% Bit-flip error correcting code example for Quack!
% by Peter Rohde

quack;

disp('Bit-flip error correcting code example');
disp(' ');

init_state(5);

% Qubit labels
redundant1 = 1;
redundant2 = 2;
redundant3 = 3;
syndrome1 = 4;
syndrome2 = 5;

% Prepare input state
prepare_plus(redundant1);

% What's the input state
disp('Input state:');
print_rdm(redundant1);
disp(' ');

% Encoding
cnot(redundant1,redundant2);
cnot(redundant1,redundant3);

% Apply a bit-flip to one of the qubits channels
n = input('Apply bit-flip to which qubit? ');
X(n);

% Syndrome measurement
cnot(redundant1,syndrome1);
cnot(redundant2,syndrome1);
cnot(redundant2,syndrome2);
cnot(redundant3,syndrome2);
outcome1 = Z_measure(syndrome1);
outcome2 = Z_measure(syndrome2);

disp(['Syndrome measured: ' num2str(outcome1) num2str(outcome2)]);
disp(' ');

% Error correction
if outcome1 == 1 && outcome2 == 1
	disp('Applied no correction');
end

if outcome1 == 1 && outcome2 == -1
	X(redundant3);
	disp('Applited X3 correction');
end

if outcome1 == -1 && outcome2 == 1
	X(redundant1);
	disp('Applied X1 correction');
end

if outcome1 == -1 && outcome2 == -1
	X(redundant2);
	disp('Applied X2 correction');
end

disp(' ');

% Decoding
cnot(redundant1,redundant3);
cnot(redundant1,redundant2);

% Output state
disp('Reduced density matrix of decoded state:');
print_rdm(redundant1);

% Show the circuit history
print_hist;