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# KMS state

The statistical mechanics of quantum field theory (see thermal quantum field theory) at the inverse temperature β and chemical potential μ can be characterized by a condition called the KMS condition.

## Preliminaries

The simplest case to study is that of a finite-dimensional Hilbert space, in which one does not encounter complications like phase transitions or spontaneous symmetry breaking. The density matrix of a thermal state is given by $\rho_{\beta,\mu}=\frac{e^{-\beta \left(H-\mu N\right)}}{Tr\left[ e^{-\beta \left(H-\mu N\right)} \right]}=\frac{e^{-\beta \left(H-\mu N\right)}}{Z(\beta,\mu)}$

where H is the Hamiltonian operator and N is the particle number operator (or charge operator, if we wish to be more general) and $Z(\beta,\mu)\ \stackrel{\mathrm{def}}{=}\ Tr\left[ e^{-\beta \left(H-\mu N\right)} \right]$

is the partition function. We assume that N commutes with H, or in other words, that particle number is conserved.

In the Heisenberg picture, the density matrix does not change with time, but the operators are time-dependent. In particular, translating an operator A by τ into the future gives the operator $\alpha_\tau(A)\ \stackrel{\mathrm{def}}{=}\ e^{iH\tau}A e^{-iH\tau}$.

A combination of time translation with an internal symmetry "rotation" gives the more general $\alpha^{\mu}_{\tau}\ \stackrel{\mathrm{def}}{=}\ e^{i\left(H-\mu N\right)\tau} A e^{-i\left(H-\mu N\right)\tau}$

A bit of algebraic manipulation shows that the expected values $\langle\alpha^\mu_\tau(A)B\rangle_{\beta,\mu}=Tr\left[\rho \alpha^\mu_\tau(A)B\right]=Tr\left[\rho B \alpha^\mu_{\tau+i\beta}(A)\right] =\langle B\alpha^\mu_{\tau+i\beta}(A)\rangle_{\beta,\mu}$

for any two operators A and B and any real τ (we are working with finite-dimensional Hilbert spaces after all). We used that fact that the density matrix commutes with any function of (HN) and that the trace is cyclic.

As hinted at earlier, with infinite dimensional Hilbert spaces, we run into a lot of problems like phase transitions, spontaneous symmetry breaking, operators which aren't trace class, divergent partition functions, etc..

The complex functions of z, $\langle\alpha^\mu_z(A)B\rangle$ and $\langle B\alpha^\mu_z(A)\rangle$ converge if $0 < \Im{z} < \beta$ if we make certain technical assumptions like the spectrum of HN is bounded from below and its density doesn't increase exponentially (see Hagedorn temperature). If the functions converge, then they have to be analytic within the $0 < \Im{z} < \beta$ strip as their derivatives, $\frac{d}{dz}\langle\alpha^\mu_z(A)B\rangle=i\langle\alpha^\mu_z\left(\left[H-\mu N,A\right]\right)B\rangle$

and $\frac{d}{dz}\langle B\alpha^\mu_z(A)\rangle = i\langle B\alpha^\mu_z\left(\left[H-\mu N,A\right]\right)\rangle$

exist.

However, we can still define a KMS state as any state satisfying $\langle \alpha^\mu_\tau(A)B\rangle=\langle B\alpha^\mu_{\tau+i\beta}(A)\rangle$

with $\langle\alpha^\mu_z(A)B\rangle$ and $\langle B\alpha^\mu_z(A)\rangle$ being analytic functions of z within the strip $0 < \Im{z} < \beta$. $\langle\alpha^\mu_\tau(A)B\rangle$ and $\langle B\alpha^\mu_{\tau+i\beta}(A)\rangle$ are the boundary distribution values of the analytic functions in question.

This gives the right large volume, large particle number thermodynamic limit. If there is a phase transition or spontaneous symmetry breaking, the KMS state isn't unique.

The density matrix of a KMS state is related to unitary transformations involving time translations (or time translations and an internal symmetry transformation for nonzero chemical potentials) via the Tomita-Takesaki theory.