend introductory (undergrad-level) material for continuous-variable, 2 particle entanglement. Stuff I have found online so far (like this...
  18. Peter Morgan

    A The relationship between random fields and quantum fields

    My paper "Classical states, quantum field measurement", arXiv:1709.06711, has been accepted by Physica Scripta, https://doi.org/10.1088/1402-4896/ab0c53. The version as submitted to Physica Scripta on November 4th, 2018 is available as arXiv:1709.06711v5. I believe that anyone who puts some...
  19. jedishrfu

    Multiverse vs Ugliverse vs Quantum Monism

    In a recent Scientific American article the notions of multi-verse and ugli-verse are examined as dangerous ideas which can stop Physics from looking for deeper truths since they are based in the universe that supports intelligent life. The notion is to extend beyond Everett's Many Worlds...
  20. Demirci Kawa

    B Does the brain use quantum mechanics?

    We need quantum physics for cognitive neuroscience ?
  21. S

    I Would a Quantum Theory of Gravity dispense with the Inverse....

    Square law? i raise this question because of recently reading some QM, and realizing that for significantly short periods of time, it becomes hard to detect the mathematical patterns. E.g. in the double slit experiment, the standard pattern doesn’t appear after just a few photons. It takes...
  22. J

    B Quantum field theory and the collapse of the wave function

    Hi everyone! Sorry for the bad english! So, just a quick doubt... Does things collapse from a wave of probability into a quantum field or is the wave in the quantum field the probabilistic wave itself? An example to make it clearer: Suppose we have an atom, it enters an atom interferometer, it...
  23. M

    I Spacetime as emergent quantum phenomenon

    Published in the peer reviewed Journal of Applied Mathematics and Physics is the intriguing paper with the following abstract: https://file.scirp.org/Html/11-1721242_88041.htm "Entanglement and the tunnel effect phenomena have been repeatedly observed and are generically accepted under...
  24. A. Neumaier

    I The thermal interpretation of quantum physics

    A complete description of the thermal interpretation of quantum physics can be found in my just finished papers (for the bare bones, see Section 2.5 of Part II) Foundations of quantum physics I. A critique of the tradition, Foundations of quantum physics II. The thermal interpretation...
  25. W

    Work Check: Simple entanglement problem

    Homework Statement Could someone assist me in skimming through my work for this problem? Many thanks! I attached an image of the problem below. Also, I only need help for the first part (part a), cheers.Homework Equations General entangled state vector of a two-particle system: $$|\psi...
  26. W

    Work Check: Neutron Scattering

    Homework Statement Hi all, could someone assist me in checking through my work? Many thanks in advance! An image of the problem is attached below (problem 1b) Homework Equations Far field approximation of a scattered wavefunction: $$\psi_s (\vec{r}) \approx \Psi_i \ r^{\frac{1-d}{2} } \...
  27. H

    I Confusion about the quantum field Lorentz transformation

    On page 59 of Peskin & Schroeder, there's a section on the lorentz transformation of field operators which I've attached. I'm confused about the part towards the end where he does a change of variable on the integration measure; it seems like he's only rewriting the lorentz-invariant integration...
  28. Demystifier

    I Find your ideal quantum interpretation

    Find the interpretation of QM that best suits your personality. You don't need to think much about quantum interpretations to decide which one is the best for you. Just choose one of the offered answers to a couple of questions on the graph. You don't need to think much about the offered...
  29. E

    The most boring quantum quench (looking for a textbook/paper)

    Hi, usually, when we talk about quantum quench dynamics we assume situation when Hamiltonian of a system has a sudden change from ##H_0## to ##H_1##. System was initially in the ground state (or more generally - eigenstate) of ##H_0##. The interesting dynamics appears when the commutator...
  30. P

    I Where to publicise some notes on quantum mechanics

    I've struggled to understand quantum mechanics for many years. I've recently written some notes on the subject that address some of the issues that I've found confusing and that I think might be helpful to others. The moderators on Physics Forums, quite reasonably, won't allow me to publicise...
  31. E

    Pairwise and joint distinguishability of quantum states

    Hi! I'm struggling with the following question: Show that if n quantum states ρ1, ..., ρn are pairwise perfectly distinguishable, they are also jointly perfectly distinguishable. Perfect distinguishability means that there is a set of psd matrices \{E_{1}, ..., E_{n}\},\, \sum_{i} E_{i} =...
  32. Geofleur

    A Is the visualization of quantum fields as harmonic oscillators accurate?

    After learning that QFT describes particles as excitations of underlying fields, I've been trying to gain a more definite understanding of what it means; I came across this paper: https://iopscience.iop.org/article/10.1088/1361-6404/aaa032 Does the author say anything that is misleading? If...
  33. L

    A Quantum amplitude for a particle falling into a black hole

    Here we consider a black hole formed by gravitational collapse classically. We also consider a scalar massless Klein-Gordon field propagating on this background. To quantize the field we expand it in appropriate modes. The three sets of modes required are: The incoming modes, appropriate for...
  34. J

    I What is Stochastic Quantum Mechanics (in simple terms)?

    Could someone, in laymen's terms explain to me what stochastic mechanics is?
  35. manojr

    I Uncertainty of momentum and position

    I was reading a book which had some comments on EPR paper (Einstein, Podolsky, Rosen - 1935) like following: In Newton's physics, when two identical billiard balls hit each other head-on, bouncing off in opposite direction, knowing one ball's position and speed will also indicate other ball's...
  36. askbbj

    What happens to a light photon when it hits glass?

    Homework Statement Using quantum mechanics, what happens when a photon of light hits glass? Homework Equations Momentum= Mass x velocity The Attempt at a Solution I am not sure of my answer but suffice to say the following: Assuming the collision between the photon particle and those in the...
  37. J

    I Energy in Everett's Many-worlds Interpretation

    Hi, Is anybody able to explain how energy is "distributed" in the many-worlds interpretation. I'm using scare quotes as I think this may be the wrong line of thought. It's tempting to imagine energy being distributed amongst subsequent branches as the wave function evolves but I'm not certain...
  38. Paulo Jardim

    B Quantum entanglement comunication

    I have a question about quantum entanglement, I see saying that it would not be possible to communicate faster than light using this technique, but for example, and I understand that it is not the information that travels, however if for example we use the interval of the measurement and read as...
  39. Demystifier

    Insights 9 Reasons Quantum Mechanics is Incomplete - Comments

    Greg Bernhardt submitted a new blog post 9 Reasons Quantum Mechanics is Incomplete Continue reading the Original Blog Post.
  40. FourEyedRaven

    I Particle Physics vs Quantum Field Theory vs Standard Model

    Hi. I'd like to ask what are the differences between Particle Physics, Quantum Field Theory and the Standard Model. I see these names of physics courses but I want to confirm if I understand the difference. My understanding is that when students learn particle physics in their undergraduate...
  41. Demystifier

    Quantum interpretations reduced to absurd, in a nutshell

    Let us introduce a new rule. You can criticize any QM interpretation you want, but to avoid too much philosophy the argument must be extremely concise. For definiteness, it may contain maximally 2 sentences, 1 equation and 1 reference. The general rule is - less is more. Here is my take. In...
  42. 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...
  43. 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...
  44. 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...
  45. 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.
  46. 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
  47. 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...
  48. 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...
  49. 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...
  50. 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...
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