Quantum gates are rotation matrices that preserve the unit magnitude of a state vector. The file quantum-gates.h implements rotation matrices using component arithmetic. See the demo programs *-demo.c for examples of how to include quantum-gates.h and implement quantum algorithms in C.

The following quantum gates are implemented in quantum-gates.h

init      Ground state
xgate     Pauli X
ygate     Pauli Y
zgate     Pauli Z
cxgate    Controlled X and Toffoli
cygate    Controlled Y
czgate    Controlled Z
hadamard  Hadamard gate
phase     S, T, and general phase gate
cphase    Controlled phase
swap      Swap qubits
cswap     Controlled swap
ft        Fourier transform
ift       Inverse fourier transform
measure   Print a histogram of the current state and rotate to a basis state

Controlled gates can have any number of control bits. Control bits are selected by setting bits in an int as shown in the following example.

#define Q0 0
#define Q1 1
#define Q2 2

// Toffoli gate

// Q2 is target, Q1 and Q0 are control

cxgate(Q2, 1 << Q1 | 1 << Q0);

Consider the following quantum circuit.

Q0 ----H----.---------M
            |
Q1 ---------X----.----M
                 |
Q2 --------------X----M

This is the circuit in C.

#define NUMQBITS 3

#include "quantum-gates.h"

#define Q0 0
#define Q1 1
#define Q2 2

int
main()
{
     init();
     hadamard(Q0);
     cxgate(Q1, 1 << Q0); // Q1 is target, Q0 is control
     cxgate(Q2, 1 << Q1); // Q2 is target, Q1 is control
     measure(NUMQBITS);
}

To build and run

make
./simple-demo

The result is a histogram of basis state probabilities.

000 0.500000 **************************************************
001 0.000000 
010 0.000000 
011 0.000000 
100 0.000000 
101 0.000000 
110 0.000000 
111 0.500000 **************************************************

The result shows a 50% chance of getting 000 from the circuit and a 50% chance getting 111.