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/* decoherence.c: Simulation of decoherence effects
Copyright 2003 Bjoern Butscher, Hendrik Weimer
This file is part of libquantum
libquantum is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published
by the Free Software Foundation; either version 3 of the License,
or (at your option) any later version.
libquantum is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received a copy of the GNU General Public License
along with libquantum; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
MA 02110-1301, USA
*/
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include "measure.h"
#include "qureg.h"
#include "gates.h"
#include "complex.h"
#include "error.h"
/* Status of the decoherence simulation. Non-zero means enabled and
decoherence effects will be simulated. */
int quantum_status = 0;
/* Decoherence parameter. The higher the value, the greater the
decoherence impact. */
float quantum_lambda = 0;
float
quantum_get_decoherence()
{
return quantum_lambda;
}
/* Initialize the decoherence simulation and set the decoherence
parameter. */
void
quantum_set_decoherence(float l)
{
if(l)
{
quantum_status = 1;
quantum_lambda = l;
}
else
quantum_status = 0;
}
/* Perform the actual decoherence of a quantum register for a single
step of time. This is done by applying a phase shift by a normal
distributed angle with the variance LAMBDA. */
void
quantum_decohere(quantum_reg *reg)
{
float u, v, s, x;
float *nrands;
float angle;
int i, j;
/* Increase the gate counter */
quantum_gate_counter(1);
if(quantum_status)
{
nrands = calloc(reg->width, sizeof(float));
if(!nrands)
quantum_error(QUANTUM_ENOMEM);
quantum_memman(reg->width * sizeof(float));
for(i=0; i<reg->width; i++)
{
/* Generate normal distributed random numbers */
do {
u = 2 * quantum_frand() - 1;
v = 2 * quantum_frand() - 1;
s = u * u + v * v;
} while (s >= 1);
x = u * sqrt(-2 * log(s) / s);
x *= sqrt(2 * quantum_lambda);
nrands[i] = x/2;
}
/* Apply the phase shifts for decoherence simulation */
for(i=0; i<reg->size; i++)
{
angle = 0;
for(j=0; j<reg->width; j++)
{
if(reg->state[i] & ((MAX_UNSIGNED) 1 << j))
angle += nrands[j];
else
angle -= nrands[j];
}
reg->amplitude[i] *= quantum_cexp(angle);
}
free(nrands);
quantum_memman(-reg->width * sizeof(float));
}
}
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