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Are Ising spins scalar or operators? I am not a condensed matter physicist hence having some confusion. I have learnt about Ising models from adiabatic quantum algorithm papers. For example this presentation or this paper encodes their quantum adiabatic algorithm for an Ising model. In an Ising model the spins are $\pm 1$. In his original adiabatic quantum computation paper, Farhi didn't mention about Ising models. Moreover he explicitly used Pauli's spin matrices and not the $\pm 1$ spins.

Can we use $\pm$ spins and Pauli's matrices interchangeability at least in the context of adiabatic quantum algorithms?

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    $\begingroup$ Copy on Physics. $\endgroup$
    – Kaveh
    Commented May 28, 2013 at 17:50
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    $\begingroup$ Please do not cross post simultaneously. It wastes effort on multiple locations. If you don't get a satisfactory answer on one site, you're welcome to repost on another one. $\endgroup$
    – Suresh Venkat
    Commented May 28, 2013 at 23:10
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    $\begingroup$ It appears that you have crossposted this question simultaneously. While we don't mind a question being reposted, our site policy prohibits simultaneous crossposting as it duplicates effort and fractures discussion. Crossposting is only permitted after sufficient time has passed and you have not obtained your desired answer elsewhere. When crossposting please summarize the relevant discussions from other sites in your question and link to the copies in both directions. $\endgroup$
    – Kaveh
    Commented May 31, 2013 at 17:18
  • $\begingroup$ @Kaveh, understood. $\endgroup$
    – Omar Shehab
    Commented May 31, 2013 at 19:59

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Neither, they are domain elements. Any two domain elements would do: $\{+1, -1\}$, $\{0,1\}$, $\{\text{cat}, \text{dog}\}$.

Physicists have wonderful imaginations and thus name certain parameters based on their physical intuition for the behavior of nature. In this case, they interpret the states of the particles interacting in the Ising model as either spinning up or down. Furthermore, opposite spins produce opposite effects, such as their resulting magnetic fields being the negative of each other. Thus, the state of the particles (i.e. the spins) are succinctly represented in the model as either $+1$ or $-1$.

Another good naming example from physics is in string theory. The equations that describe the motion of these exceedingly small "objects" reminded the physicists of the equations that describe the motion of strings. Thus the name.

The Potts model is a generalization of the Ising model. One of the generalizations is that each particle can take one of $q$ values (i.e. the domain size is now $q$). By continuing the intuitive naming from the Ising model, these $q$ states are also called spins.

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  • $\begingroup$ not sure whether I get it. So, do you mean that I cannot use them interchangeably? $\endgroup$
    – Omar Shehab
    Commented May 28, 2013 at 12:52
  • $\begingroup$ The symbols used to describe the domain elements are arbitrary. You can use whatever you want. I think your confusion stems from the fact that physicists use "spin" to describe both concepts. Mathematically, the domain elements are completely different from any set of matrices, including the Pauli matrices. $\endgroup$
    – Tyson Williams
    Commented May 28, 2013 at 16:33

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