def test_vec():
"""Test standard vec operation on a matrix."""
expected_res = np.array([[1], [3], [2], [4]])
test_input_mat = np.array([[1, 2], [3, 4]])
res = vec(test_input_mat)
bool_mat = np.isclose(res, expected_res)
np.testing.assert_equal(np.all(bool_mat), True)
def apply_channel(
mat: np.ndarray, phi_op: Union[np.ndarray, List[List[np.ndarray]]]
) -> np.ndarray:
r"""
Apply a quantum channel to an operator [WatAChan18]_.
Specifically, an application of the channel is defined as
.. math::
\Phi(X) = \text{Tr}_{\mathcal{X}} \left(J(\Phi)
\left(\mathbb{I}_{\mathcal{Y}} \otimes X^{T}\right)\right),
where
.. math::
J(\Phi): \text{T}(\mathcal{X}, \mathcal{Y}) \rightarrow
\text{L}(\mathcal{Y} \otimes \mathcal{X})
is the Choi representation of :math:`\Phi`.
We assume the quantum channel given as :code:`phi_op` is provided as either
the Choi matrix of the channel or a set of Kraus operators that define the
quantum channel.
This function is adapted from the QETLAB package.
Examples
==========
The swap operator is the Choi matrix of the transpose map. The following is
a (non-ideal, but illustrative) way of computing the transpose of a matrix.
Consider the following matrix
.. math::
X = \begin{pmatrix}
1 & 4 & 7 \\
2 & 5 & 8 \\
3 & 6 & 9
\end{pmatrix}
Applying the swap operator given as
.. math::
\Phi =
\begin{pmatrix}
1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 \\
0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 \\
0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1
\end{pmatrix}
to the matrix :math:`X`, we have the resulting matrix of
.. math::
\Phi(X) = \begin{pmatrix}
1 & 2 & 3 \\
4 & 5 & 6 \\
7 & 8 & 9
\end{pmatrix}
Using :code:`toqito`, we can obtain the above matrices as follows.
>>> from toqito.channel_ops import apply_channel
>>> from toqito.perms import swap_operator
>>> import numpy as np
>>> test_input_mat = np.array([[1, 4, 7], [2, 5, 8], [3, 6, 9]])
>>> apply_channel(test_input_mat, swap_operator(3))
[[1., 2., 3.],
[4., 5., 6.],
[7., 8., 9.]]
References
==========
.. [WatAChan18] Watrous, John.
The theory of quantum information.
Section: Representations and characterizations of channels.
Cambridge University Press, 2018.
:param mat: A matrix.
:param phi_op: A superoperator. :code:`phi_op` should be provided either as
a Choi matrix, or as a list of numpy arrays with either 1 or
2 columns whose entries are its Kraus operators.
:return: The result of applying the superoperator :code:`phi_op` to the
operator :code:`mat`.
"""
# Both of the following methods of applying the superoperator are much
# faster than naively looping through the Kraus operators or constructing
# eigenvectors of a Choi matrix.
# The superoperator was given as a list of Kraus operators:
if isinstance(phi_op, list):
s_phi_op = [len(phi_op), len(phi_op[0])]
# Map is completely positive.
if s_phi_op[1] == 1 or (s_phi_op[0] == 1 and s_phi_op[1] > 2):
for i in range(s_phi_op[0]):
phi_op[i][1] = phi_op[i][0].conj().T
else:
for i in range(s_phi_op[0]):
phi_op[i][1] = phi_op[i][1].conj().T
phi_0_list = []
phi_1_list = []
for i in range(s_phi_op[0]):
phi_0_list.append(phi_op[i][0])
phi_1_list.append(phi_op[i][1])
k_1 = np.concatenate(phi_0_list, axis=1)
k_2 = np.concatenate(phi_1_list, axis=0)
a_mat = np.kron(np.identity(len(phi_op)), mat)
return k_1 @ a_mat @ k_2
# The superoperator was given as a Choi matrix:
if isinstance(phi_op, np.ndarray):
mat_size = np.array(list(mat.shape))
phi_size = np.array(list(phi_op.shape)) / mat_size
a_mat = np.kron(vec(mat).T[0], np.identity(int(phi_size[0])))
b_mat = np.reshape(
swap(
phi_op.T,
[1, 2],
[[mat_size[1], phi_size[1]], [mat_size[0], phi_size[0]]],
True,
).T,
(int(phi_size[0] * np.prod(mat_size)), int(phi_size[1])),
)
return a_mat @ b_mat
raise ValueError(
"Invalid: The variable `phi_op` must either be a list of "
"Kraus operators or as a Choi matrix."
)
def apply_map(mat: np.ndarray,
phi_op: Union[np.ndarray, List[List[np.ndarray]]]) -> np.ndarray:
r"""
Apply a superoperator to an operator.
Examples
==========
The swap operator is the Choi matrix of the transpose map. The following is
a (non-ideal, but illustrative) way of computing the transpose of a matrix.
Consider the following matrix
.. math::
X = \begin{pmatrix}
1 & 4 & 7 \\
2 & 5 & 8 \\
3 & 6 & 9
\end{pmatrix}
Applying the swap operator given as
.. math::
\Phi =
\begin{pmatrix}
1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 \\
0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 \\
0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1
\end{pmatrix}
to the matrix :math:`X`, we have the resulting matrix of
.. math::
\Phi(X) = \begin{pmatrix}
1 & 2 & 3 \\
4 & 5 & 6 \\
7 & 8 & 9
\end{pmatrix}
Using :code:`toqito`, we can obtain the above matrices as follows.
>>> from toqito.channel_ops import apply_map
>>> from toqito.perms import swap_operator
>>> import numpy as np
>>> test_input_mat = np.array([[1, 4, 7], [2, 5, 8], [3, 6, 9]])
>>> apply_map(test_input_mat, swap_operator(3))
[[1., 2., 3.],
[4., 5., 6.],
[7., 8., 9.]]
:param mat: A matrix.
:param phi_op: A superoperator. :code:`phi_op` should be provided either as
a Choi matrix, or as a list of numpy arrays with either 1 or
2 columns whose entries are its Kraus operators.
:return: The result of applying the superoperator :code:`phi_op` to the
operator :code:`mat`.
"""
# Both of the following methods of applying the superoperator are much
# faster than naively looping through the Kraus operators or constructing
# eigenvectors of a Choi matrix.
# The superoperator was given as a list of Kraus operators:
if isinstance(phi_op, list):
s_phi_op = [len(phi_op), len(phi_op[0])]
# Map is completely positive.
if s_phi_op[1] == 1 or (s_phi_op[0] == 1 and s_phi_op[1] > 2):
for i in range(s_phi_op[0]):
phi_op[i][1] = phi_op[i][0].conj().T
else:
for i in range(s_phi_op[0]):
phi_op[i][1] = phi_op[i][1].conj().T
phi_0_list = []
phi_1_list = []
for i in range(s_phi_op[0]):
phi_0_list.append(phi_op[i][0])
phi_1_list.append(phi_op[i][1])
k_1 = np.concatenate(phi_0_list, axis=1)
k_2 = np.concatenate(phi_1_list, axis=0)
a_mat = np.kron(np.identity(len(phi_op)), mat)
return np.matmul(np.matmul(k_1, a_mat), k_2)
# The superoperator was given as a Choi matrix:
if isinstance(phi_op, np.ndarray):
mat_size = np.array(list(mat.shape))
phi_size = np.array(list(phi_op.shape)) / mat_size
a_mat = np.kron(vec(mat).T[0], np.identity(int(phi_size[0])))
b_mat = np.reshape(
swap(
phi_op.T,
[1, 2],
[[mat_size[1], phi_size[1]], [mat_size[0], phi_size[0]]],
True,
).T,
(int(phi_size[0] * np.prod(mat_size)), int(phi_size[1])),
)
return np.matmul(a_mat, b_mat)
raise ValueError("Invalid: The variable `phi_op` must either be a list of "
"Kraus operators or as a Choi matrix.")
def gen_bell(k_1: int, k_2: int, dim: int) -> np.ndarray:
r"""
Produce a generalized Bell state [DL09]_.
Produces a generalized Bell state. Note that the standard Bell states can be recovered as:
- `bell(0)` : `gen_bell(0, 0, 2)`
- `bell(1)` : `gen_bell(0, 1, 2)`
- `bell(2)` : `gen_bell(1, 0, 2)`
- `bell(3)` : `gen_bell(1, 1, 2)`
Examples
==========
For :math:`d = 2` and :math:`k_1 = k_2 = 0`, this generates the following matrix
.. math::
G = \frac{1}{2} \begin{pmatrix}
1 & 0 & 0 & 1 \\
0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 \\
1 & 0 & 0 & 1
\end{pmatrix}
which is equivalent to :math:`|\phi_0 \rangle \langle \phi_0 |` where
.. math::
|\phi_0\rangle = \frac{1}{\sqrt{2}} \left( |00 \rangle + |11 \rangle \right)
is one of the four standard Bell states. This can be computed via :code:`toqito` as follows.
>>> from toqito.states import gen_bell
>>> dim = 2
>>> k_1 = 0
>>> k_2 = 0
>>> gen_bell(k_1, k_2, dim)
[[0.5+0.j, 0. +0.j, 0. +0.j, 0.5+0.j],
[0. +0.j, 0. +0.j, 0. +0.j, 0. +0.j],
[0. +0.j, 0. +0.j, 0. +0.j, 0. +0.j],
[0.5+0.j, 0. +0.j, 0. +0.j, 0.5+0.j]]
It is possible for us to consider higher dimensional Bell states. For instance, we can consider
the :math:`3`-dimensional Bell state for :math:`k_1 = k_2 = 0` as follows.
>>> from toqito.states import gen_bell
>>> dim = 3
>>> k_1 = 0
>>> k_2 = 0
>>> gen_bell(k_1, k_2, dim)
[[0.33333333+0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0.33333333+0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0.33333333+0.j],
[0. +0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0. +0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0. +0.j],
[0. +0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0. +0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0. +0.j],
[0. +0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0. +0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0. +0.j],
[0.33333333+0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0.33333333+0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0.33333333+0.j],
[0. +0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0. +0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0. +0.j],
[0. +0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0. +0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0. +0.j],
[0. +0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0. +0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0. +0.j],
[0.33333333+0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0.33333333+0.j, 0. +0.j, 0. +0.j, 0. +0.j,
0.33333333+0.j]]
References
==========
.. [DL09] Sych, Denis, and Gerd Leuchs.
"A complete basis of generalized Bell states."
New Journal of Physics 11.1 (2009): 013006.
:param k_1: An integer 0 <= k_1 <= n.
:param k_2: An integer 0 <= k_2 <= n.
:param dim: The dimension of the generalized Bell state.
"""
gen_pauli_w = gen_pauli(k_1, k_2, dim)
return 1 / dim * vec(gen_pauli_w) * vec(gen_pauli_w).conj().T
def permute_systems(
input_mat: np.ndarray,
perm: Union[np.ndarray, List[int]],
dim: Union[np.ndarray, List[int]] = None,
row_only: bool = False,
inv_perm: bool = False,
) -> np.ndarray:
r"""
Permute subsystems within a state or operator.
Permutes the order of the subsystems of the vector or matrix :code:`input_mat` according to the
permutation vector :code:`perm`, where the dimensions of the subsystems are given by the vector
:code:`dim`. If :code:`input_mat` is non-square and not a vector, different row and column
dimensions can be specified by putting the row dimensions in the first row of :code:`dim` and
the columns dimensions in the second row of :code:`dim`.
If :code:`row_only = True`, then only the rows of :code:`input_mat` are permuted, but not the
columns -- this is equivalent to multiplying :code:`input_mat` on the left by the corresponding
permutation operator, but not on the right.
If :code:`row_only = False`, then :code:`dim` only needs to contain the row dimensions of the
subsystems, even if :code:`input_mat` is not square. If :code:`inv_perm = True`, then the
inverse permutation of :code:`perm` is applied instead of :code:`perm` itself.
Examples
==========
For spaces :math:`\mathcal{A}` and :math:`\mathcal{B}` where
:math:`\text{dim}(\mathcal{A}) = \text{dim}(\mathcal{B}) = 2` we may consider an operator
:math:`X \in \mathcal{A} \otimes \mathcal{B}`. Applying the `permute_systems` function with
vector :math:`[2,1]` on :math:`X`, we may reorient the spaces such that
:math:`X \in \mathcal{B} \otimes \mathcal{A}`.
For example, if we define :math:`X \in \mathcal{A} \otimes \mathcal{B}` as
.. math::
X = \begin{pmatrix}
1 & 2 & 3 & 4 \\
5 & 6 & 7 & 8 \\
9 & 10 & 11 & 12 \\
13 & 14 & 15 & 16
\end{pmatrix},
then applying the `permute_systems` function on :math:`X` to obtain
:math:`X \in \mathcal{B} \otimes \mathcal{A}` yield the following matrix
.. math::
X_{[2,1]} = \begin{pmatrix}
1 & 3 & 2 & 4 \\
9 & 11 & 10 & 12 \\
5 & 7 & 6 & 8 \\
13 & 15 & 14 & 16
\end{pmatrix}.
>>> from toqito.perms import permute_systems
>>> import numpy as np
>>> test_input_mat = np.array(
>>> [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], [13, 14, 15, 16]]
>>> )
>>> permute_systems(test_input_mat, [2, 1])
[[ 1 3 2 4]
[ 9 11 10 12]
[ 5 7 6 8]
[13 15 14 16]]
For spaces :math:`\mathcal{A}, \mathcal{B}`, and :math:`\mathcal{C}` where
:math:`\text{dim}(\mathcal{A}) = \text{dim}(\mathcal{B}) = \text{dim}(\mathcal{C}) = 2` we may
consider an operator :math:`X \in \mathcal{A} \otimes \mathcal{B} \otimes \mathcal{C}`. Applying
the :code:`permute_systems` function with vector :math:`[2,3,1]` on :math:`X`, we may reorient
the spaces such that :math:`X \in \mathcal{B} \otimes \mathcal{C} \otimes \mathcal{A}`.
For example, if we define :math:`X \in \mathcal{A} \otimes \mathcal{B} \otimes \mathcal{C}` as
.. math::
X =
\begin{pmatrix}
1 & 2 & 3 & 4, 5 & 6 & 7 & 8 \\
9 & 10 & 11 & 12 & 13 & 14 & 15 & 16 \\
17 & 18 & 19 & 20 & 21 & 22 & 23 & 24 \\
25 & 26 & 27 & 28 & 29 & 30 & 31 & 32 \\
33 & 34 & 35 & 36 & 37 & 38 & 39 & 40 \\
41 & 42 & 43 & 44 & 45 & 46 & 47 & 48 \\
49 & 50 & 51 & 52 & 53 & 54 & 55 & 56 \\
57 & 58 & 59 & 60 & 61 & 62 & 63 & 64
\end{pmatrix},
then applying the `permute_systems` function on :math:`X` to obtain
:math:`X \in \mathcal{B} \otimes \mathcal{C} \otimes \mathcal{C}` yield the following matrix
.. math::
X_{[2, 3, 1]} =
\begin{pmatrix}
1 & 5 & 2 & 6 & 3 & 7 & 4, 8 \\
33 & 37 & 34 & 38 & 35 & 39 & 36 & 40 \\
9 & 13 & 10 & 14 & 11 & 15 & 12 & 16 \\
41 & 45 & 42 & 46 & 43 & 47 & 44 & 48 \\
17 & 21 & 18 & 22 & 19 & 23 & 20 & 24 \\
49 & 53 & 50 & 54 & 51 & 55 & 52 & 56 \\
25 & 29 & 26 & 30 & 27 & 31 & 28 & 32 \\
57 & 61 & 58 & 62 & 59 & 63 & 60 & 64
\end{pmatrix}.
>>> from toqito.perms import permute_systems
>>> import numpy as np
>>> test_input_mat = np.array(
>>> [
>>> [1, 2, 3, 4, 5, 6, 7, 8],
>>> [9, 10, 11, 12, 13, 14, 15, 16],
>>> [17, 18, 19, 20, 21, 22, 23, 24],
>>> [25, 26, 27, 28, 29, 30, 31, 32],
>>> [33, 34, 35, 36, 37, 38, 39, 40],
>>> [41, 42, 43, 44, 45, 46, 47, 48],
>>> [49, 50, 51, 52, 53, 54, 55, 56],
>>> [57, 58, 59, 60, 61, 62, 63, 64],
>>> ]
>>> )
>>> permute_systems(test_input_mat, [2, 3, 1])
[[ 1 5 2 6 3 7 4 8]
[33 37 34 38 35 39 36 40]
[ 9 13 10 14 11 15 12 16]
[41 45 42 46 43 47 44 48]
[17 21 18 22 19 23 20 24]
[49 53 50 54 51 55 52 56]
[25 29 26 30 27 31 28 32]
[57 61 58 62 59 63 60 64]]
:param input_mat: The vector or matrix.
:param perm: A permutation vector.
:param dim: The default has all subsystems of equal dimension.
:param row_only: Default: :code:`False`
:param inv_perm: Default: :code:`True`
:return: The matrix or vector that has been permuted.
"""
if len(input_mat.shape) == 1:
input_mat_dims = (1, input_mat.shape[0])
else:
input_mat_dims = input_mat.shape
is_vec = np.min(input_mat_dims) == 1
num_sys = len(perm)
if dim is None:
x_tmp = input_mat_dims[0] ** (1 / num_sys) * np.ones(num_sys)
y_tmp = input_mat_dims[1] ** (1 / num_sys) * np.ones(num_sys)
dim = np.array([x_tmp, y_tmp])
if isinstance(dim, list):
dim = np.array(dim)
if is_vec:
# 1 if column vector
if len(input_mat.shape) > 1:
vec_orien = 0
# 2 if row vector
elif len(input_mat.shape) == 1:
vec_orien = 1
else:
raise ValueError(
"InvalidMat: Length of tuple of dimensions "
"specifying the input matrix can only be of "
"length 1 or length 2."
)
if len(dim.shape) == 1:
# Force dim to be a row vector.
dim_tmp = dim[:].T
if is_vec:
dim = np.ones((2, len(dim)))
dim[vec_orien, :] = dim_tmp
else:
dim = np.array([[dim_tmp], [dim_tmp]])
prod_dim_r = int(np.prod(dim[0, :]))
prod_dim_c = int(np.prod(dim[1, :]))
if len(perm) != num_sys:
raise ValueError("InvalidPerm: `len(perm)` must be equal to " "`len(dim)`.")
if sorted(perm) != list(range(1, num_sys + 1)):
raise ValueError("InvalidPerm: `perm` must be a permutation vector.")
if input_mat_dims[0] != prod_dim_r or (not row_only and input_mat_dims[1] != prod_dim_c):
raise ValueError(
"InvalidDim: The dimensions specified in DIM do not " "agree with the size of X."
)
if is_vec:
if inv_perm:
permuted_mat_1 = input_mat.reshape(dim[vec_orien, ::-1].astype(int), order="F")
permuted_mat = vec(np.transpose(permuted_mat_1, num_sys - np.array(perm[::-1]))).T
# We need to flatten out the array.
permuted_mat = functools.reduce(operator.iconcat, permuted_mat, [])
else:
permuted_mat_1 = input_mat.reshape(dim[vec_orien, ::-1].astype(int), order="F")
permuted_mat = vec(np.transpose(permuted_mat_1, num_sys - np.array(perm[::-1]))).T
# We need to flatten out the array.
permuted_mat = functools.reduce(operator.iconcat, permuted_mat, [])
return np.array(permuted_mat)
vec_arg = np.array(list(range(0, input_mat_dims[0])))
# If the dimensions are specified, ensure they are given to the
# recursive calls as flattened lists.
if len(dim[0][:]) == 1:
dim = functools.reduce(operator.iconcat, dim, [])
row_perm = permute_systems(vec_arg, perm, dim[0][:], False, inv_perm)
# This condition is only necessary if the `input_mat` variable is sparse.
if isinstance(input_mat, (sparse.csr_matrix, sparse.dia_matrix)):
input_mat = input_mat.toarray()
permuted_mat = input_mat[row_perm, :]
permuted_mat = np.array(permuted_mat)
else:
permuted_mat = input_mat[row_perm, :]
if not row_only:
vec_arg = np.array(list(range(0, input_mat_dims[1])))
col_perm = permute_systems(vec_arg, perm, dim[1][:], False, inv_perm)
permuted_mat = permuted_mat[:, col_perm]
return permuted_mat
