## Reduced density matrix

Suppose we have two quantum systems $$a, b$$, with dimension $$N_a, N_b$$ respectively. Then the Hilbert space of $$a+b$$ is of dimension $$N=N_aN_b$$. Suppose we have a density matrix

$$\hat\rho=\sum_{i,j}\rho_{ij}\lvert i\rangle \langle j\rvert=\sum_{i,j,k,l}\rho_{ijkl}\lvert i\rangle_a\lvert j\rangle_b \langle k\rvert_a\langle l\rvert_b$$

Then the reduced density matrix of $$a$$ is defined as

$$\hat\rho_a=\mathrm{tr}_b\hat\rho=\sum_i \langle i\rvert_b\hat\rho\lvert i\rangle_b$$

i.e. reduced density matrix problem is equivalent to partial trace problem.

### Tensor

In fact, if we take $$\hat\rho$$ as a 4-tensor $$\rho_{ijkl}$$, then the reduced density matrix is

$$\rho^{(a)}_{ij}=\delta^{\mu\nu}\rho_{i\mu k\nu}$$

For simple density matrix $$\rho=\lvert \psi\rangle \langle \psi\rvert$$, the reduced matrix is

$$\rho^{(a)}_{ik}=\delta^{jl}\rho_{ijkl}=\delta^{jl}\psi_{ij}\psi^+_{lk}=\sum_i |\langle i_b\lvert \psi\rangle|^2=[\psi\psi^+]_{ik}$$

Here we are taking $$\psi$$ as an $$N_a\times N_b$$ matrix.

For general case, if we find decomposition

$$\rho=\sum_c \lambda_c\lvert \psi_c\rangle \langle \psi_c\rvert,\quad \sum_c \lambda_c=1$$

then we have

$$\rho^{(a)}_{ik}=\left[\sum_c\lambda_c\psi_c\psi^+_c\right]_{ik}$$

## ipynb2pelican Plugin released!

Yet another Pelican Plugin for blogging with Jupyter Notebooks using MetaCell to store metadata.

Thanks to super cow power of python, we can finally publish ipynb easily! Below is the README.md from the project at this time.

## 24 Game Solver

24 game is an arithmetic game with a simple rule: Given 4 numbers and use + - * / to get 24.

• A simple example is 1, 2, 3, 4, and you find 1*2*3*4=24
• A more difficult one is 5, 5, 5, 1, the answer is 5*(5-(1/5))=24, which includes fractions.

24game project provides a powerful C++ solver for the 24 game. And you can play with the PyQt5 based graphical front end.

## 默认目录

jupyter notebook --generate-config
vim /home/zpj/.jupyter/jupyter_notebook_config.py#查找dir设定notebook根目录


## NaCl离子晶体晶格能的研究

$$k=1.74756459463, \quad E_1=kE_0=-8.95\rm{eV}$$

#include <stdio.h>
#include <math.h>
#define N 1000//计算的层数
int main(){
double a=0.,b=0.,c=0.,s=0.,t=0.;
int m, i, j;
for(m=1; m<N+1; m++){
a=(1-(m%2)*2)/(double)m;//*6 面
b=(1-(m%2)*2)/sqrt(3*m*m);//*8 点
c=1/sqrt(2*m*m);//*12 边
for(i=1;i<m;i++){
for(j=0;j<m;j++){
a+=4*(1-((m+i+j)%2)*2)/sqrt(m*m+i*i+j*j);//*24
}
c+=2*(1-(i%2)*2)/sqrt(2*m*m+i*i);
}
t=a*6+b*8+c*12;
if (m%10 == 0){
printf("t[%4d]=%9lf\t\ts=%.15lf\t\ts2=%lf\n",m,t,s+a*3+b+c*3,s+t);
}
s+=t;
}
s=s-t+a*3+b+c*3;
printf("s[%4d]=%.15lf\n", N, s);
return 0;
}