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. K

    I Is there some agreement on properties of a theory of quantum gravity?

    Having read many times that there is no theory of quantum gravity, yet physicists at Physics Forums must have some ideas of what a theory of quantum gravity will contain. Is it allowed to discuss these questions at Physics Forums? Wikipedia does allow some current theoretical work to be...
  2. E

    Projective measurements of quantum processor

    Am I correct in thinking that the system measures the probability |<f|1>|^2 for some state <f|? Then the probabilities for each of the six states would be: |<0|1>|^2= 0 |<1|1>|^2= 1 |<+x|1>|^2= |(1/√2)|^2 = 1/2 |<-x|1>|^2= |(-1/√2)|^2 = 1/2 |<+y|1>|^2= |(-i/√2)|^2 = 1/2 |<-y|1>|^2= |(i/√2)|^2...
  3. E

    Qubit Operations: Finding U Gates & Probabilities

    Part a: Gate H X Y Z S T R_x R_y Theta pi pi pi pi pi/2 pi/4 pi/2 pi/2 n_alpha (1/sqrt(2))*(1,0,1) (1,0,0) (0,1,0) (0,0,1) (0,0,1) (0,0,1) (1,0,0) (0,1,0) Using the info from the table and equation 1, I find: U_H=(i/sqrt(2))*[1,1;1,-1] U_X=i*[0,1;1,0] U_Y=i*[0,-i;i,0] U_Z=i*[1,0;0,-1]...
  4. patric44

    Perturbation treatment of hydrogen molecular ion

    hi guys i am a the third year undergrad student and in this 2nd semester in my collage we should start taking quantum mechanics along with molecular physics , our molecular physics professor choose a book that we are going to take which is " molecular physics by wolfgang Demtroder " when i...
  5. S

    Quantum physics and philosophy (books)

    Summary:: Looking for recommendations Are there any books on the philosophical implications of quantum theory by people who know what they're talking about?
  6. S

    I How could a quantum system "predict the future"?

    What is nonlocality? How could a quantum system (like a superposition - is that right?) "know" an interaction will take place? Source...
  7. Quark Effect

    B Quantum Computing for Beginners: Understanding Double Hadamard Gates

    Hi guys, I am an absolute beginner in quantum computing. I am really curious about its mechanics but my lack of knowledge in mathematics makes me struggle. I cannot entirely understand why numbers |1> cancel each other out while double Hadamard gates are applied. If the rule is to cancel out the...
  8. Quark Effect

    B What is the significance of 'i' in quantum computation notation?

    Hi guys, I am currently having some difficulties with this quantum state. I don't entirely understand what that letter 'i' means, where it comes from and why it appears in brackets [1, i]. Shouldn't there be a '0' instead? I am an absolute beginner in quantum computation. I've been following a...
  9. V

    I Getting rid of nonlocality from quantum physics

    The main message of this post is that quantum theory is local, that "``spooky action at a distance'' was just shicky Einstein's slogan from a letter to Born in 1947. Einstein directed it against the individual interpretation of a quantum state. This interpretation is often referred as the...
  10. TopologyisGeometry

    Plotting the radial wave function of Deuteron in a finite well

    To plot ##u(r)## we need to find the solutions for each region. Which is in the relevant equations part. Now, I have to do this numerically. Using python 3.7 I made an ##u## which is filled with zeros and a for loop with if/elseif statement, basically telling it to plot values for whenever...
  11. Adwit

    I Rotation problem in Quantum Mechanics

    How do "sinϕ - cosϕ sinϕ" become 0 ?
  12. J

    Quantum Journal on hardware used to build quantum computers?

    Hi folks, This time is not a book, I would like to know if exists a journal explaining how a qubit is actually built, I mean the hardware not the theory. Indeed I am interested in any hardware related with these technologies. I know this field is still in an early stage and they are still...
  13. jisbon

    Quantum Physics (Electron in infinite deep quantum well)

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  14. Pispi Choudhury

    Courses Math & Physics Courses for Quantum & Statistical Field Theory

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  15. jisbon

    Engineering Calculating the number of quantum states in the valence and conduction bands

    Hi all, This question asks me to calculate the number of quantum states, as well as electrons per cm^3 of the crystal in the room temperature. The problem is I only dealt with a single element before without any calculation for 1cm^3 whatsoever. For example for a Silicon semiconductor, I can...
  16. J

    Quantum Books teaching maths more advanced than what is used in quantum physics

    Hello friends. I was thinking that if you study physics, you learn first the basic math to understand it, but you don't go beyond, or deeper, you just learn the minimum math necessary to understand what you are going to see in your physics leassons. For example, if you are going to learn...
  17. entropy1

    I What does it mean to influence a quantum measurement's outcome?

    This just occurred to me and I don't expect to be the first one to address it: It is said that in a specific measurement basis, the outcome of a measurement in this basis is determined by chance. But in how far is this the case, since if the eigenvectors are for example ##\overrightarrow{A}##...
  18. Quantum Alchemy

    I What exactly is a measurement device and how does it work?

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  19. pinball1970

    A Quantum measurement of a Strontium ion

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  20. G

    Programs Quantum Computation and Information Master's Degree

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  21. K

    I Quantum fluctuation and classical physics

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  22. MichPod

    I Why the statistics for a real gas are not quantum in nature?

    A gas of bosons or fermion particles follows a particular quantum statistics. Then why a molecular gas (say, H2) follows a classical distribution statistics? Is it not the case that the molecules should be indistinguishable one from another and be either bosons or fermions? What is exactly the...
  23. ngrunenberg

    B How do quantum computers find a solution?

    Apologies in advance if this is a stupid question, I'm not the brightest. I recently listened to Scott Aaronson's conversation with Lex Fridman, and an interview he did for Scientific American, regarding quantum computing (QC from now on) and have a question regarding how a QC finds a solution...
  24. A

    A Quantum biology - photosynthesis

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  25. Replusz

    I Topological quantum physics - spin statistics

  26. T

    Quantum 3-Qubit Error Correction Circuit

    Hello! I have this problem regarding the analysis of an error correction circuit. This problem set has been set for hand-in before the lectures regarding error-correction take place, so I am struggling. With that in mind I have read about the initialisation of the first qubit with two others...
  27. Cetus

    Five State Quantum System, understanding the question

    I’ve never worked with a quantum system with more that two states 1, -1, and I’ve just gotten this homework problem. I'm not sure what it means. Does this mean it has five states? Why are there two 0’s and two 1’s?
  28. PeroK

    I What do we do with the massive scalar quantum field in QFT?

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  29. B

    Traveling quantum Gaussian wave packet

    I tried plugging Psi into the right of the Schrodinger equation but can't get anything close to the solution or anything that is usable. How should I solve this?
  30. cookiemnstr510510

    Linear operators, quantum mechanics

    Hello, I am struggling with what each piece of these equations are. I generally know the two rules that need to hold for an operator to be linear, but I am struggling with what each piece of each equation is/means. Lets look at one of the three operators in question. A(f(x))=(∂f/∂x)+3f(x) I...
  31. G

    I Does classical mechanics apply to the quantum world at all?

    I know we can't use classical mechanics to describe or measure the quantum. That is not what I'm asking. I am asking whether particles still follow the same rules like action/reaction if there is a force involved. If electron A interacts with electron B, is Newton's 3rd law still being applied...
  32. X

    I Null Spacetime Intervals and Quantum Superposition

    In Abner Shimony's paper "The Reality of the Quantum World", the choice between particle detector and wave interference detector is said to be made "after the photon had interacted with the beam splitter". A: Isn't it true that, at light speed, time is not passing for the photon? And so, with...
  33. Greg Bernhardt

    Quantum Coherent Quantum Physics by PF Member A. Neumaier

    Coherent Quantum Physics: A Reinterpretation of the Tradition by @A. Neumaier https://www.amazon.com/dp/3110667290/?tag=pfamazon01-20
  34. Jamister

    Quantum Book for quantum measurement theory

    I'm looking for a book about the theory of measurement in quantum mechanics. A book that goes deep into understanding different kinds and ways. thank you
  35. W

    I Understanding Loop Quantum Cosmology

    As I understand it there have been different attempts to use LQG to make a theory of cosmology. The first one being done by Martin Bojowald and then later one being performed by Ashtekar, Pawloski and Singh. there is a description of what they did that was different but as a non cosmologist I...
  36. jisbon

    What are the possible quantum states of an electron relaxing from n=4 to n=3?

    Hi all, I'm right now confused about this. As far as I know, when changing from a level to another, the change in l (subshell) can only be a difference of 1, and ##m_{l}## can be the same or a difference of 1. In this case, since the question wants me to state possible quantum states of...
  37. B

    I New interpretation of quantum mechanics

    https://www.nature.com/articles/d41586-020-00120-6
  38. atyy

    I Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

    Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime Sean Carroll https://www.amazon.com/dp/1524743011/?tag=pfamazon01-20 Review of the book by Matt Leifer Does the many-worlds interpretation hold the key to spacetime? https://physicstoday.scitation.org/doi/10.1063/PT.3.4366
  39. T

    B Layman's question re quantum entanglement

    Hi I hope that you will humour me as I try, in layman’s terms to explain a question I have. I recently watched BBC4’s excellent program about quantum entanglement and it got me thinking. I am aware that there are currently experiments ongoing to prove that quantum entanglement is real and...
  40. Demystifier

    A How ‘spooky’ is quantum physics? The answer could be incalculable

    A new mathematical result seems to have very important implications for quantum mechanics. https://www.nature.com/articles/d41586-020-00120-6?fbclid=IwAR0DxBdyEW10Te3C1Vfyn17TZshyCRrO8nX6D5Iq4CGahZ_DXgfeQ_Ii0Z8 The original paper is a very hard read: https://arxiv.org/abs/2001.04383
  41. S

    What did Hans Bethe think of von Neumann's quantum logic?

    Nobel laureate Hans Bethe was a friend of mathematician-physicist John von Neumann, and he once said: "I have sometimes wondered whether a brain like von Neumann's does not indicate a species superior to that of man" and "[von Neumann's] brain indicated a new species, an evolution beyond man"...
  42. jisbon

    Electron excited from the ground state to a quantum state

    Hi there, popping by here to check my answer because another online platform has already answered it but my answer appears to be wrong. I can't seem to understand why though :/ Since I can find the energy at a state to be ##E_{n}=\dfrac {-13.6z^{2}}{n^{2}}eV## At ground state where n=1...
  43. Frigus

    B Quantum mechanical model of an atom

    We say that principal quantum number tells us the average distance from the nucleus, so the sub shells of some principal quantum number say 3 has 3 sub Shells 0,1,2 and in 3p sub shell their will probability of finding the electron near the nucleus which doesn't mean electron will find in region...
  44. I

    Quantum Mechanics Infinite Potential Well -- Check Answers please

    I'm self studying so I just want to ensure my answers are correct so I know I truly understand the material as it's easy to trick yourself in thinking you do! A particle of mass m is in a 1-D infinite potential well of width a given by the potential: V= 0 for 0##\leq## x ##\leq## a =...
  45. S

    A According to Wilczek, how does quantum mechanics arise?

    According to Nobel laureate Frank Wilczek, the universe emerges from a Grid. This was proposed in his book "The Lightness of Being: Mass, Ether, and the Unification of Forces". He also likes the idea that the universe emerged from a state of "nothingness" (or rather, a quantum vacuum) where...
  46. Tiptoeingelephants

    I Quantum Field Theory question

    Trying to better understand quantum field theory, I've read that particles are created when it becomes an exitation of its quantum field. Would it then be right to think of a particle as the manifested kinetic energy of its field?
  47. jedishrfu

    The Perils of Quantum Supremacy and Research

    Optical quantum computer goes big in new quest for “quantum supremacy” https://arstechnica.com/science/2019/12/optical-quantum-computer-goes-big-in-new-quest-for-quantum-supremacy/
  48. T

    A First Chip-to-Chip Quantum Teleportation

    ...researchers at the University of Bristol's Quantum Engineering Technology Labs (QET Labs) demonstrate the quantum teleportation of information between two programmable chip for the first time, which they remark is a cornerstone of quantum communications and quantum computing. Seems they did...
  49. CrosisBH

    Courses Taking both Classical Physics and Quantum Physics at the Same Time.

    Hello! I'll be entering my second semester of my junior year of my Physics Degree. My uni doesn't have a bad physics program, but it's really small so I have to take my upper level classes as soon as they're offered or I won't take them for another 2 years or so. They are on a cycle so I will be...
  50. MathematicalPhysicist

    Problem 7.7 and its solution from A Quantum Approach to Condensed Matter

    Well, I don't understand the integral part of ##1/(VD) = \int_0^{\hbar \omega_D}\frac{\tanh(\beta E/2}{E}dE## and ##\tanh(\beta E/2) \approx 1-2\exp(-\beta E)##, then he writes the following (which I don't understand how did he get it): $$\frac{1}{VD} = \sinh^{-1} (\hbar \omega/\Delta(0)) =...
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