# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2022.
#
# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.
"""
N-Qubit Sparse Pauli Operator class.
"""
from __future__ import annotations
from collections import defaultdict
from collections.abc import Mapping, Sequence
from numbers import Number
from typing import Dict, Optional
from copy import deepcopy
import numpy as np
import rustworkx as rx
from qiskit._accelerate.sparse_pauli_op import unordered_unique
from qiskit.circuit.parameter import Parameter
from qiskit.circuit.parameterexpression import ParameterExpression
from qiskit.circuit.parametertable import ParameterView
from qiskit.exceptions import QiskitError
from qiskit.quantum_info.operators.custom_iterator import CustomIterator
from qiskit.quantum_info.operators.linear_op import LinearOp
from qiskit.quantum_info.operators.mixins import generate_apidocs
from qiskit.quantum_info.operators.operator import Operator
from qiskit.quantum_info.operators.symplectic.pauli import BasePauli
from qiskit.quantum_info.operators.symplectic.pauli_list import PauliList
from qiskit.quantum_info.operators.symplectic.pauli_utils import pauli_basis
[docs]class SparsePauliOp(LinearOp):
"""Sparse N-qubit operator in a Pauli basis representation.
This is a sparse representation of an N-qubit matrix
:class:`~qiskit.quantum_info.Operator` in terms of N-qubit
:class:`~qiskit.quantum_info.PauliList` and complex coefficients.
It can be used for performing operator arithmetic for hundred of qubits
if the number of non-zero Pauli basis terms is sufficiently small.
The Pauli basis components are stored as a
:class:`~qiskit.quantum_info.PauliList` object and can be accessed
using the :attr:`~SparsePauliOp.paulis` attribute. The coefficients
are stored as a complex Numpy array vector and can be accessed using
the :attr:`~SparsePauliOp.coeffs` attribute.
.. rubric:: Data type of coefficients
The default ``dtype`` of the internal ``coeffs`` Numpy array is ``complex128``. Users can
configure this by passing ``np.ndarray`` with a different dtype. For example, a parameterized
:class:`SparsePauliOp` can be made as follows:
.. code-block:: python
>>> import numpy as np
>>> from qiskit.circuit import ParameterVector
>>> from qiskit.quantum_info import SparsePauliOp
>>> SparsePauliOp(["II", "XZ"], np.array(ParameterVector("a", 2)))
SparsePauliOp(['II', 'XZ'],
coeffs=[ParameterExpression(1.0*a[0]), ParameterExpression(1.0*a[1])])
.. note::
Parameterized :class:`SparsePauliOp` does not support the following methods:
- ``to_matrix(sparse=True)`` since ``scipy.sparse`` cannot have objects as elements.
- ``to_operator()`` since :class:`~.quantum_info.Operator` does not support objects.
- ``sort``, ``argsort`` since :class:`.ParameterExpression` does not support comparison.
- ``equiv`` since :class:`.ParameterExpression` cannot be converted into complex.
- ``chop`` since :class:`.ParameterExpression` does not support absolute value.
"""
def __init__(self, data, coeffs=None, *, ignore_pauli_phase=False, copy=True):
"""Initialize an operator object.
Args:
data (PauliList or SparsePauliOp or Pauli or list or str): Pauli list of
terms. A list of Pauli strings or a Pauli string is also allowed.
coeffs (np.ndarray): complex coefficients for Pauli terms.
.. note::
If ``data`` is a :obj:`~SparsePauliOp` and ``coeffs`` is not ``None``, the value
of the ``SparsePauliOp.coeffs`` will be ignored, and only the passed keyword
argument ``coeffs`` will be used.
ignore_pauli_phase (bool): if true, any ``phase`` component of a given :obj:`~PauliList`
will be assumed to be zero. This is more efficient in cases where a
:obj:`~PauliList` has been constructed purely for this object, and it is already
known that the phases in the ZX-convention are zero. It only makes sense to pass
this option when giving :obj:`~PauliList` data. (Default: False)
copy (bool): copy the input data if True, otherwise assign it directly, if possible.
(Default: True)
Raises:
QiskitError: If the input data or coeffs are invalid.
"""
if ignore_pauli_phase and not isinstance(data, PauliList):
raise QiskitError("ignore_pauli_list=True is only valid with PauliList data")
if isinstance(data, SparsePauliOp):
if coeffs is None:
coeffs = data.coeffs
data = data._pauli_list
# `SparsePauliOp._pauli_list` is already compatible with the internal ZX-phase
# convention. See `BasePauli._from_array` for the internal ZX-phase convention.
ignore_pauli_phase = True
pauli_list = PauliList(data.copy() if copy and hasattr(data, "copy") else data)
if isinstance(coeffs, np.ndarray) and coeffs.dtype == object:
dtype = object
elif coeffs is not None:
if not isinstance(coeffs, (np.ndarray, Sequence)):
coeffs = [coeffs]
if any(isinstance(coeff, ParameterExpression) for coeff in coeffs):
dtype = object
else:
dtype = complex
if coeffs is None:
coeffs = np.ones(pauli_list.size, dtype=complex)
else:
coeffs = np.array(coeffs, copy=copy, dtype=dtype)
if ignore_pauli_phase:
# Fast path used in copy operations, where the phase of the PauliList is already known
# to be zero. This path only works if the input data is compatible with the internal
# ZX-phase convention.
self._pauli_list = pauli_list
self._coeffs = coeffs
else:
# move the phase of `pauli_list` to `self._coeffs`
phase = pauli_list._phase
count_y = pauli_list._count_y()
self._coeffs = np.asarray((-1j) ** (phase - count_y) * coeffs, dtype=coeffs.dtype)
pauli_list._phase = np.mod(count_y, 4)
self._pauli_list = pauli_list
if self._coeffs.shape != (self._pauli_list.size,):
raise QiskitError(
"coeff vector is incorrect shape for number"
" of Paulis {} != {}".format(self._coeffs.shape, self._pauli_list.size)
)
# Initialize LinearOp
super().__init__(num_qubits=self._pauli_list.num_qubits)
def __array__(self, dtype=None):
if dtype:
return np.asarray(self.to_matrix(), dtype=dtype)
return self.to_matrix()
def __repr__(self):
prefix = "SparsePauliOp("
pad = len(prefix) * " "
return "{}{},\n{}coeffs={})".format(
prefix,
self.paulis.to_labels(),
pad,
np.array2string(self.coeffs, separator=", "),
)
def __eq__(self, other):
"""Entrywise comparison of two SparsePauliOp operators"""
return (
super().__eq__(other)
and self.coeffs.dtype == other.coeffs.dtype
and self.coeffs.shape == other.coeffs.shape
and self.paulis == other.paulis
and (
np.allclose(self.coeffs, other.coeffs)
if self.coeffs.dtype != object
else (self.coeffs == other.coeffs).all()
)
)
[docs] def equiv(self, other, atol: Optional[float] = None):
"""Check if two SparsePauliOp operators are equivalent.
Args:
other (SparsePauliOp): an operator object.
atol: Absolute numerical tolerance for checking equivalence.
Returns:
bool: True if the operator is equivalent to ``self``.
"""
if not super().__eq__(other):
return False
if atol is None:
atol = self.atol
return np.allclose((self - other).simplify().coeffs, 0.0, atol=atol)
@property
def settings(self) -> Dict:
"""Return settings."""
return {"data": self._pauli_list, "coeffs": self._coeffs}
# ---------------------------------------------------------------------
# Data accessors
# ---------------------------------------------------------------------
@property
def size(self):
"""The number of Pauli of Pauli terms in the operator."""
return self._pauli_list.size
def __len__(self):
"""Return the size."""
return self.size
@property
def paulis(self):
"""Return the the PauliList."""
return self._pauli_list
@paulis.setter
def paulis(self, value):
if not isinstance(value, PauliList):
value = PauliList(value)
self._pauli_list = value
@property
def coeffs(self):
"""Return the Pauli coefficients."""
return self._coeffs
@coeffs.setter
def coeffs(self, value):
"""Set Pauli coefficients."""
self._coeffs[:] = value
def __getitem__(self, key):
"""Return a view of the SparsePauliOp."""
# Returns a view of specified rows of the PauliList
# This supports all slicing operations the underlying array supports.
if isinstance(key, (int, np.integer)):
key = [key]
return SparsePauliOp(self.paulis[key], self.coeffs[key])
def __setitem__(self, key, value):
"""Update SparsePauliOp."""
# Modify specified rows of the PauliList
if not isinstance(value, SparsePauliOp):
value = SparsePauliOp(value)
self.paulis[key] = value.paulis
self.coeffs[key] = value.coeffs
# ---------------------------------------------------------------------
# LinearOp Methods
# ---------------------------------------------------------------------
[docs] def conjugate(self):
# Conjugation conjugates phases and also Y.conj() = -Y
# Hence we need to multiply conjugated coeffs by -1
# for rows with an odd number of Y terms.
# Find rows with Ys
ret = self.transpose()
ret._coeffs = ret._coeffs.conj()
return ret
[docs] def transpose(self):
# The only effect transposition has is Y.T = -Y
# Hence we need to multiply coeffs by -1 for rows with an
# odd number of Y terms.
ret = self.copy()
minus = (-1) ** ret.paulis._count_y()
ret._coeffs *= minus
return ret
[docs] def adjoint(self):
# Pauli's are self adjoint, so we only need to
# conjugate the phases
ret = self.copy()
ret._coeffs = ret._coeffs.conj()
return ret
[docs] def compose(self, other, qargs=None, front=False):
if qargs is None:
qargs = getattr(other, "qargs", None)
if not isinstance(other, SparsePauliOp):
other = SparsePauliOp(other)
# Validate composition dimensions and qargs match
self._op_shape.compose(other._op_shape, qargs, front)
if qargs is not None:
x1, z1 = self.paulis.x[:, qargs], self.paulis.z[:, qargs]
else:
x1, z1 = self.paulis.x, self.paulis.z
x2, z2 = other.paulis.x, other.paulis.z
num_qubits = other.num_qubits
# This method is the outer version of `BasePauli.compose`.
# `x1` and `z1` have shape `(self.size, num_qubits)`.
# `x2` and `z2` have shape `(other.size, num_qubits)`.
# `x1[:, no.newaxis]` results in shape `(self.size, 1, num_qubits)`.
# `ar = ufunc(x1[:, np.newaxis], x2)` will be in shape `(self.size, other.size, num_qubits)`.
# So, `ar.reshape((-1, num_qubits))` will be in shape `(self.size * other.size, num_qubits)`.
# Ref: https://numpy.org/doc/stable/user/theory.broadcasting.html
phase = np.add.outer(self.paulis._phase, other.paulis._phase).reshape(-1)
if front:
q = np.logical_and(x1[:, np.newaxis], z2).reshape((-1, num_qubits))
else:
q = np.logical_and(z1[:, np.newaxis], x2).reshape((-1, num_qubits))
# `np.mod` will be applied to `phase` in `SparsePauliOp.__init__`
phase = phase + 2 * q.sum(axis=1, dtype=np.uint8)
x3 = np.logical_xor(x1[:, np.newaxis], x2).reshape((-1, num_qubits))
z3 = np.logical_xor(z1[:, np.newaxis], z2).reshape((-1, num_qubits))
if qargs is None:
pauli_list = PauliList(BasePauli(z3, x3, phase))
else:
x4 = np.repeat(self.paulis.x, other.size, axis=0)
z4 = np.repeat(self.paulis.z, other.size, axis=0)
x4[:, qargs] = x3
z4[:, qargs] = z3
pauli_list = PauliList(BasePauli(z4, x4, phase))
# note: the following is a faster code equivalent to
# `coeffs = np.kron(self.coeffs, other.coeffs)`
# since `self.coeffs` and `other.coeffs` are both 1d arrays.
coeffs = np.multiply.outer(self.coeffs, other.coeffs).ravel()
return SparsePauliOp(pauli_list, coeffs, copy=False)
[docs] def tensor(self, other):
if not isinstance(other, SparsePauliOp):
other = SparsePauliOp(other)
return self._tensor(self, other)
[docs] def expand(self, other):
if not isinstance(other, SparsePauliOp):
other = SparsePauliOp(other)
return self._tensor(other, self)
@classmethod
def _tensor(cls, a, b):
paulis = a.paulis.tensor(b.paulis)
coeffs = np.kron(a.coeffs, b.coeffs)
return SparsePauliOp(paulis, coeffs, ignore_pauli_phase=True, copy=False)
def _add(self, other, qargs=None):
if qargs is None:
qargs = getattr(other, "qargs", None)
if not isinstance(other, SparsePauliOp):
other = SparsePauliOp(other)
self._op_shape._validate_add(other._op_shape, qargs)
paulis = self.paulis._add(other.paulis, qargs=qargs)
coeffs = np.hstack((self.coeffs, other.coeffs))
return SparsePauliOp(paulis, coeffs, ignore_pauli_phase=True, copy=False)
def _multiply(self, other):
if not isinstance(other, Number):
raise QiskitError("other is not a number")
if other == 0:
# Check edge case that we deleted all Paulis
# In this case we return an identity Pauli with a zero coefficient
paulis = PauliList.from_symplectic(
np.zeros((1, self.num_qubits), dtype=bool),
np.zeros((1, self.num_qubits), dtype=bool),
)
coeffs = np.array([0j])
return SparsePauliOp(paulis, coeffs, ignore_pauli_phase=True, copy=False)
# Otherwise we just update the phases
return SparsePauliOp(
self.paulis.copy(), other * self.coeffs, ignore_pauli_phase=True, copy=False
)
# ---------------------------------------------------------------------
# Utility Methods
# ---------------------------------------------------------------------
[docs] def is_unitary(self, atol=None, rtol=None):
"""Return True if operator is a unitary matrix.
Args:
atol (float): Optional. Absolute tolerance for checking if
coefficients are zero (Default: 1e-8).
rtol (float): Optional. relative tolerance for checking if
coefficients are zero (Default: 1e-5).
Returns:
bool: True if the operator is unitary, False otherwise.
"""
# Get default atol and rtol
if atol is None:
atol = self.atol
if rtol is None:
rtol = self.rtol
# Compose with adjoint
val = self.compose(self.adjoint()).simplify()
# See if the result is an identity
return (
val.size == 1
and np.isclose(val.coeffs[0], 1.0, atol=atol, rtol=rtol)
and not np.any(val.paulis.x)
and not np.any(val.paulis.z)
)
[docs] def simplify(self, atol=None, rtol=None):
"""Simplify PauliList by combining duplicates and removing zeros.
Args:
atol (float): Optional. Absolute tolerance for checking if
coefficients are zero (Default: 1e-8).
rtol (float): Optional. relative tolerance for checking if
coefficients are zero (Default: 1e-5).
Returns:
SparsePauliOp: the simplified SparsePauliOp operator.
"""
# Get default atol and rtol
if atol is None:
atol = self.atol
if rtol is None:
rtol = self.rtol
# Filter non-zero coefficients
if self.coeffs.dtype == object:
def to_complex(coeff):
if not hasattr(coeff, "sympify"):
return coeff
sympified = coeff.sympify()
return complex(sympified) if sympified.is_Number else np.nan
non_zero = np.logical_not(
np.isclose([to_complex(x) for x in self.coeffs], 0, atol=atol, rtol=rtol)
)
else:
non_zero = np.logical_not(np.isclose(self.coeffs, 0, atol=atol, rtol=rtol))
paulis_x = self.paulis.x[non_zero]
paulis_z = self.paulis.z[non_zero]
nz_coeffs = self.coeffs[non_zero]
# Pack bool vectors into np.uint8 vectors by np.packbits
array = np.packbits(paulis_x, axis=1) * 256 + np.packbits(paulis_z, axis=1)
indexes, inverses = unordered_unique(array)
if np.all(non_zero) and indexes.shape[0] == array.shape[0]:
# No zero operator or duplicate operator
return self.copy()
coeffs = np.zeros(indexes.shape[0], dtype=self.coeffs.dtype)
np.add.at(coeffs, inverses, nz_coeffs)
# Delete zero coefficient rows
if self.coeffs.dtype == object:
is_zero = np.array(
[np.isclose(to_complex(coeff), 0, atol=atol, rtol=rtol) for coeff in coeffs]
)
else:
is_zero = np.isclose(coeffs, 0, atol=atol, rtol=rtol)
# Check edge case that we deleted all Paulis
# In this case we return an identity Pauli with a zero coefficient
if np.all(is_zero):
x = np.zeros((1, self.num_qubits), dtype=bool)
z = np.zeros((1, self.num_qubits), dtype=bool)
coeffs = np.array([0j], dtype=self.coeffs.dtype)
else:
non_zero = np.logical_not(is_zero)
non_zero_indexes = indexes[non_zero]
x = paulis_x[non_zero_indexes]
z = paulis_z[non_zero_indexes]
coeffs = coeffs[non_zero]
return SparsePauliOp(
PauliList.from_symplectic(z, x), coeffs, ignore_pauli_phase=True, copy=False
)
[docs] def argsort(self, weight=False):
"""Return indices for sorting the rows of the table.
Returns the composition of permutations in the order of sorting
by coefficient and sorting by Pauli.
By using the `weight` kwarg the output can additionally be sorted
by the number of non-identity terms in the Pauli, where the set of
all Pauli's of a given weight are still ordered lexicographically.
**Example**
Here is an example of how to use SparsePauliOp argsort.
.. code-block::
import numpy as np
from qiskit.quantum_info import SparsePauliOp
# 2-qubit labels
labels = ["XX", "XX", "XX", "YI", "II", "XZ", "XY", "XI"]
# coeffs
coeffs = [2.+1.j, 2.+2.j, 3.+0.j, 3.+0.j, 4.+0.j, 5.+0.j, 6.+0.j, 7.+0.j]
# init
spo = SparsePauliOp(labels, coeffs)
print('Initial Ordering')
print(spo)
# Lexicographic Ordering
srt = spo.argsort()
print('Lexicographically sorted')
print(srt)
# Lexicographic Ordering
srt = spo.argsort(weight=False)
print('Lexicographically sorted')
print(srt)
# Weight Ordering
srt = spo.argsort(weight=True)
print('Weight sorted')
print(srt)
.. parsed-literal::
Initial Ordering
SparsePauliOp(['XX', 'XX', 'XX', 'YI', 'II', 'XZ', 'XY', 'XI'],
coeffs=[2.+1.j, 2.+2.j, 3.+0.j, 3.+0.j, 4.+0.j, 5.+0.j, 6.+0.j, 7.+0.j])
Lexicographically sorted
[4 7 0 1 2 6 5 3]
Lexicographically sorted
[4 7 0 1 2 6 5 3]
Weight sorted
[4 7 3 0 1 2 6 5]
Args:
weight (bool): optionally sort by weight if True (Default: False).
By using the weight kwarg the output can additionally be sorted
by the number of non-identity terms in the Pauli.
Returns:
array: the indices for sorting the table.
"""
sort_coeffs_inds = np.argsort(self._coeffs, kind="stable")
pauli_list = self._pauli_list[sort_coeffs_inds]
sort_pauli_inds = pauli_list.argsort(weight=weight, phase=False)
return sort_coeffs_inds[sort_pauli_inds]
[docs] def sort(self, weight=False):
"""Sort the rows of the table.
After sorting the coefficients using numpy's argsort, sort by Pauli.
Pauli sort takes precedence.
If Pauli is the same, it will be sorted by coefficient.
By using the `weight` kwarg the output can additionally be sorted
by the number of non-identity terms in the Pauli, where the set of
all Pauli's of a given weight are still ordered lexicographically.
**Example**
Here is an example of how to use SparsePauliOp sort.
.. code-block::
import numpy as np
from qiskit.quantum_info import SparsePauliOp
# 2-qubit labels
labels = ["XX", "XX", "XX", "YI", "II", "XZ", "XY", "XI"]
# coeffs
coeffs = [2.+1.j, 2.+2.j, 3.+0.j, 3.+0.j, 4.+0.j, 5.+0.j, 6.+0.j, 7.+0.j]
# init
spo = SparsePauliOp(labels, coeffs)
print('Initial Ordering')
print(spo)
# Lexicographic Ordering
srt = spo.sort()
print('Lexicographically sorted')
print(srt)
# Lexicographic Ordering
srt = spo.sort(weight=False)
print('Lexicographically sorted')
print(srt)
# Weight Ordering
srt = spo.sort(weight=True)
print('Weight sorted')
print(srt)
.. parsed-literal::
Initial Ordering
SparsePauliOp(['XX', 'XX', 'XX', 'YI', 'II', 'XZ', 'XY', 'XI'],
coeffs=[2.+1.j, 2.+2.j, 3.+0.j, 3.+0.j, 4.+0.j, 5.+0.j, 6.+0.j, 7.+0.j])
Lexicographically sorted
SparsePauliOp(['II', 'XI', 'XX', 'XX', 'XX', 'XY', 'XZ', 'YI'],
coeffs=[4.+0.j, 7.+0.j, 2.+1.j, 2.+2.j, 3.+0.j, 6.+0.j, 5.+0.j, 3.+0.j])
Lexicographically sorted
SparsePauliOp(['II', 'XI', 'XX', 'XX', 'XX', 'XY', 'XZ', 'YI'],
coeffs=[4.+0.j, 7.+0.j, 2.+1.j, 2.+2.j, 3.+0.j, 6.+0.j, 5.+0.j, 3.+0.j])
Weight sorted
SparsePauliOp(['II', 'XI', 'YI', 'XX', 'XX', 'XX', 'XY', 'XZ'],
coeffs=[4.+0.j, 7.+0.j, 3.+0.j, 2.+1.j, 2.+2.j, 3.+0.j, 6.+0.j, 5.+0.j])
Args:
weight (bool): optionally sort by weight if True (Default: False).
By using the weight kwarg the output can additionally be sorted
by the number of non-identity terms in the Pauli.
Returns:
SparsePauliOp: a sorted copy of the original table.
"""
indices = self.argsort(weight=weight)
return SparsePauliOp(self._pauli_list[indices], self._coeffs[indices])
[docs] def chop(self, tol=1e-14):
"""Set real and imaginary parts of the coefficients to 0 if ``< tol`` in magnitude.
For example, the operator representing ``1+1e-17j X + 1e-17 Y`` with a tolerance larger
than ``1e-17`` will be reduced to ``1 X`` whereas :meth:`.SparsePauliOp.simplify` would
return ``1+1e-17j X``.
If a both the real and imaginary part of a coefficient is 0 after chopping, the
corresponding Pauli is removed from the operator.
Args:
tol (float): The absolute tolerance to check whether a real or imaginary part should
be set to 0.
Returns:
SparsePauliOp: This operator with chopped coefficients.
"""
realpart_nonzero = np.abs(self.coeffs.real) > tol
imagpart_nonzero = np.abs(self.coeffs.imag) > tol
remaining_indices = np.logical_or(realpart_nonzero, imagpart_nonzero)
remaining_real = realpart_nonzero[remaining_indices]
remaining_imag = imagpart_nonzero[remaining_indices]
if len(remaining_real) == 0: # if no Paulis are left
x = np.zeros((1, self.num_qubits), dtype=bool)
z = np.zeros((1, self.num_qubits), dtype=bool)
coeffs = np.array([0j], dtype=complex)
else:
coeffs = np.zeros(np.count_nonzero(remaining_indices), dtype=complex)
coeffs.real[remaining_real] = self.coeffs.real[realpart_nonzero]
coeffs.imag[remaining_imag] = self.coeffs.imag[imagpart_nonzero]
x = self.paulis.x[remaining_indices]
z = self.paulis.z[remaining_indices]
return SparsePauliOp(
PauliList.from_symplectic(z, x), coeffs, ignore_pauli_phase=True, copy=False
)
[docs] @staticmethod
def sum(ops):
"""Sum of SparsePauliOps.
This is a specialized version of the builtin ``sum`` function for SparsePauliOp
with smaller overhead.
Args:
ops (list[SparsePauliOp]): a list of SparsePauliOps.
Returns:
SparsePauliOp: the SparsePauliOp representing the sum of the input list.
Raises:
QiskitError: if the input list is empty.
QiskitError: if the input list includes an object that is not SparsePauliOp.
QiskitError: if the numbers of qubits of the objects in the input list do not match.
"""
if len(ops) == 0:
raise QiskitError("Input list is empty")
if not all(isinstance(op, SparsePauliOp) for op in ops):
raise QiskitError("Input list includes an object that is not SparsePauliOp")
for other in ops[1:]:
ops[0]._op_shape._validate_add(other._op_shape)
z = np.vstack([op.paulis.z for op in ops])
x = np.vstack([op.paulis.x for op in ops])
phase = np.hstack([op.paulis._phase for op in ops])
pauli_list = PauliList(BasePauli(z, x, phase))
coeffs = np.hstack([op.coeffs for op in ops])
return SparsePauliOp(pauli_list, coeffs, ignore_pauli_phase=True, copy=False)
# ---------------------------------------------------------------------
# Additional conversions
# ---------------------------------------------------------------------
[docs] @staticmethod
def from_operator(obj, atol=None, rtol=None):
"""Construct from an Operator objector.
Note that the cost of this construction is exponential as it involves
taking inner products with every element of the N-qubit Pauli basis.
Args:
obj (Operator): an N-qubit operator.
atol (float): Optional. Absolute tolerance for checking if
coefficients are zero (Default: 1e-8).
rtol (float): Optional. relative tolerance for checking if
coefficients are zero (Default: 1e-5).
Returns:
SparsePauliOp: the SparsePauliOp representation of the operator.
Raises:
QiskitError: if the input operator is not an N-qubit operator.
"""
# Get default atol and rtol
if atol is None:
atol = SparsePauliOp.atol
if rtol is None:
rtol = SparsePauliOp.rtol
if not isinstance(obj, Operator):
obj = Operator(obj)
# Check dimension is N-qubit operator
num_qubits = obj.num_qubits
if num_qubits is None:
raise QiskitError("Input Operator is not an N-qubit operator.")
data = obj.data
# Index of non-zero basis elements
inds = []
# Non-zero coefficients
coeffs = []
# Non-normalized basis factor
denom = 2**num_qubits
# Compute coefficients from basis
basis = pauli_basis(num_qubits)
for i, mat in enumerate(basis.matrix_iter()):
coeff = np.trace(mat.dot(data)) / denom
if not np.isclose(coeff, 0, atol=atol, rtol=rtol):
inds.append(i)
coeffs.append(coeff)
# Get Non-zero coeff PauliList terms
paulis = basis[inds]
return SparsePauliOp(paulis, coeffs, copy=False)
[docs] @staticmethod
def from_list(obj, dtype=complex):
"""Construct from a list of Pauli strings and coefficients.
For example, the 5-qubit Hamiltonian
.. math::
H = Z_1 X_4 + 2 Y_0 Y_3
can be constructed as
.. code-block:: python
# via tuples and the full Pauli string
op = SparsePauliOp.from_list([("XIIZI", 1), ("IYIIY", 2)])
Args:
obj (Iterable[Tuple[str, complex]]): The list of 2-tuples specifying the Pauli terms.
dtype (type): The dtype of coeffs (Default complex).
Returns:
SparsePauliOp: The SparsePauliOp representation of the Pauli terms.
Raises:
QiskitError: If the list of Paulis is empty.
"""
obj = list(obj) # To convert zip or other iterable
size = len(obj) # number of Pauli terms
if size == 0:
raise QiskitError("Input Pauli list is empty.")
# determine the number of qubits
num_qubits = len(obj[0][0])
coeffs = np.zeros(size, dtype=dtype)
labels = np.zeros(size, dtype=f"<U{num_qubits}")
for i, item in enumerate(obj):
labels[i] = item[0]
coeffs[i] = item[1]
paulis = PauliList(labels)
return SparsePauliOp(paulis, coeffs, copy=False)
[docs] @staticmethod
def from_sparse_list(obj, num_qubits, do_checks=True, dtype=complex):
"""Construct from a list of local Pauli strings and coefficients.
Each list element is a 3-tuple of a local Pauli string, indices where to apply it,
and a coefficient.
For example, the 5-qubit Hamiltonian
.. math::
H = Z_1 X_4 + 2 Y_0 Y_3
can be constructed as
.. code-block:: python
# via triples and local Paulis with indices
op = SparsePauliOp.from_sparse_list([("ZX", [1, 4], 1), ("YY", [0, 3], 2)], num_qubits=5)
# equals the following construction from "dense" Paulis
op = SparsePauliOp.from_list([("XIIZI", 1), ("IYIIY", 2)])
Args:
obj (Iterable[Tuple[str, List[int], complex]]): The list 3-tuples specifying the Paulis.
num_qubits (int): The number of qubits of the operator.
do_checks (bool): The flag of checking if the input indices are not duplicated.
dtype (type): The dtype of coeffs (Default complex).
Returns:
SparsePauliOp: The SparsePauliOp representation of the Pauli terms.
Raises:
QiskitError: If the list of Paulis is empty.
QiskitError: If the number of qubits is incompatible with the indices of the Pauli terms.
QiskitError: If the designated qubit is already assigned.
"""
obj = list(obj) # To convert zip or other iterable
size = len(obj) # number of Pauli terms
if size == 0:
raise QiskitError("Input Pauli list is empty.")
coeffs = np.zeros(size, dtype=dtype)
labels = np.zeros(size, dtype=f"<U{num_qubits}")
for i, (paulis, indices, coeff) in enumerate(obj):
if do_checks and len(indices) != len(set(indices)):
raise QiskitError("Input indices are duplicated.")
# construct the full label based off the non-trivial Paulis and indices
label = ["I"] * num_qubits
for pauli, index in zip(paulis, indices):
if index >= num_qubits:
raise QiskitError(
f"The number of qubits ({num_qubits}) is smaller than a required index {index}."
)
label[~index] = pauli
labels[i] = "".join(label)
coeffs[i] = coeff
paulis = PauliList(labels)
return SparsePauliOp(paulis, coeffs, copy=False)
[docs] def to_list(self, array=False):
"""Convert to a list Pauli string labels and coefficients.
For operators with a lot of terms converting using the ``array=True``
kwarg will be more efficient since it allocates memory for
the full Numpy array of labels in advance.
Args:
array (bool): return a Numpy array if True, otherwise
return a list (Default: False).
Returns:
list or array: List of pairs (label, coeff) for rows of the PauliList.
"""
# Dtype for a structured array with string labels and complex coeffs
pauli_labels = self.paulis.to_labels(array=True)
labels = np.zeros(
self.size, dtype=[("labels", pauli_labels.dtype), ("coeffs", self.coeffs.dtype)]
)
labels["labels"] = pauli_labels
labels["coeffs"] = self.coeffs
if array:
return labels
return labels.tolist()
[docs] def to_matrix(self, sparse=False):
"""Convert to a dense or sparse matrix.
Args:
sparse (bool): if True return a sparse CSR matrix, otherwise
return dense Numpy array (Default: False).
Returns:
array: A dense matrix if `sparse=False`.
csr_matrix: A sparse matrix in CSR format if `sparse=True`.
"""
mat = None
for i in self.matrix_iter(sparse=sparse):
if mat is None:
mat = i
else:
mat += i
return mat
[docs] def to_operator(self):
"""Convert to a matrix Operator object"""
return Operator(self.to_matrix())
# ---------------------------------------------------------------------
# Custom Iterators
# ---------------------------------------------------------------------
[docs] def label_iter(self):
"""Return a label representation iterator.
This is a lazy iterator that converts each term in the SparsePauliOp
into a tuple (label, coeff). To convert the entire table to labels
use the :meth:`to_labels` method.
Returns:
LabelIterator: label iterator object for the SparsePauliOp.
"""
class LabelIterator(CustomIterator):
"""Label representation iteration and item access."""
def __repr__(self):
return f"<SparsePauliOp_label_iterator at {hex(id(self))}>"
def __getitem__(self, key):
coeff = self.obj.coeffs[key]
pauli = self.obj.paulis.label_iter()[key]
return (pauli, coeff)
return LabelIterator(self)
[docs] def matrix_iter(self, sparse=False):
"""Return a matrix representation iterator.
This is a lazy iterator that converts each term in the SparsePauliOp
into a matrix as it is used. To convert to a single matrix use the
:meth:`to_matrix` method.
Args:
sparse (bool): optionally return sparse CSR matrices if True,
otherwise return Numpy array matrices
(Default: False)
Returns:
MatrixIterator: matrix iterator object for the PauliList.
"""
class MatrixIterator(CustomIterator):
"""Matrix representation iteration and item access."""
def __repr__(self):
return f"<SparsePauliOp_matrix_iterator at {hex(id(self))}>"
def __getitem__(self, key):
coeff = self.obj.coeffs[key]
mat = self.obj.paulis[key].to_matrix(sparse)
return coeff * mat
return MatrixIterator(self)
def _create_graph(self, qubit_wise):
"""Transform measurement operator grouping problem into graph coloring problem
Args:
qubit_wise (bool): whether the commutation rule is applied to the whole operator,
or on a per-qubit basis.
Returns:
rustworkx.PyGraph: A class of undirected graphs
"""
edges = self.paulis._noncommutation_graph(qubit_wise)
graph = rx.PyGraph()
graph.add_nodes_from(range(self.size))
graph.add_edges_from_no_data(edges)
return graph
[docs] def group_commuting(self, qubit_wise=False):
"""Partition a SparsePauliOp into sets of commuting Pauli strings.
Args:
qubit_wise (bool): whether the commutation rule is applied to the whole operator,
or on a per-qubit basis. For example:
.. code-block:: python
>>> op = SparsePauliOp.from_list([("XX", 2), ("YY", 1), ("IZ",2j), ("ZZ",1j)])
>>> op.group_commuting()
[SparsePauliOp(["IZ", "ZZ"], coeffs=[0.+2.j, 0.+1j]),
SparsePauliOp(["XX", "YY"], coeffs=[2.+0.j, 1.+0.j])]
>>> op.group_commuting(qubit_wise=True)
[SparsePauliOp(['XX'], coeffs=[2.+0.j]),
SparsePauliOp(['YY'], coeffs=[1.+0.j]),
SparsePauliOp(['IZ', 'ZZ'], coeffs=[0.+2.j, 0.+1.j])]
Returns:
List[SparsePauliOp]: List of SparsePauliOp where each SparsePauliOp contains
commuting Pauli operators.
"""
graph = self._create_graph(qubit_wise)
# Keys in coloring_dict are nodes, values are colors
coloring_dict = rx.graph_greedy_color(graph)
groups = defaultdict(list)
for idx, color in coloring_dict.items():
groups[color].append(idx)
return [self[group] for group in groups.values()]
@property
def parameters(self) -> ParameterView:
r"""Return the free ``Parameter``\s in the coefficients."""
ret = set()
for coeff in self.coeffs:
if isinstance(coeff, ParameterExpression):
ret |= coeff.parameters
return ParameterView(ret)
[docs] def assign_parameters(
self,
parameters: Mapping[Parameter, complex | ParameterExpression]
| Sequence[complex | ParameterExpression],
inplace: bool = False,
) -> SparsePauliOp | None:
r"""Bind the free ``Parameter``\s in the coefficients to provided values.
Args:
parameters: The values to bind the parameters to.
inplace: If ``False``, a copy of the operator with the bound parameters is returned.
If ``True`` the operator itself is modified.
Returns:
A copy of the operator with bound parameters, if ``inplace`` is ``False``, otherwise
``None``.
"""
if inplace:
bound = self
else:
bound = deepcopy(self)
# turn the parameters to a dictionary
if isinstance(parameters, Sequence):
free_parameters = bound.parameters
if len(parameters) != len(free_parameters):
raise ValueError(
f"Mismatching number of values ({len(parameters)}) and parameters "
f"({len(free_parameters)}). For partial binding please pass a dictionary of "
"{parameter: value} pairs."
)
parameters = dict(zip(free_parameters, parameters))
for i, coeff in enumerate(bound.coeffs):
if isinstance(coeff, ParameterExpression):
for key in coeff.parameters & parameters.keys():
coeff = coeff.assign(key, parameters[key])
if len(coeff.parameters) == 0:
coeff = complex(coeff)
bound.coeffs[i] = coeff
return None if inplace else bound
# Update docstrings for API docs
generate_apidocs(SparsePauliOp)