Simulating Measurements

Two Qubits - measure both

//
//  0.0       ... r ...                       1.0
//   |<------->|<-------->|<------->|<-------->|
//       p00        p01       p10       p11
//
int Mk( Complex a[]) { // measure overall state

   double r = rand() / (RAND_MAX + 1.0); // 0.0 <= r < 1.0

   double p = 0; // cumulative probability

   int k = 3; for( int i = 0; i < 4; ++i) { p += abs2( a[i]); if( r < p) { k = i; break; } }

   printf( "k = %i\n", k); return k;
}

void M10( Complex a[]) { // measure qubits 1 and 0

   int k = Mk( a); a[0]=a[1]=a[2]=a[3]=0; a[k] = 1;
}