// Copyright (C) 2008 Davis E. King (davis@dlib.net)
// License: Boost Software License See LICENSE.txt for the full license.
#ifndef DLIB_QUANTUM_COMPUTINg_1_
#define DLIB_QUANTUM_COMPUTINg_1_
#include <complex>
#include <cmath>
#include "../matrix.h"
#include "../rand.h"
#include "../enable_if.h"
#include "../algs.h"
#include "quantum_computing_abstract.h"
namespace dlib
{
template <typename T>
struct gate_traits {};
namespace qc_helpers
{
// ------------------------------------------------------------------------------------
// This is a template to compute the value of 2^n at compile time
template <long n>
struct exp_2_n
{
COMPILE_TIME_ASSERT(0 <= n && n <= 30);
static const long value = exp_2_n<n-1>::value*2;
};
template <>
struct exp_2_n<0>
{
static const long value = 1;
};
// ------------------------------------------------------------------------------------
}
typedef std::complex<double> qc_scalar_type;
// ----------------------------------------------------------------------------------------
class quantum_register
{
public:
quantum_register()
{
set_num_bits(1);
}
int num_bits (
) const
{
return num_bits_in_register;
}
void set_num_bits (
int num_bits
)
{
// make sure requires clause is not broken
DLIB_CASSERT(1 <= num_bits && num_bits <= 30,
"\tvoid quantum_register::set_num_bits()"
<< "\n\tinvalid arguments to this function"
<< "\n\tnum_bits: " << num_bits
<< "\n\tthis: " << this
);
num_bits_in_register = num_bits;
unsigned long size = 1;
for (int i = 0; i < num_bits; ++i)
size *= 2;
state.set_size(size);
zero_all_bits();
}
void zero_all_bits()
{
set_all_elements(state,0);
state(0) = 1;
}
void append (
const quantum_register& reg
)
{
num_bits_in_register += reg.num_bits_in_register;
state = tensor_product(state, reg.state);
}
template <typename rand_type>
bool measure_bit (
int bit,
rand_type& rnd
)
{
// make sure requires clause is not broken
DLIB_CASSERT(0 <= bit && bit < num_bits(),
"\tbool quantum_register::measure_bit()"
<< "\n\tinvalid arguments to this function"
<< "\n\tbit: " << bit
<< "\n\tnum_bits(): " << num_bits()
<< "\n\tthis: " << this
);
const bool value = (rnd.get_random_double() < probability_of_bit(bit));
// Next we set all the states where this bit doesn't have the given value to 0
// But first make a mask that selects our bit
unsigned long mask = 1;
for (int i = 0; i < bit; ++i)
mask <<= 1;
// loop over all the elements in the state vector and zero out those that
// conflict with the measurement we just made.
for (long r = 0; r < state.nr(); ++r)
{
const unsigned long field = r;
// if this state indicates that the bit should be set and it isn't
if ((field & mask) && !value)
{
state(r) = 0;
}
// else if this state indicates that the bit should not be set and it is
else if (!(field & mask) && value)
{
state(r) = 0;
}
}
// normalize the state
state = state/(std::sqrt(sum(norm(state))));
return value;
}
template <typename rand_type>
bool measure_and_remove_bit (
int bit,
rand_type& rnd
)
{
// make sure requires clause is not broken
DLIB_CASSERT(0 <= bit && bit < num_bits() && num_bits() > 0,
"\tbool quantum_register::measure_and_remove_bit()"
<< "\n\tinvalid arguments to this function"
<< "\n\tbit: " << bit
<< "\n\tnum_bits(): " << num_bits()
<< "\n\tthis: " << this
);
const bool value = (rnd.get_random_double() < probability_of_bit(bit));
quantum_register temp;
temp.set_num_bits(num_bits()-1);
// Next we set all the states where this bit doesn't have the given value to 0
// But first make a mask that selects our bit
unsigned long mask = 1;
for (int i = 0; i < bit; ++i)
mask <<= 1;
long count = 0;
for (long r = 0; r < state.nr(); ++r)
{
const unsigned long field = r;
// if this basis vector is one that matches the measured state then keep it
if (((field & mask) != 0) == value)
{
temp.state(count) = state(r);
++count;
}
}
// normalize the state
temp.state = temp.state/std::sqrt(sum(norm(temp.state)));
temp.swap(*this);
return value;
}
double probability_of_bit (
int bit
) const
{
// make sure requires clause is not broken
DLIB_CASSERT(0 <= bit && bit < num_bits(),
"\tdouble quantum_register::probability_of_bit()"
<< "\n\tinvalid arguments to this function"
<< "\n\tbit: " << bit
<< "\n\tnum_bits(): " << num_bits()
<< "\n\tthis: " << this
);
// make a mask that selects our bit
unsigned long mask = 1;
for (int i = 0; i < bit; ++i)
mask <<= 1;
// now find the total probability of all the states that have the given bit set
double prob = 0;
for (long r = 0; r < state.nr(); ++r)
{
const unsigned long field = r;
if (field & mask)
{
prob += std::norm(state(r));
}
}
return prob;
}
const matrix<qc_scalar_type,0,1>& state_vector() const { return state; }
matrix<qc_scalar_type,0,1>& state_vector() { return state; }
void swap (
quantum_register& item
)
{
exchange(num_bits_in_register, item.num_bits_in_register);
state.swap(item.state);
}
private:
int num_bits_in_register;
matrix<qc_scalar_type,0,1> state;
};
inline void swap (
quantum_register& a,
quantum_register& b
) { a.swap(b); }
// ----------------------------------------------------------------------------------------
template <typename T>
class gate_exp
{
public:
static const long num_bits = gate_traits<T>::num_bits;
static const long dims = gate_traits<T>::dims;
gate_exp(T& exp_) : exp(exp_) {}
const qc_scalar_type operator() (long r, long c) const { return exp(r,c); }
const matrix<qc_scalar_type> mat (
) const
{
matrix<qc_scalar_type,dims,dims> m;
for (long r = 0; r < m.nr(); ++r)
{
for (long c = 0; c < m.nc(); ++c)
{
m(r,c) = exp(r,c);
}
}
return m;
}
void apply_gate_to (quantum_register& reg) const
{
// make sure requires clause is not broken
DLIB_CASSERT(reg.num_bits() == num_bits,
"\tvoid gate_exp::apply_gate_to()"
<< "\n\tinvalid arguments to this function"
<< "\n\treg.num_bits(): " << reg.num_bits()
<< "\n\tnum_bits: " << num_bits
<< "\n\tthis: " << this
);
quantum_register temp(reg);
// check if any of the elements of the register are 1 and if so then
// we don't have to do the full matrix multiply. Or check if only a small number are non-zero.
long non_zero_elements = 0;
for (long r = 0; r < dims; ++r)
{
if (reg.state_vector()(r) != qc_scalar_type(0))
++non_zero_elements;
reg.state_vector()(r) = 0;
}
if (non_zero_elements > 3)
{
// do a full matrix multiply to compute the output state
for (long r = 0; r < dims; ++r)
{
reg.state_vector()(r) = compute_state_element(temp.state_vector(),r);
}
}
else
{
// do a matrix multiply but only use the columns in the gate matrix
// that correspond to the non-zero register elements
for (long r = 0; r < dims; ++r)
{
if (temp.state_vector()(r) != qc_scalar_type(0))
{
for (long i = 0; i < dims; ++i)
{
reg.state_vector()(i) += temp.state_vector()(r)*exp(i,r);
}
}
}
}
}
template <typename exp>
qc_scalar_type compute_state_element (
const matrix_exp<exp>& reg,
long row_idx
) const
{
// make sure requires clause is not broken
DLIB_ASSERT(reg.nr() == dims && reg.nc() == 1 &&
0 <= row_idx && row_idx < dims,
"\tqc_scalar_type gate_exp::compute_state_element(reg,row_idx)"
<< "\n\tinvalid arguments to this function"
<< "\n\treg.nr(): " << reg.nr()
<< "\n\treg.nc(): " << reg.nc()
<< "\n\tdims: " << dims
<< "\n\trow_idx: " << row_idx
<< "\n\tthis: " << this
);
return this->exp.compute_state_element(reg,row_idx);
}
const T& ref() const { return exp; }
private:
T& exp;
};
// ----------------------------------------------------------------------------------------
template <typename T, typename U>
class composite_gate;
template <typename T, typename U>
struct gate_traits<composite_gate<T,U> >
{
static const long num_bits = T::num_bits + U::num_bits;
static const long dims = qc_helpers::exp_2_n<num_bits>::value;
};
template <typename T, typename U>
class composite_gate : public gate_exp<composite_gate<T,U> >
{
public:
typedef T lhs_type;
typedef U rhs_type;
composite_gate(const composite_gate& g) : gate_exp<composite_gate>(*this), lhs(g.lhs), rhs(g.rhs) {}
composite_gate(
const gate_exp<T>& lhs_,
const gate_exp<U>& rhs_
) : gate_exp<composite_gate>(*this), lhs(lhs_.ref()), rhs(rhs_.ref()) {}
static const long num_bits = gate_traits<composite_gate>::num_bits;
static const long dims = gate_traits<composite_gate>::dims;
const qc_scalar_type operator() (long r, long c) const { return lhs(r/U::dims,c/U::dims)*rhs(r%U::dims, c%U::dims); }
template <typename exp>
qc_scalar_type compute_state_element (
const matrix_exp<exp>& reg,
long row_idx
) const
{
// make sure requires clause is not broken
DLIB_ASSERT(reg.nr() == dims && reg.nc() == 1 &&
0 <= row_idx && row_idx < dims,
"\tqc_scalar_type composite_gate::compute_state_element(reg,row_idx)"
<< "\n\tinvalid arguments to this function"
<< "\n\treg.nr(): " << reg.nr()
<< "\n\treg.nc(): " << reg.nc()
<< "\n\tdims: " << dims
<< "\n\trow_idx: " << row_idx
<< "\n\tthis: " << this
);
qc_scalar_type result = 0;
for (long c = 0; c < T::dims; ++c)
{
if (lhs(row_idx/U::dims,c) != qc_scalar_type(0))
{
result += lhs(row_idx/U::dims,c) * rhs.compute_state_element(subm(reg,c*U::dims,0,U::dims,1), row_idx%U::dims);
}
}
return result;
}
const T lhs;
const U rhs;
};
// ----------------------------------------------------------------------------------------
template <long bits>
class gate;
template <long bits>
struct gate_traits<gate<bits> >
{
static const long num_bits = bits;
static const long dims = qc_helpers::exp_2_n<num_bits>::value;
};
// ----------------------------------------------------------------------------------------
template <long bits>
class gate : public gate_exp<gate<bits> >
{
public:
gate() : gate_exp<gate>(*this) { set_all_elements(data,0); }
gate(const gate& g) :gate_exp<gate>(*this), data(g.data) {}
template <typename T>
explicit gate(const gate_exp<T>& g) : gate_exp<gate>(*this)
{
COMPILE_TIME_ASSERT(T::num_bits == num_bits);
for (long r = 0; r < dims; ++r)
{
for (long c = 0; c < dims; ++c)
{
data(r,c) = g(r,c);
}
}
}
static const long num_bits = gate_traits<gate>::num_bits;
static const long dims = gate_traits<gate>::dims;
const qc_scalar_type& operator() (long r, long c) const { return data(r,c); }
qc_scalar_type& operator() (long r, long c) { return data(r,c); }
template <typename exp>
qc_scalar_type compute_state_element (
const matrix_exp<exp>& reg,
long row_idx
) const
{
// make sure requires clause is not broken
DLIB_ASSERT(reg.nr() == dims && reg.nc() == 1 &&
0 <= row_idx && row_idx < dims,
"\tqc_scalar_type gate::compute_state_element(reg,row_idx)"
<< "\n\tinvalid arguments to this function"
<< "\n\treg.nr(): " << reg.nr()
<< "\n\treg.nc(): " << reg.nc()
<< "\n\tdims: " << dims
<< "\n\trow_idx: " << row_idx
<< "\n\tthis: " << this
);
return (data*reg)(row_idx);
}
private:
matrix<qc_scalar_type,dims,dims> data;
};
// ----------------------------------------------------------------------------------------
namespace qc_helpers
{
// This is the maximum number of bits used for cached sub-matrices in composite_gate expressions
const int qc_block_chunking_size = 8;
template <typename T>
struct is_composite_gate { const static bool value = false; };
template <typename T, typename U>
struct is_composite_gate<composite_gate<T,U> > { const static bool value = true; };
// These overloads all deal with intelligently composing chains of composite_gate expressions
// such that the resulting expression has the form:
// (gate_exp,(gate_exp,(gate_exp,(gate_exp()))))
// and each gate_exp contains a cached gate matrix for a gate of at most qc_block_chunking_size bits.
// This facilitates the optimizations in the compute_state_element() function.
template <typename T, typename U, typename V, typename enabled = void>
struct combine_gates;
// This is a base case of this recursive template. It takes care of converting small composite_gates into
// cached gate objects.
template <typename T, typename U, typename V>
struct combine_gates<T,U,V,typename enable_if_c<(T::num_bits + U::num_bits <= qc_block_chunking_size)>::type >
{
typedef composite_gate<gate<T::num_bits + U::num_bits>,V> result_type;
static const result_type eval (
const composite_gate<T,U>& lhs,
const gate_exp<V>& rhs
)
{
typedef gate<T::num_bits + U::num_bits> gate_type;
return composite_gate<gate_type,V>(gate_type(lhs), rhs);
}
};
// this is the recursive step of this template
template <typename T, typename U, typename V>
struct combine_gates<T,U,V,typename enable_if_c<(is_composite_gate<U>::value == true)>::type >
{
typedef typename combine_gates<typename U::lhs_type, typename U::rhs_type, V>::result_type inner_type;
typedef composite_gate<T,inner_type> result_type;
static const result_type eval (
const composite_gate<T,U>& lhs,
const gate_exp<V>& rhs
)
{
return composite_gate<T,inner_type>(lhs.lhs, combine_gates<typename U::lhs_type, typename U::rhs_type, V>::eval(lhs.rhs,rhs));
}
};
// This is a base case of this recursive template. It takes care of adding new gates when the left
// hand side is too big to just turn it into a cached gate object.
template <typename T, typename U, typename V>
struct combine_gates<T,U,V,typename enable_if_c<(T::num_bits + U::num_bits > qc_block_chunking_size &&
is_composite_gate<U>::value == false)>::type >
{
typedef composite_gate<T,composite_gate<U, V> > result_type;
static const result_type eval (
const composite_gate<T,U>& lhs,
const gate_exp<V>& rhs
)
{
return result_type(lhs.lhs, composite_gate<U,V>(lhs.rhs, rhs));
}
};
}
template <typename T, typename U>
const composite_gate<T,U> operator, (
const gate_exp<T>& lhs,
const gate_exp<U>& rhs
)
{
return composite_gate<T,U>(lhs,rhs);
}
template <typename T, typename U, typename V>
const typename qc_helpers::combine_gates<T,U,V>::result_type operator, (
const composite_gate<T,U>& lhs,
const gate_exp<V>& rhs
)
{
return qc_helpers::combine_gates<T,U,V>::eval(lhs,rhs);
}
// If you are getting an error here then it means that you are trying to combine a gate expression
// with an integer somewhere (and that is an error).
template <typename T> void operator, ( const gate_exp<T>&, int) { COMPILE_TIME_ASSERT(sizeof(T) > 100000000); }
template <typename T> void operator, ( int, const gate_exp<T>&) { COMPILE_TIME_ASSERT(sizeof(T) > 100000000); }
// ----------------------------------------------------------------------------------------
namespace quantum_gates
{
template <int control_bit, int target_bit>
class cnot;
template <int control_bit1, int control_bit2, int target_bit>
class toffoli;
}
template <int control_bit, int target_bit>
struct gate_traits<quantum_gates::cnot<control_bit, target_bit> >
{
static const long num_bits = tabs<control_bit-target_bit>::value+1;
static const long dims = qc_helpers::exp_2_n<num_bits>::value;
};
template <int control_bit1, int control_bit2, int target_bit>
struct gate_traits<quantum_gates::toffoli<control_bit1, control_bit2, target_bit> >
{
static const long num_bits = tmax<tabs<control_bit1-target_bit>::value,
tabs<control_bit2-target_bit>::value>::value+1;
static const long dims = qc_helpers::exp_2_n<num_bits>::value;
};
// ----------------------------------------------------------------------------------------
namespace quantum_gates
{
inline const gate<1> hadamard(
)
{
gate<1> h;
h(0,0) = std::sqrt(1/2.0);
h(0,1) = std::sqrt(1/2.0);
h(1,0) = std::sqrt(1/2.0);
h(1,1) = -std::sqrt(1/2.0);
return h;
}
// ------------------------------------------------------------------------------------
inline const gate<1> x(
)
{
gate<1> x;
x(0,1) = 1;
x(1,0) = 1;
return x;
}
// ------------------------------------------------------------------------------------
inline const gate<1> y(
)
{
gate<1> x;
qc_scalar_type i(0,1);
x(0,1) = -i;
x(1,0) = i;
return x;
}
// ------------------------------------------------------------------------------------
inline const gate<1> z(
)
{
gate<1> z;
z(0,0) = 1;
z(1,1) = -1;
return z;
}
// ------------------------------------------------------------------------------------
inline const gate<1> noop(
)
{
gate<1> i;
i(0,0) = 1;
i(1,1) = 1;
return i;
}
// ------------------------------------------------------------------------------------
template <int control_bit, int target_bit>
class cnot : public gate_exp<cnot<control_bit, target_bit> >
{
public:
COMPILE_TIME_ASSERT(control_bit != target_bit);
cnot() : gate_exp<cnot>(*this)
{
const int min_bit = std::min(control_bit, target_bit);
control_mask = 1;
target_mask = 1;
// make the masks so that their only on bit corresponds to the given control_bit and target_bit bits
for (int i = 0; i < control_bit-min_bit; ++i)
control_mask <<= 1;
for (int i = 0; i < target_bit-min_bit; ++i)
target_mask <<= 1;
}
static const long num_bits = gate_traits<cnot>::num_bits;
static const long dims = gate_traits<cnot>::dims;
const qc_scalar_type operator() (long r, long c) const
{
unsigned long output;
// if the input control bit is set
if (control_mask&c)
{
output = c^target_mask;
}
else
{
output = c;
}
if ((unsigned long)r == output)
return 1;
else
return 0;
}
template <typename exp>
qc_scalar_type compute_state_element (
const matrix_exp<exp>& reg,
long row_idx
) const
{
// make sure requires clause is not broken
DLIB_ASSERT(reg.nr() == dims && reg.nc() == 1 &&
0 <= row_idx && row_idx < dims,
"\tqc_scalar_type cnot::compute_state_element(reg,row_idx)"
<< "\n\tinvalid arguments to this function"
<< "\n\treg.nr(): " << reg.nr()
<< "\n\treg.nc(): " << reg.nc()
<< "\n\tdims: " << dims
<< "\n\trow_idx: " << row_idx
<< "\n\tthis: " << this
);
unsigned long output = row_idx;
// if the input control bit is set
if (control_mask&output)
{
output = output^target_mask;
}
return reg(output);
}
private:
unsigned long control_mask;
unsigned long target_mask;
};
// ------------------------------------------------------------------------------------
template <int control_bit1, int control_bit2, int target_bit>
class toffoli : public gate_exp<toffoli<control_bit1, control_bit2, target_bit> >
{
public:
COMPILE_TIME_ASSERT(control_bit1 != target_bit && control_bit2 != target_bit && control_bit1 != control_bit2);
COMPILE_TIME_ASSERT((control_bit1 < target_bit && control_bit2 < target_bit) ||(control_bit1 > target_bit && control_bit2 > target_bit) );
toffoli() : gate_exp<toffoli>(*this)
{
const int min_bit = std::min(std::min(control_bit1, control_bit2), target_bit);
control1_mask = 1;
control2_mask = 1;
target_mask = 1;
// make the masks so that their only on bit corresponds to the given control_bit1 and target_bit bits
for (int i = 0; i < control_bit1-min_bit; ++i)
control1_mask <<= 1;
for (int i = 0; i < control_bit2-min_bit; ++i)
control2_mask <<= 1;
for (int i = 0; i < target_bit-min_bit; ++i)
target_mask <<= 1;
}
static const long num_bits = gate_traits<toffoli>::num_bits;
static const long dims = gate_traits<toffoli>::dims;
const qc_scalar_type operator() (long r, long c) const
{
unsigned long output;
// if the input control bits are set
if ((control1_mask&c) && (control2_mask&c))
{
output = c^target_mask;
}
else
{
output = c;
}
if ((unsigned long)r == output)
return 1;
else
return 0;
}
template <typename exp>
qc_scalar_type compute_state_element (
const matrix_exp<exp>& reg,
long row_idx
) const
{
// make sure requires clause is not broken
DLIB_ASSERT(reg.nr() == dims && reg.nc() == 1 &&
0 <= row_idx && row_idx < dims,
"\tqc_scalar_type toffoli::compute_state_element(reg,row_idx)"
<< "\n\tinvalid arguments to this function"
<< "\n\treg.nr(): " << reg.nr()
<< "\n\treg.nc(): " << reg.nc()
<< "\n\tdims: " << dims
<< "\n\trow_idx: " << row_idx
<< "\n\tthis: " << this
);
unsigned long output;
// if the input control bits are set
if ((control1_mask&row_idx) && (control2_mask&row_idx))
{
output = row_idx^target_mask;
}
else
{
output = row_idx;
}
return reg(output);
}
private:
unsigned long control1_mask;
unsigned long control2_mask;
unsigned long target_mask;
};
// ------------------------------------------------------------------------------------
}
// ----------------------------------------------------------------------------------------
}
#endif // DLIB_QUANTUM_COMPUTINg_1_