Update and rename anderson-model to anderson-model.md

Added some text on the SIAM model

docs/src/examples/anderson-model.md

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+# The Anderson Impurity Model 
+
+We use the fermionic chain mapping proposed in [1] to perform tensor network simulations of the Single Impurity Anderson Model (SIAM). The SIAM Hamiltonian is defined as:
+\begin{equation}
+    \hat H^\text{SIAM}  = \hat H_\text{loc} + \hat H_\text{hyb} + \hat H_\text{cond} = \overbrace{\epsilon_d \hat d^\dagger \hat d}^{\hat H_\text{loc}} + \underbrace{\sum_{k} V_k \Big( \hat d^\dagger \hat c_k + \hat c_k^\dagger \hat d \Big)}_{H_\text{hyb}} + \underbrace{\sum_k \epsilon_k \hat c_k^\dagger \hat c_k}_{H_I^\text{chain}}.
+\end{equation}
+All of the operators obey to the usual fermionic anti-commutation relations: $\{\hat c_i, \hat c_j^\dagger \} = \delta_{ij}$, $\{\hat c_i, \hat c_j \} =\{\hat c_i^\dagger, \hat c_j^\dagger \} =0$ $\forall i,j$. The chain mapping is based on a thermofield-like transformation [2], performed with fermions: ancillary fermionic operators $\hat c_{2k}$ are defined, one for each of the original fermionic modes $\hat c_{1k}$. A Bogoliubov transformation is then applied, so that two new fermionic modes $\hat f_{1k}$ and $\hat f_{2k}$ are defined as a linear combination of $\hat c_{1k}$ and $\hat c_{2k}$. Two chains are defined: the chain labelled $1$ for the empty modes, the chain labelled $2$ for the filled modes.
+The following relations are used to define the functions equivalent to the spectral density of the bosonic case, one for each chain:
+\begin{equation}
+\begin{split}
+   &V_{1k} = V_{k} \sin \theta_k = \sqrt{\frac{1}{e^{\beta \epsilon_k}+1}} \\
+   &V_{2k} = V_{k} \cos \theta_k = \sqrt{\frac{1}{e^{-\beta \epsilon_k}+1}}, 
+\end{split}
+\end{equation}    
+where we choose the spectral function that characterizes the fermionic bath to be: $V_k= \sqrt{1-k^2}$, and we define the dispersion relation as: $e_k = k$, that is, a linear dispersion relation with propagation speed equal to $1$. This latter choice corresponds to a model of metals (gapless energy spectrum). We select a filled state as the initial state of the defect.
+Using the mapping proposed in [1], the chain Hamiltonian becomes:
+\begin{equation}
+    \begin{split}
+    \hat H^\text{chain}  = \hat H_\text{loc} &+ \sum_{i = \{1,2\}}\bigg[ J_{i,0} \Big(\hat d^\dagger \hat a_{i,0} + \hat d \hat a_{i,0}^\dagger \Big) + \\ &+ \sum_{n=1}^\infty  \Big( J_{i,n} \hat a_{i,n}^\dagger \hat a_{i,n-1} +  J_{i,n} \hat  a_{i,n-1}^\dagger \hat a_{i,n} \Big) + \sum_{n=0}^\infty E_{i,n} \hat  a_{i,n}^\dagger \hat a_{i,n} \bigg],
+    \end{split}
+\end{equation}
+where the $J_{i,n}$ coefficients are the couplings between the chain sites and the $E_{i,n}$ coefficients are the energies associated to each chain site. Clearly, the interactions are between nearest neighbors. This, combined with the fact that the fermions in our model are spinless, enables a straightforward mapping into fermionic operators of the bosonic creation and annihilation operators, that on their part obey to the bosonic commutation relations: $[\hat b_i, \hat b_j^\dagger] = \delta_{ij}$, $[\hat b_i, \hat b_j] =[\hat b_i^\dagger, \hat b_j^\dagger] =0$ $\forall i,j$. The mapping derived from Jordan-Wigner transformations for spinless fermions is:
+\begin{equation}
+    \hat a_{i}^\dagger \hat a_{i+1} + \hat a_{i+1}^\dagger \hat a_{i} = \hat b_{i}^\dagger \hat b_{i+1} + \hat b_{i+1}^\dagger \hat b_{i}.  
+\end{equation}
+
+
+## Double chain mapping
+
+The corresponding MPO representation is:
+\begin{equation}
+\begin{split}
+&
+\begin{bmatrix}
+ \hat{\mathbb I} & J_{2,N} \hat b_{2,N}^\dagger & J_{2,N} \hat b_{2,N} & E_{2,N} \hat b_{2,N}^\dagger \hat b_{2,N} 
+\end{bmatrix}\cdot ... \cdot
+\begin{bmatrix}
+ \hat{ \mathbb I} & J_{2,0} \hat b_{2,0}^\dagger & J_{2,0} \hat b_{2,0} & E_{2,0} \hat b_{2,0}^\dagger \hat b_{2,0}\\
+0 &0 & 0 & \hat b_{2,0} \\
+0 &0 & 0 & \hat b_{2,0}^\dagger \\
+0 &0 & 0 & \hat{\mathbb I}
+\end{bmatrix}
+\cdot \\ \cdot &
+\begin{bmatrix}
+ \hat{ \mathbb I} & \hat d^\dagger & \hat d & \epsilon_d \hat d^\dagger \hat d\\
+0 &0 & 0 & \hat d \\
+0 &0 & 0 & \hat d^\dagger \\
+0 &0 & 0 & \hat{\mathbb I}
+\end{bmatrix}
+\cdot 
+\begin{bmatrix}
+ \hat{ \mathbb I} & \hat b_{1,0}^\dagger & \hat b_{1,0} & E_{1,0} \hat b_{1,0}^\dagger \hat b_{1,0}\\
+0 &0 & 0 & \hat J_{1,0}b_{1,0} \\
+0 &0 & 0 & \hat J_{1,0}b_{1,0}^\dagger \\
+0 &0 & 0 & \hat{\mathbb I}
+\end{bmatrix}
+\cdot  ... \cdot
+\begin{bmatrix}
+ E_{2,N} \hat b_{2,N}^\dagger \hat b_{2,N} \\ J_{2,N} \hat b_{2,N} \\ J_{2,N} \hat b_{2,N}^\dagger \\ \hat{\mathbb I}
+\end{bmatrix}
+\end{split}
+\end{equation}
+
+The system starts from a filled state, the chain starts as in the Fermi sea.
+
+## Interleaved chain mapping
+
+The drawback of such a representation though, is that the particle-hole pairs are spatially separated in the MPS, creating correlations and therefore leading to a dramatic increase in the bond dimensions. This is why Kohn and Santoro propose an interleaved geometry, the advantages of which are thoroughly explained in \cite{Kohn_Santoro_2021b}. Exploiting the interleaved representation, the interaction comes to be between next-nearest neighbors: a string operator appears in the Jordan-Wigner transformation from bosons to fermions:
+\begin{equation}
+    \hat a_{i}^\dagger \hat a_{i+2} + \hat a_{i+2}^\dagger \hat a_{i} = \hat b_{i}^\dagger \hat F_{i+1} \hat b_{i+2} + \hat b_{i} \hat F_{i+1} \hat b_{i+2}^\dagger,
+\end{equation}
+where the string operator $\hat F_i$ is defined as: $\hat F_i = (-1)^{\hat n_i} = \hat{\mathbb I} -2 \hat n_i = \hat{\mathbb I}-2 \hat b_i^\dagger \hat b_i$. It is possible to find the analytical form also for MPOs with long range interaction \cite{mpo}. In the case of next-nearest neighbors interactions between spinless fermions, the MPO representation will require a bond dimension $\chi=6$. We explicitly write it as:
+\begin{equation}
+\begin{split}
+&
+\begin{bmatrix}
+ \hat{\mathbb I} & \hat d & \hat d^\dagger & 0 & 0 & E_{d} \hat d^\dagger \hat d 
+\end{bmatrix}\cdot
+\begin{bmatrix}
+ \hat{ \mathbb I} & \hat b_{2,0} & \hat b_{2,0}^\dagger & 0 & 0 & E_{2,0} \hat b_{2,0}^\dagger \hat b_{2,0}\\
+0 &0 & 0 & \hat{F}_{2,0} & 0 & J_{2,0} \hat b_{2,0}^\dagger \\
+0 &0 & 0 & 0 & \hat{F}_{2,0} & J_{2,0} \hat b_{2,0} \\
+0 &0 & 0 & 0 & 0 &  0\\
+0 &0 & 0 & 0 & 0 & 0 \\
+0 &0 & 0 & 0 & 0 & \hat{\mathbb I}
+\end{bmatrix}
+\cdot \\ \cdot &
+\begin{bmatrix}
+ \hat{ \mathbb I} & \hat b_{1,0} & \hat b_{1,0}^\dagger & 0 & 0 & E_{1,0} \hat b_{1,0}^\dagger \hat b_{1,0}\\
+0 &0 & 0 & \hat{ F}_{1,0} & 0 & 0 \\
+0 &0 & 0 & 0 & \hat{F}_{1,0} & 0 \\
+0 &0 & 0 & 0 & 0 & J_{1,0} \hat b_{1,0}^\dagger \\
+0 &0 & 0 & 0 & 0 & J_{1,0} \hat b_{1,0} \\
+0 &0 & 0 & 0 & 0 & \hat{\mathbb I}
+\end{bmatrix}
+\cdot ... \cdot 
+\begin{bmatrix}
+ \hat{ \mathbb I} & \hat b_{2,N} & \hat b_{2,N}^\dagger & 0 & 0 & E_{2,N} \hat b_{2,N}^\dagger \hat b_{2,N}\\
+0 &0 & 0 & \hat{F}_{2,N} & 0 & 0 \\
+0 &0 & 0 & 0 & \hat{F}_{2,N} & 0 \\
+0 &0 & 0 & 0 & 0 & J_{2,N} \hat b_{2,N}^\dagger \\
+0 &0 & 0 & 0 & 0 & J_{2,N} \hat b_{2,N} \\
+0 &0 & 0 & 0 & 0 & \hat{\mathbb I}
+\end{bmatrix}
+\cdot \\ \cdot &
+\begin{bmatrix}
+ E_{1,N} \hat b_{1,N}^\dagger \hat b_{1,N} \\ 0 \\0 \\ J_{1,N} \hat b_{1,N}^\dagger \\ J_{1,N} \hat b_{1,N} \\ \hat{\mathbb I}
+\end{bmatrix} 
+\end{split}
+\end{equation}
+________________
+### References
+
+[1] Lucas Kohn and Giuseppe E. Santoro. Efficient mapping for anderson impurity problems with matrix product states. Physical Review B, 104(1):014303, Jul 2021. arXiv: [2012.01424](https://arxiv.org/abs/2012.01424).
+
+
+[2] Ines de Vega and Mari-Carmen Banuls. Thermofield-based chain mapping approach for open quantum systems. Physical Review A, 92(5):052116, Nov 2015. arXiv:[1504.07228](https://arxiv.org/abs/1504.07228).
+
+[3] L. Kohn and G. E. Santoro. Quenching the anderson impurity model at finite temperature: Entanglement and bath dynamics using matrix product states. arXiv:2107.02807 [cond-mat, physics:quant-ph], Jul 2021. arXiv: [2107.02807](https://arxiv.org/abs/2107.02807)