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/* measure.c: Quantum register measurement
Copyright 2003, 2004 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 <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <math.h>
#include <unistd.h>
#include <stdio.h>
#include "qureg.h"
#include "complex.h"
#include "config.h"
#include "objcode.h"
#include "error.h"
/* Generate a uniformly distributed random number between 0 and 1 */
double
quantum_frand()
{
return (double) rand() / RAND_MAX;
}
/* Measure the contents of a quantum register */
MAX_UNSIGNED
quantum_measure(quantum_reg reg)
{
double r;
int i;
if(quantum_objcode_put(MEASURE))
return 0;
/* Get a random number between 0 and 1 */
r = quantum_frand();
for (i=0; i<reg.size; i++)
{
/* If the random number is less than the probability of the
given base state - r, return the base state as the
result. Otherwise, continue with the next base state. */
r -= quantum_prob_inline(reg.node[i].amplitude);
if(0 >= r)
return reg.node[i].state;
}
/* The sum of all probabilities is less than 1. Usually, the cause
for this is the application of a non-normalized matrix, but there
is a slim chance that rounding errors may lead to this as
well. */
return -1;
}
/* Measure a single bit of a quantum register. The bit measured is
indicated by its position POS, starting with 0 as the least
significant bit. The new state of the quantum register depends on
the result of the measurement. */
int
quantum_bmeasure(int pos, quantum_reg *reg)
{
int i;
int result=0;
double pa=0, r;
MAX_UNSIGNED pos2;
quantum_reg out;
if(quantum_objcode_put(BMEASURE, pos))
return 0;
pos2 = (MAX_UNSIGNED) 1 << pos;
/* Sum up the probability for 0 being the result */
for(i=0; i<reg->size; i++)
{
if(!(reg->node[i].state & pos2))
pa += quantum_prob_inline(reg->node[i].amplitude);
}
/* Compare the probability for 0 with a random number and determine
the result of the measurement */
r = quantum_frand();
if (r > pa)
result = 1;
out = quantum_state_collapse(pos, result, *reg);
quantum_delete_qureg_hashpreserve(reg);
*reg = out;
return result;
}
/* Measure a single bit, but do not remove it from the quantum
register */
int
quantum_bmeasure_bitpreserve(int pos, quantum_reg *reg)
{
int i, j;
int size=0, result=0;
double d=0, pa=0, r;
MAX_UNSIGNED pos2;
quantum_reg out;
if(quantum_objcode_put(BMEASURE_P, pos))
return 0;
pos2 = (MAX_UNSIGNED) 1 << pos;
/* Sum up the probability for 0 being the result */
for(i=0; i<reg->size; i++)
{
if(!(reg->node[i].state & pos2))
pa += quantum_prob_inline(reg->node[i].amplitude);
}
/* Compare the probability for 0 with a random number and determine
the result of the measurement */
r = quantum_frand();
if (r > pa)
result = 1;
/* Eradicate all amplitudes of base states which have been ruled out
by the measurement and get the absolute of the new register */
for(i=0;i<reg->size;i++)
{
if(reg->node[i].state & pos2)
{
if(!result)
reg->node[i].amplitude = 0;
else
{
d += quantum_prob_inline(reg->node[i].amplitude);
size++;
}
}
else
{
if(result)
reg->node[i].amplitude = 0;
else
{
d += quantum_prob_inline(reg->node[i].amplitude);
size++;
}
}
}
/* Build the new quantum register */
out.size = size;
out.node = calloc(size, sizeof(quantum_reg_node));
if(!out.node)
quantum_error(QUANTUM_ENOMEM);
quantum_memman(size * sizeof(quantum_reg_node));
out.hashw = reg->hashw;
out.hash = reg->hash;
out.width = reg->width;
/* Determine the numbers of the new base states and norm the quantum
register */
for(i=0, j=0; i<reg->size; i++)
{
if(reg->node[i].amplitude)
{
out.node[j].state = reg->node[i].state;
out.node[j].amplitude = reg->node[i].amplitude * 1 / (float) sqrt(d);
j++;
}
}
quantum_delete_qureg_hashpreserve(reg);
*reg = out;
return result;
}
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