% Higher number state |20> (Sec. II.A)
% To represent |n>, the field dimension d must be larger than n.

clear;              % Clear memory
clc;                % Clear the command window/screen
d = 21;             % dimension of the field (must exceed 20)
I = eye(d);         % Identity matrix of dimension d
Ket20 = I(:,21)     % number state |20>: 21st column of the identity matrix