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 What makes the interpretations of Quantum Mechanics so important?

    How valid is the statement "It means physics is ultimately concerned with descriptions of the real world" in the realm of QM? Heretic question, what is "real" besides the outcome of the measurement?
  2. Michael Price

    A Resolving power of a radio telescope array: Quantum or classical?

    My question is: is the resolving power of an array of radio telescopes a quantum or a classical effect? The increase in resolving power of a single telescope, as aperture size increases, is easy to explain in terms of Heisenberg's uncertainty principle. But when we go an array of telescopes are...
  3. adosar

    I Momentum operator in quantum mechanics

    The momentum operator for one spation dimension is -iħd/dx (which isn't a vector operator) but for 3 spatial dimensions is -iħ∇ which is a vector operator. So is it a vector or a scalar operator ?
  4. M

    A Is time irrelevant in quantum mechanics?

    Could one come to think that time is irrelevant in quantum mechanics? we know that the QM equations are written with the time variable, (schrodinger equation). Yet everything suggests that time is irrelevant, as the search for loop quantum gravity seems to indicate
  5. B

    I How to use a quantum bit, or several

    Its all in the summary.
  6. Demystifier

    A Is quantum theory a microscopic theory?

    Quantum theory is widely thought to be a theory of the fundamental microscopic constituents of matter. It is supposed to tell us something about how matter behaves at the fundamental microscopic level, from which the classical macroscopic behavior should somehow emerge as an approximation based...
  7. M

    A Tensor and vector product for Quantum

    Hello, I am calculating the krauss operators to find the new density matrix after the interaction between environment and the qubit. My question is: Is there an operational order between matrix multiplication and tensor product? Because apparently author is first applying I on |0> and X on |0>...
  8. B

    B Can quantum entangled particles be split and sent into a black hole

    I am new here so apologies in advance. When a virtual particle and anti particle appear at the event horizon of a black hole, before they destroy each other, they are split with one being sucked into the black hole and the other becoming exhaust. Are these the same particles as the quantum...
  9. D

    Quantum Should I Get Both of Dirac's Quantum Mechanics Books?

    Hello, I remembered once hearing of a must-have quantum mechanics book by Paul Dirac. I don't remember if it was his Principles of QM or Lectures on QM. Based on the table of contents, I believe it was the Principles of QM book; however, looking at both I was thinking about getting his Lectures...
  10. T

    A What does the first ever photo of quantum entanglement look like?

    A photo: https://petapixel.com/assets/uploads/2019/07/quantum.jpg The popular press version (with above photo): https://petapixel.com/2019/07/13/this-is-the-first-ever-photo-of-quantum-entanglement/ The full paper in Science Magazine: https://advances.sciencemag.org/content/5/7/eaaw2563
  11. W

    I Quantum Causal Modelling: Mathematics

    I have been trying to read through the following paper: https://iopscience.iop.org/article/10.1088/1367-2630/18/6/063032/pdf, but am stuck at the parts from page 6 onwards. What is the "approach/formalism" used in the following sections of the paper, and are there any gentle introductions...
  12. W

    I Relativistic Quantum Mechanics & Localized Particles

    A lecturer today told the class that relativistic QM for single particles is flawed by showing us that for a state centered at the origin, it was possible that ##Pr(\vec{x}>ct)>0##. He said that this was down to the fact that we should be considering multi-particle states in relativistic...
  13. Haorong Wu

    Quantum Introductory textbook for cavity quantum electrodynamics

    Hi. I'm having a hard time learning the physical realization of quantum computers. I got stuck with the section of optical cavity quantum eletrodynamics. There are some concepts I am not familiar with. I think I should read some introductory textbooks which cover cavity quantum electrodynamics...
  14. P

    Other Submission to Classical and Quantum Gravity: Awaiting Decision After 1 Month

    I submitt a paper to journal Classical and Quantum Gravity. The status "awaiting referee reports" lasted one mont. However, now , the paper is on status "awaiting decision" two weaks up to now. Is a ill signal the delayed on the status "awaiting decision"?
  15. L

    I The Cellular Automaton Interpretation of Quantum Mechanics

    Why aren't you guys discussing this? http://de.arxiv.org/abs/1405.1548 The paper is 259 pages. And it will take me a year to read it. The Cellular Automaton Interpretation of Quantum Mechanics doesn't use any wave function. Just please tell me. How does it explain for example the double slit...
  16. Haorong Wu

    I Questions about an optical photon quantum computer

    Hi. I'm learning the optical photon quantum computer from Nielsen's QCQI. Since I'm not familiar with quantum optics, I got some questions about it.Q1. In page 288, the book reads: A laser outputs a state known as a coherent state ##\left | \alpha \right > = e^{- \left | \alpha \right | ^2 /2 }...
  17. M

    A Developments to unify quantum theory with General Relativity?

    Have there been any recent developments in the attempt to unify the standard model of quantum theory with General Relativity? It appears the no progress has been made recently in string theory or loop quantum gravity.
  18. patric44

    Quantum What is the most suitable Quantum Mechanics book for these questions?

    i want to know what is the most suitable quantum mechanics that will enables me after studying it to answer these specific questions ? i mean based on the hardness of these questions ( what book would you suggest to study these subjects ) . and by the way what is the level of these QM questions...
  19. A

    I Quantum Interpretations of this optical effect

    I generally stay away from discussion about the interpretations of QM, but just for this time I would like to know what's the point of view from Bohmian, many worlds, thermal and other interpretations of the following: https://arxiv.org/abs/1305.0168
  20. Demystifier

    I What is the current experimental evidence for quantum trajectory theory?

    [Moderator's note: Spin off from previous thread due to topic change.] Recent experimental confirmation of the quantum trajectory theory: https://www.quantamagazine.org/how-quantum-trajectory-theory-lets-physicists-understand-whats-going-on-during-wave-function-collapse-20190703...
  21. M

    I Quantum Gravimeter: Exploiting the Born rule

    Among the last of the classical tests of general relativity was the Pound–Rebka experiment performed in 1959. This experiment employed a variation of Mössbauer spectroscopy in which a moving emitter was used to counteract a gravitational redshift. The idea here is to exploit QED to measure the...
  22. F

    A Random Quantum Walk: Learn & Use w/ Quantum Gates

    I am an undergraduate doing research on QC/QI. My current topic to learn is continuous-time quantum walks, but first I must learn the random quantum walk. That being said, I was wondering if someone could simply explain what a random quantum walk is and then explain how they could be useful with...
  23. Haorong Wu

    I In quantum search algorithm, how to interpret the effect of U(t)？

    In Nielsen's QCQI, in page 259, it reads, $$U \left ( \Delta t \right ) = \left ( \cos^2 \left ( \frac {\Delta t} 2 \right ) - \sin ^2 \left ( \frac {\Delta t} 2 \right ) \vec \psi \cdot \hat z \right ) I \\ -2 i \sin \left ( \frac {\Delta t} 2 \right ) \left ( \cos \left ( \frac {\Delta t} 2...
  24. P

    I Quantum Computing - projection operators

    Assume ##P_1## and ##P_2## are two projection operators. I want to show that if their commutator ##[P_1,P_2]=0##, then their product ##P_1P_2## is also a projection operator. My first idea was: $$P_1=|u_1\rangle\langle u_1|, P_2=|u_2\rangle\langle u_2|$$ $$P_1P_2= |u_1\rangle\langle...
  25. J

    B Quantum Entanglement Violates the Laws of Physics

    The problem with this is that only one push's worth of energy was expended. One push's worth of input cannot produce two push's worth of output, for this would violate the law that says you can't get more movement out of something than the amount of force you exert onto it (to put is very...
  26. bhanum

    B The Cat's Perspective: Can Macroscopic Bodies Be in Superposition?

    Hi, In the Schrodinger's Cat thought experiment, we say that the cat is in a superposition of states of being either dead or alive. But isn't that opinion biased from being outside of the system? From the cat's point of view, it is either dead or alive but never both. The same argument can be...
  27. A

    A What quantum effects that involve gravity can be studied without QFT?

    So there is no a full quantum theory of gravitation. However, there are instances where quantum effects due to gravitation have been studied. Like Gravitational neutron interferometry https://arxiv.org/pdf/1701.00259.pdf or maybe gravitational decoherence...
  28. A

    A What topics in Quantum Gravity don't require QFT or string theory?

    Full quantization of gravity is a big issue, but that's not what I'm asking here. I'm asking about quantum effects that involve any form of gravitation (Newtonian or GR) but that don't require a full quantization of GR or anything like that. Things like gravitational neutron interference or the...
  29. Haorong Wu

    I Confusion about factoring in quantum computation

    Hi, I'm currently learning QC with Nielsen's QCQI. I've written a program in Matlab following the factoring algorithm in page 233 and 235. I run the program for factoring 15, 18, and 25. I got the proper results several times, but I also got error reports sometime, especially when factoring 18...
  30. Andy_K

    B Quantum Wave & Principle of Least Action

    Dear All, I would like to better understand how the Principle of Least Action applies in observations / measurements in quantum physics. Does the wave function of a particle correspond directly to the principle of least action, as in, the positions with higher probability of detecting the...
  31. Quotidian

    B Understanding Quantum: The Particle/Number Duality

    I am watching a good introductory account of quantum physics for a lay audience. I’m up to the part where it explains how Einstein introduced the idea of the quantum as the description of the particulate nature of light, as opposed to the generally-assumed wave nature of light. The narrative...
  32. eliUCV

    I Quantum Mechanics, The Delta-Function potential

    Hi, I have a question, why when we study the Delta-Function Potencial we can treat with ##E < V##, since the following relation says ##\frac{d^2 \psi}{dx^2} = \frac{2m}{\hbar^2} (V - E) \psi## And do not allow it? or it is just ##E <...
  33. Haorong Wu

    Circuit for the inverse quantum Fourier transform

    First, the inverse quantum Fourier transform is ##\left | k \right > =\frac 1 {\sqrt {2^n}} \sum_{j=0}^{2^n-1} e^{-2 \pi ijk / 2^n} \left | j \right >##, and it is equal to ##\left | k_1 , k_2 , \dots , k_n \right > \rightarrow \frac { \left ( \left | 0 \right > + e^{-2 \pi i 0.k_n} \left...
  34. J

    A Which is the quantized variable in quantum gravity?

    In Quantum Mechanics, the position (or momentum) variable is quantized. I define "quantization" as promoting a variable into a probability distribution. For example, with the double slit experiment, the classical assumption that the position/path of a particle is "unique" cannot explain...
  35. Sasho Andonov

    A Uncertainty vs Randomness in Quantum Physics

    In Quanta Magazine there is article on this link...
  36. Haorong Wu

    Quantum A book about problems of quantum computation?

    Summary: A book about problems of quantum computation? I remember that there is a book containing many problems of quantum computation, but I can't find the page or post now. I have hard time learning quantum computation. Thus, I want to try to solve problems in order to get a solid...
  37. Auto-Didact

    Quantum Best books on Quantum Foundations?

    What are the best books on QT Foundations out there? This seems to be a difficult question which has very different answers depending on who you happen to ask. There seem to be at least two different levels, e.g. intermediate texts and advanced texts, with a very wide gap in between, which is...
  38. SJay16

    B What’s more “important": GR or QM?

    Let’s say to the average Astronomer conducting research; generalizing the research to “astronomy” ; which would be more useful for the “average” Astronomer on a day to day basis: Quantum Mechanics or General Relativity? Obviously most are, but which so more? Which would be referred to more?
  39. Z

    Eigenstates of a free electron in a uniform magnetic field

    I started with the first of the relevant equations, replacing the p with the operator -iħ∇ and expanding the squared term to yield: H = (-ħ^2 / 2m)∇^2 + (iqħ/m)A·∇ + (q^2 / 2m)A^2 + qV But since A = (1/2)B x r (iqħ/m)A·∇ = (iqħ / 2m)(r x ∇)·B = -(q / 2m)L·B = -(qB_0 / 2m)L_z and A^2 =...
  40. Gbox

    What are energy values and how do they relate to eigenvectors in quantum states?

    Summary: Finding state at t=0, energy values and more So this is my first question in quantum mechanics (please understand). 1. So we have a system, and to describe the state of the system we have to measure, A is an hermitian matrix, that each physical measurable quantity has. To find the...
  41. W

    A Quantum Optics statistics

    Hi everyone, I am following along with the MIT OCW quantum optical communication course. I have a question about this chapter, concerning the linear attenuators and amplifiers. Specifically, the chapter mentions that they are not going to get ##\rho_{out}##, but I am interested in this. More...
  42. IonReactor

    I Role of mass and quantum gravitational effects in friction?

    Friction is commonly thought to arise from the electromagnetic forces of the atoms at the boundary between two surfaces in contact. However it occurs to me that, in addition to charge quanta, there are also mass quanta present in this system and they could very well play a role. Now, I'm just...
  43. R

    I Question About The Role of Observation in Quantum Mechanics

    In the double-slit experiment when a detector was placed before the two slits, a 2 strip pattern was produced after the two slits. When there was no detector placed before the two slits, a different pattern was produced after the two slits. Why does the presence of a detector before the two...
  44. R

    I Communicating Via Quantum Entanglement Using Time Differentiated Pulse

    Which of these premises is impossible or incorrect according to our current understanding of quantum entanglement? Given 2 entangled particles, p1 and p2: Observing paired particle p1 induces a change in spin on paired particle p2. There a way of detecting a change in spin on particle p2...
  45. Haorong Wu

    Schools Are there any good groups about quantum computation?

    Hi. After learning quantum computation for months, it fascinates me. Quantum computation expands my view about computational methods. I believe that many future achievements can be obtained from quantum computing, especially the combination of AI and quantum computation. Meanwhile, I think...
  46. I

    Programs Topics at the junction of AdS/CFT and quantum information

    I have been asked to draft a (informal) research proposal for a PhD thesis. I have some background in quantum information and my interests are leaning towards the AdS/CFT correspondence. I was wondering if you could suggest a few (preferably recent) theory papers at the junction of quantum...
  47. T

    I Quantum Jumps and Schrodinger's Cat are predictable

    Quantum Jumps are predicted using microwave monitoring. Weird. https://phys.org/news/2019-06-physicists-schrodinger-cat.html
  48. P

    I According to quantum physics, can our universe be a simulation?

    What does quantum physics say about the simulation hypothesis?
  49. A

    Quantum Theory Of Semiconductor Quantum Dots + other books on this topic

    Hello, I have two questions into one. First I would like to know what books are considered the best to introduce the theory of quantum dots, so for example with the k.p method, tight-binding, empirical pseudopotentials, and other techniques, analytical derivations, optical properties, band...
  50. Auto-Didact

    A Experimental evidence of non-local quantum forces

    Becker et al. 2019, Asymmetry and non-dispersivity in the Aharonov-Bohm effect The paper is open access and published in Nature Communications.
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