# What is Quantum spin: Definition and 35 Discussions

Spin is an intrinsic form of angular momentum carried by elementary particles, composite particles (hadrons), and atomic nuclei.Spin is one of two types of angular momentum in quantum mechanics, the other being orbital angular momentum. The orbital angular momentum operator is the quantum-mechanical counterpart to the classical angular momentum of orbital revolution and appears when there is periodic structure to its wavefunction as the angle varies. For photons, spin is the quantum-mechanical counterpart of the polarization of light; for electrons, the spin has no classical counterpart.The existence of electron spin angular momentum is inferred from experiments, such as the Stern–Gerlach experiment, in which silver atoms were observed to possess two possible discrete angular momenta despite having no orbital angular momentum. The existence of the electron spin can also be inferred theoretically from spin–statistics theorem and from the Pauli exclusion principle—and vice versa, given the particular spin of the electron, one may derive the Pauli exclusion principle.
Spin is described mathematically as a vector for some particles such as photons, and as spinors and bispinors for other particles such as electrons. Spinors and bispinors behave similarly to vectors: they have definite magnitudes and change under rotations; however, they use an unconventional "direction". All elementary particles of a given kind have the same magnitude of spin angular momentum, though its direction may change. These are indicated by assigning the particle a spin quantum number.The SI unit of spin is the same as classical angular momentum (i.e. N·m·s or kg·m2·s−1). In practice, spin is given as a dimensionless spin quantum number by dividing the spin angular momentum by the reduced Planck constant ħ, which has the same dimensions as angular momentum, although this is not the full computation of this value. Very often, the "spin quantum number" is simply called "spin". The fact that it is a quantum number is implicit.
Wolfgang Pauli in 1924 was the first to propose a doubling of the number of available electron states due to a two-valued non-classical "hidden rotation". In 1925, George Uhlenbeck and Samuel Goudsmit at Leiden University suggested the simple physical interpretation of a particle spinning around its own axis, in the spirit of the old quantum theory of Bohr and Sommerfeld. Ralph Kronig anticipated the Uhlenbeck–Goudsmit model in discussion with Hendrik Kramers several months earlier in Copenhagen, but did not publish. The mathematical theory was worked out in depth by Pauli in 1927. When Paul Dirac derived his relativistic quantum mechanics in 1928, electron spin was an essential part of it.

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1. ### I Understanding Bell Inequality Proof: Explained from a Probability Perspective

Starting from this link my understanding of Bell inequality proof goes as follows: Suppose we have a model of local pre-determinate hidden variables for QM. This amounts to say QM objects are in pre-determinate given states even if we do not measure it. Locality just means that spacelike...
2. ### Quantum spin superposition interpretation

Here is my workings out:  If a particle's spin of magnitude ##\frac {\hbar}{2}## is prepared along direction ##\vec r_1## and subsequently its spin is measured along direction ##\vec r_2 ## at an angle ##\vec \theta ## to ##\vec r_1##, the probability of its being found "spin up" along is...
3. ### A In What Space Does Quantum Spin Take Place

Quantum spin is orientable so it takes place in a space with an even number of dimensions. What is that space? If the space had an odd number of dimensions, then spin in that space wouldn't be orientable. But quantum spin is orientable. We could say that it is Minkowski space, but that space...
4. ### I Quantum Spin - If It’s Not Spinning, What Is It Doing?

I read about quantum spin a while ago. If it’s not spinning, what is it doing?
5. ### I Quantum spin, wavelength and frequency

Greetings, I'm new here, I have an interest in the nature of reality, and a question. Does the quantum spin of a particle (its intrinsic angular momentum) have anything to do with its wavelength and frequency? One of the experts on Quora said no, and I cannot find anything about it on the web...
6. ### I What is the quantum spin of a single electron in an (atomic) orbital?

What is the quantum spin of the valence electron in the silver atom in the furnace in the Stern-Gerlach experiment? . Up, down, at random, alternating, in a (quantum) superposition (of both), or none? Does it even have/get one until it's measured/observed /needed? . Does the second electron, in...
7. ### 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...
8. ### A Why do we study quantum spin liquids?

What kind of research is currently going on in the field of quantum spin liquids and why do we study spin liquids?
9. ### How to develop a method to reconstruct state in quantum mech

Hi all, There is this question; Firstly, I am new to quantum mechanics and there are a lot of terms I am unfamiliar with. So there is this question that asks me to develop a method to reconstruct state and I have no clue how I should start with. Any help or steps to solve this type of...
10. ### B What is Quantum Spin? Understanding Basics

Hey guy! Can you guy pleas tell me what is quantum spin? I read and watch many stuff but i still don't understand it. Thanks for help!
11. ### How do you visualize quantum spin?

I have previously read that the spin of a particle is comparable to angular momentum of a system or object on the macroscopic level. However, the quantum world is much different and all I can imagine is a minuscule ball rotating on it's axis.
12. ### I How are quantum bits implememted?

Its very easy to pull a logic gate or transistor circuit of the internet for example and begin to understand how it represents physical bits, by either having 5V across the circuit or below 3.3V. But I was wondering what are some of the methods for representing qubits from a hardware...
13. ### B What is the relationship between spin and angular momentum in quantum mechanics?

The "color force" doesn't actually change the color of quarks because they don't actually have any color. We use the word "color" non-literally here. Now, I'm told that quarks don't "actually" "spin" either, so again, I infer that it's a term used out of convenience, but I'm also told that they...
14. ### Prove Quantum Spin Problem: θ, Su11, Su22, Sz1, Sz2, Sx2

Homework Statement It was noted that <00|Su11Su22|00> = -ħ2/4 * u1⋅u2. where u1 and us are unit vectors along which the two spin operators are measured and θ is the angle between. |00> is the singlet state that the two electrons are entangled in (corresponding to total spin values ). Prove...
15. ### What are S, L and J for the following states....?

Homework Statement What are S, L and J for the following states: ##^1S_0, ^2D_{5/2} ^5F_1, ^3F_4## Homework Equations The superscript is defined as: 2S + 1 The subscript is defined as: J = L + S The letter denotes the angular momentum number (s, p, d, f...) starting at s = 0. The Attempt at a...
16. ### I Quantum Spin Liquid: New State of Matter Explained

I just read an article stating physicists have discovered a new state of matter. What is a quantum spin liquid?

Hello I am joining as a Project assistant on a project called "Search for spin liquid and other novel ground states arising from an interplay between electronic correlations, spin-orbit coupling and geometric magnetic frustration" what can i read to get onto the topic for it.
18. ### Expectation value of spin 1/2 particles along different axes

Homework Statement Show that for a two spin 1/2 particle system, the expectation value is \langle S_{z1} S_{n2} \rangle = -\frac{\hbar^2}{4}\cos \theta when the system is prepared to be in the singlet state...
19. ### Free floating electron quantum spin axis locked to space?

This is my attempt at understanding any real world physical effects from quantum spin. I will start with the short version first. A ... When we have a free floating electron in space is the physical orientation of the quantum spin axis locked into a fixed 3D orientation and from there can only...
20. ### Elementary quantum spin in Sakurai

Homework Statement I am currently working on a seemingly straightforward eigenvalue problem appearing as problem 1.8 in Sakurai's Modern QM. He asks us to find an eigenket \vert\vec S\cdot\hat n;+\rangle with \vec S\cdot\hat n\vert\vec S\cdot\hat n;+\rangle = \frac\hbar 2\vert\vec S\cdot\hat...
21. ### Good references for quantum spin chains

Hi folks, I work in theoretical high energy physics. Recently I got interested in condensed matter theory. But each time I see some spin system I get scared thinking another one must be waiting for me! Can some one suggest me some reference(s) where these spin chains, specially those which are...
22. ### Quantum spin numbers for ground-state electron configurations

Homework Statement [/B] Please redraw this figure by assuming that an electron can have spin quantum number ms = +1/2 (arrow up), ms = 0 (marked as "I"), or ms = -1/2 (arrow down). It is important to clearly state your arguments/reasoning. http://s30.postimg.org/jz7tfeha9/wow.png Homework...
23. ### Quantum Spin: Exploring the Mysterious Angular Momentum

Question about quantum spin: So for the Schrodinger equation, the angular momentum of a wave-packet, whether it is free or confined to a potential well, appears to be calculated in exactly the manner you would expect it to be: L(x) = m*rxv[Psi(x)]. Aside from quantization (decomposing the...
24. ### Quantum Spin vs Spin - Understand the Difference

quantum spin vs ...spin I keep hearing people say, "comparing particle spin to a spinning top will lead you in the wrong direction." I'm curious.. what exactly is the difference between the two?
25. ### Help Where can I start if I want to learn Quantum Spin Hall Effect?

Help please! Where can I start if I want to learn Quantum Spin Hall Effect? I have learned the some fundamental ideas of integer quantum hall effect by myself. Under this background, what more should I read? cause I really can't find any notes or videos that are teaching about QSHE. I am now in...
26. ### Polarized Light and Quantum Spin

Can there be EM radiation with spin zero? I don't mean that the sum of all spins is zero, as it usually is. My thinking is that all light spins and usually the left and right component is of equal intensity. Am I correct in thinking that light reflecting off of a suitable horizontal...
27. ### Quantum Spin up spin down with magnetic field

There is an external magnetic field along the positive z direction B = (0,0, B) and the initial spin direction at time t0= 0 is specified by the direction cosines Ŝ0= (cos α0, cos β0, cos γ0) where α0, β0, γ0 are the angles between the initial spin and the x, y, z axes, respectively. This...
28. ### Understanding Quantum Spin: A Layman's Guide to Particle Spin of 1/2

Please help a poor stupid layman. When a particle has a spin of 1/2, what does that really mean? 1/2 of WHAT?
29. ### Constructing Hamiltonian Matrix from Sz Basis States for Quantum Spin Chains

Hallo! My question relates to the use of basis states to form operator matrices... In the context of quantum spin chains, where the Hamiltonian on a chain of N sites is defined periodically as:H = sumk=0N-1[ S(k) dot S(k+1) ] (apologies for the notation) so there is a sum over k=0 to N-1...
30. ### Angular momentum and quantum spin

Is there an invariance relation for angular momentum similar to E^2 = p^2 + m^2 ? How about quantum spin ?
31. ### Exploring Quantum Spin: Visualizing Spin Observables in Real and C-2 Space

I'm reading Hughes book The Structure and Interpretation of Quantum Mechanics. I was wondering if people knew of any good web resources that graphically (maybe with Java applets, etc.) illustrate how the spin observables Sx Sy and Sz are related to each other, either in real space or in their...
32. ### Quantum Spin Chain Problem: An Overview

what is the quantum spin chain problem? From my research on the net I can see that its solved through exact diagonlization of some sort of matrix, but I can't work out exactly what the problem is...
33. ### Quantum Spin: Forgetting, Entanglement & Change

My understanding is that when you measure an electron's spin on on Axis A, and then on Axis B, the spin on Axis A is "forgotten" and can be something different next time you measure it. Is this correct? If it is, then how does this work across entanglement? If you measure electron A's spin...
34. ### What is the relationship between quantum spin and classical spin?

What is the relationship between quantum spin and classical spin? I have seen this question asked many times but have never seen an answer that I was happy with. The most common answer I have seen is that there is no connection. The word "spin" has more to do with history than physics and...
35. ### Understanding Quantum Spin - A Beginner's Guide

What is quantum spin? I've read lots of things about it and describing it, I still don't quite understand it.