% Number states (Sec. II.A)
% A number state |n> is a column of the identity matrix: the (n+1)th column.

clear;             % Clear memory
clc;               % Clear the command window/screen
d = 5;             % dimension of the field
I = eye(d);        % Identity matrix of dimension d
Vacuum = I(:,1)    % First column of identity matrix: vacuum state |0>
Ket1 = I(:,2)      % Second column of identity matrix: single-photon state |1>
Ket2 = I(:,3)      % Third column of identity matrix: two-photon state |2>