"""Quantum channels and noise models."""
from __future__ import annotations
import copy
import typing
from typing import TYPE_CHECKING, SupportsIndex, TypeVar
import numpy as np
import numpy.typing as npt
from graphix import linalg_validations as lv
from graphix.ops import Ops
if TYPE_CHECKING:
from collections.abc import Iterable, Iterator
_T = TypeVar("_T", bound=np.generic)
def _ilog2(n: int) -> int:
"""Return the integer base-2 logarithm of ``n``.
Parameters
----------
n : int
Positive integer.
Returns
-------
int
``floor(log2(n))`` for ``n > 0``.
"""
if n <= 0:
raise ValueError("n must be positive.")
return (n - 1).bit_length()
class KrausData:
"""Kraus operator data.
Attributes
----------
coef : complex
Scalar prefactor of the operator.
operator : npt.NDArray[np.complex128]
Operator.
"""
__coef: complex
__operator: npt.NDArray[np.complex128]
def __init__(self, coef: complex, operator: npt.NDArray[_T]) -> None:
if not lv.is_square(operator):
raise ValueError("Operator must be a square matrix.")
if not lv.is_qubitop(operator):
raise ValueError("Operator must be a qubit operator.")
self.__coef = coef
self.__operator = operator.astype(np.complex128, copy=True)
self.__operator.flags.writeable = False
@property
def coef(self) -> complex:
"""Return the scalar prefactor."""
return self.__coef
@property
def operator(self) -> npt.NDArray[np.complex128]:
"""Return the operator."""
return self.__operator.view()
@property
def nqubit(self) -> int:
"""Validate the data."""
size, _ = self.__operator.shape
return _ilog2(size)
[docs]
class KrausChannel:
r"""Quantum channel class in the Kraus representation.
Defined by Kraus operators :math:`K_i` with scalar prefactors :code:`coef`) :math:`c_i`,
where the channel act on density matrix as :math:`\rho' = \sum_i K_i^\dagger \rho K_i`.
The data should satisfy :math:`\sum K_i^\dagger K_i = I`.
"""
__nqubit: int
__data: list[KrausData]
@staticmethod
def _nqubit(kraus_data: Iterable[KrausData]) -> int:
"""Return the number of qubits acted on by ``kraus_data``."""
# MEMO: ``kraus_data`` is not empty.
it = iter(kraus_data)
nqubit = next(it).nqubit
if any(data.nqubit != nqubit for data in it):
raise ValueError("All operators must have the same shape.")
return nqubit
[docs]
def __init__(self, kraus_data: Iterable[KrausData]) -> None:
"""Initialize `KrausChannel` given a Kraus operator.
Parameters
----------
kraus_data : Iterable[KrausData]
Iterable of Kraus operator data.
Raises
------
ValueError
If kraus_data is empty.
"""
kraus_data = [copy.deepcopy(kdata) for kdata in kraus_data]
if not kraus_data:
raise ValueError("Cannot instantiate the channel with empty data.")
self.__nqubit = self._nqubit(kraus_data)
self.__data = kraus_data
if len(self.__data) > 4**self.__nqubit:
raise ValueError("len(kraus_data) cannot exceed 4**nqubit.")
# Check that the channel is properly normalized, i.e., \sum_K_i^\dagger K_i = Identity.
data = next(iter(self.__data))
work = np.zeros_like(data.operator, dtype=np.complex128)
for data in self.__data:
m = data.coef * data.operator
work += m.conj().T @ m
if not np.allclose(work, np.eye(2**self.__nqubit)):
raise ValueError("The specified channel is not normalized.")
@typing.overload
def __getitem__(self, index: SupportsIndex, /) -> KrausData: ...
@typing.overload
def __getitem__(self, index: slice, /) -> list[KrausData]: ...
def __getitem__(self, index: SupportsIndex | slice, /) -> KrausData | list[KrausData]:
"""Return the Kraus operator at the given index."""
return self.__data[index]
def __len__(self) -> int:
"""Return the number of Kraus operators."""
return len(self.__data)
def __iter__(self) -> Iterator[KrausData]:
"""Iterate over Kraus operators."""
return iter(self.__data)
@property
def nqubit(self) -> int:
"""Return the number of qubits."""
return self.__nqubit
[docs]
def dephasing_channel(prob: float) -> KrausChannel:
r"""Single-qubit dephasing channel, :math:`(1-p) \rho + p Z \rho Z`.
Parameters
----------
prob : float
The probability associated to the channel
Returns
-------
:class:`graphix.channels.KrausChannel` object
containing the corresponding Kraus operators
"""
return KrausChannel(
[
KrausData(np.sqrt(1 - prob), Ops.I),
KrausData(np.sqrt(prob), Ops.Z),
]
)
[docs]
def depolarising_channel(prob: float) -> KrausChannel:
r"""Single-qubit depolarizing channel.
.. math::
(1-p) \rho + \frac{p}{3} (X \rho X + Y \rho Y + Z \rho Z) = (1 - 4 \frac{p}{3}) \rho + 4 \frac{p}{3} id
Parameters
----------
prob : float
The probability associated to the channel
"""
return KrausChannel(
[
KrausData(np.sqrt(1 - prob), Ops.I),
KrausData(np.sqrt(prob / 3.0), Ops.X),
KrausData(np.sqrt(prob / 3.0), Ops.Y),
KrausData(np.sqrt(prob / 3.0), Ops.Z),
]
)
[docs]
def pauli_channel(px: float, py: float, pz: float) -> KrausChannel:
r"""Single-qubit Pauli channel.
.. math::
(1-p_X-p_Y-p_Z) \rho + p_X X \rho X + p_Y Y \rho Y + p_Z Z \rho Z)
"""
if px + py + pz > 1:
raise ValueError("The sum of probabilities must not exceed 1.")
p_i = 1 - px - py - pz
return KrausChannel(
[
KrausData(np.sqrt(1 - p_i), Ops.I),
KrausData(np.sqrt(px / 3.0), Ops.X),
KrausData(np.sqrt(py / 3.0), Ops.Y),
KrausData(np.sqrt(pz / 3.0), Ops.Z),
]
)
[docs]
def two_qubit_depolarising_channel(prob: float) -> KrausChannel:
r"""Two-qubit depolarising channel.
.. math::
\mathcal{E} (\rho) = (1-p) \rho + \frac{p}{15} \sum_{P_i \in \{id, X, Y ,Z\}^{\otimes 2}/(id \otimes id)}P_i \rho P_i
Parameters
----------
prob : float
The probability associated to the channel
Returns
-------
:class:`graphix.channels.KrausChannel` object
containing the corresponding Kraus operators
"""
return KrausChannel(
[
KrausData(np.sqrt(1 - prob), np.kron(Ops.I, Ops.I)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.X, Ops.X)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.Z, Ops.Z)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.X, Ops.I)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.Y, Ops.Y)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.Y, Ops.I)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.Z, Ops.I)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.I, Ops.X)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.I, Ops.Y)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.I, Ops.Z)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.X, Ops.Y)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.X, Ops.Z)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.Y, Ops.X)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.Y, Ops.Z)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.Z, Ops.Y)),
KrausData(np.sqrt(prob / 15.0), np.kron(Ops.Z, Ops.X)),
]
)
[docs]
def two_qubit_depolarising_tensor_channel(prob: float) -> KrausChannel:
r"""Two-qubit tensor channel of single-qubit depolarising channels with same probability.
Kraus operators:
.. math::
\Big\{ \sqrt{(1-p)} id, \sqrt{(p/3)} X, \sqrt{(p/3)} Y , \sqrt{(p/3)} Z \Big\} \otimes \Big\{ \sqrt{(1-p)} id, \sqrt{(p/3)} X, \sqrt{(p/3)} Y , \sqrt{(p/3)} Z \Big\}
Parameters
----------
prob : float
The probability associated to the channel
Returns
-------
:class:`graphix.channels.KrausChannel` object
containing the corresponding Kraus operators
"""
return KrausChannel(
[
KrausData(1 - prob, np.kron(Ops.I, Ops.I)),
KrausData(prob / 3.0, np.kron(Ops.X, Ops.X)),
KrausData(prob / 3.0, np.kron(Ops.Y, Ops.Y)),
KrausData(prob / 3.0, np.kron(Ops.Z, Ops.Z)),
KrausData(np.sqrt(1 - prob) * np.sqrt(prob / 3.0), np.kron(Ops.X, Ops.I)),
KrausData(np.sqrt(1 - prob) * np.sqrt(prob / 3.0), np.kron(Ops.Y, Ops.I)),
KrausData(np.sqrt(1 - prob) * np.sqrt(prob / 3.0), np.kron(Ops.Z, Ops.I)),
KrausData(np.sqrt(1 - prob) * np.sqrt(prob / 3.0), np.kron(Ops.I, Ops.X)),
KrausData(np.sqrt(1 - prob) * np.sqrt(prob / 3.0), np.kron(Ops.I, Ops.Y)),
KrausData(np.sqrt(1 - prob) * np.sqrt(prob / 3.0), np.kron(Ops.I, Ops.Z)),
KrausData(prob / 3.0, np.kron(Ops.X, Ops.Y)),
KrausData(prob / 3.0, np.kron(Ops.X, Ops.Z)),
KrausData(prob / 3.0, np.kron(Ops.Y, Ops.X)),
KrausData(prob / 3.0, np.kron(Ops.Y, Ops.Z)),
KrausData(prob / 3.0, np.kron(Ops.Z, Ops.X)),
KrausData(prob / 3.0, np.kron(Ops.Z, Ops.Y)),
]
)