Source code for pennylane.pauli.pauli_vspace

# Copyright 2025 Xanadu Quantum Technologies Inc.

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# you may not use this file except in compliance with the License.
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"""A class for the linearly independent basis of a vector space in operator space."""
# pylint: disable=too-many-arguments
from copy import copy
from functools import reduce

import numpy as np
import scipy

import pennylane as qml
from pennylane.operation import Operator

from .pauli_arithmetic import PauliSentence


class PauliVSpace:
    r"""
    Class representing the linearly independent basis of a vector space in operator space.

    The main purpose of this class is to store and process ``M``, which
    is a dictionary-of-keys (DOK) style sparse representation of the set of basis vectors. You can
    think of it as the numpy-equivalent of a PauliSentence: each :class:`~pennylane.pauli.PauliWord` (key of :class:`~pennylane.pauli.PauliSentence`)
    represents one row of ``M`` with the coefficient (value of :class:`~pennylane.pauli.PauliSentence`).
    For example the set of 3 linearly independent generators ``X(0) + X(1), X(0) + X(2), X(0) + 0.5 * Y(0)``
    can be represented as

    .. code-block:: python3

        [
            [1, 1, 1],
            [1, 0, 0],
            [0, 1, 0],
            [0, 0, 0.5]
        ]

    where each column represents one sentence, and each row represents the coefficient of the respective word in the sentence.
    To make sense of this representation one additionally needs to keep track of the mapping between keys and rows. In this case we have

    .. code-block:: python3

        pw_to_idx = {
            X(0) : 0,
            X(1) : 1,
            X(2) : 2,
            Y(0) : 3
        }

    where we have set the numbering based on appearance in the list of generators. This mapping is in general not unique.

    Args:
        generators (Iterable[Union[PauliWord, PauliSentence, Operator]]): Operators that span the vector space.
        dtype (type): ``dtype`` of the underlying DOK sparse matrix ``M``. Default is ``float``.
        tol (float): Numerical tolerance for the linear independence check. If the norm of the projection of the candidate vector
            onto :math:`M^\perp` is greater than ``tol``, then it is deemed to be linearly independent.

    **Example**

    Take a linearly dependent set of operators and span the PauliVSpace.

    .. code-block:: python3

        ops = [
            X(0) @ X(1) + Y(0) @ Y(1),
            X(0) @ X(1),
            Y(0) @ Y(1)
        ]

        vspace = PauliVSpace(ops)

    It automatically detects that the third operator is linearly dependent on the former two, so it does not add the third operator to the basis.

    >>> vspace.basis
    [1.0 * X(0) @ X(1)
     + 1.0 * Y(0) @ Y(1),
     1.0 * X(0) @ X(1)]

    We can also retrospectively add operators.

    >>> vspace.add(qml.X(0))
    [1.0 * X(0) @ X(1)
     + 1.0 * Y(0) @ Y(1),
     1.0 * X(0) @ X(1),
     1.0 * X(0)]

    Again, checks of linear independence are always performed. So in the following example no operator is added.

    >>> vspace.add(Y(0) @ Y(1))
    [1.0 * X(0) @ X(1)
     + 1.0 * Y(0) @ Y(1),
     1.0 * X(0) @ X(1),
     1.0 * X(0)]
    """

    def __init__(self, generators, dtype=float, tol=None):

        self.dtype = dtype

        if any(not isinstance(g, PauliSentence) for g in generators):
            generators = [
                qml.pauli.pauli_sentence(g) if not isinstance(g, PauliSentence) else g
                for g in generators
            ]

        # Get all Pauli words that are present in at least one Pauli sentence
        if len(generators) != 0:
            all_pws = list(reduce(set.__or__, [set(ps.keys()) for ps in generators]))
        else:
            all_pws = []

        num_pw = len(all_pws)
        # Create a dictionary mapping from PauliWord to row index
        self._pw_to_idx = {pw: i for i, pw in enumerate(all_pws)}

        # Initialize PauliVSpace properties trivially
        self._basis = []
        rank = 0

        self._M = np.zeros((num_pw, rank), dtype=self.dtype)
        self._rank = rank
        self._num_pw = num_pw

        self.tol = np.finfo(self._M.dtype).eps * 100 if tol is None else tol

        # Add all generators that are linearly independent
        self.add(generators, tol=tol)

    @property
    def basis(self):
        """List of basis operators of PauliVSpace"""
        return self._basis

    def __len__(self):
        return len(self.basis)

    def add(self, other, tol=None):
        r"""Adding Pauli sentences if they are linearly independent.

        Args:
            other (List[:class:`~.PauliWord`, :class:`~.PauliSentence`, :class:`~.Operator`]): List of candidate operators to add to the ``PauliVSpace``, if they are linearly independent.
            tol (float): Numerical tolerance for linear independence check. Defaults to ``1e-15``.

        Returns:
            List: New basis vectors after adding the linearly independent ones from ``other``.

        **Example**

        We can generate a ``PauliVSpace`` and add a linearly independent operator to its basis.

        >>> ops = [X(0), X(1)]
        >>> vspace = qml.pauli.PauliVSpace(ops)
        >>> vspace.add(Y(0))
        >>> vspace
        [1.0 * X(0), 1.0 * X(1), 1.0 * Y(0)]

        We can add a list of operators at once. Only those that are linearly dependent with the current ``PauliVSpace`` are added.

        >>> vspace.add([Z(0), X(0)])
        [1.0 * X(0), 1.0 * X(1), 1.0 * Y(0), 1.0 * Z(0)]

        """
        if tol is None:
            tol = self.tol

        if isinstance(other, (qml.pauli.PauliWord, qml.pauli.PauliSentence, Operator)):
            other = [other]

        other = [
            qml.pauli.pauli_sentence(op) if not isinstance(op, qml.pauli.PauliSentence) else op
            for op in other
        ]

        for ps in other:
            # TODO: Potential speed-up by computing the maximal linear independent set for all current basis vectors + other, essentially algorithm1 in https://arxiv.org/abs/1012.5256
            (
                self._M,
                self._pw_to_idx,
                self._rank,
                self._num_pw,
                is_independent,
            ) = self._check_independence(
                self._M, ps, self._pw_to_idx, self._rank, self._num_pw, tol
            )
            if is_independent:
                self._basis.append(ps)
        return self._basis

    def is_independent(self, pauli_sentence, tol=None):
        r"""Check if the ``pauli_sentence`` is linearly independent of the basis of ``PauliVSpace``.

        Args:
            pauli_sentence (`~.PauliSentence`): Candidate Pauli sentence to check against the ``PauliVSpace`` basis for linear independence.
            tol (float): Numerical tolerance for linear independence check. Defaults to ``1e-15``.

        Returns:
            bool: whether ``pauli_sentence`` was linearly independent

        **Example**

        >>> ops = [X(0), X(1)]
        >>> vspace = PauliVSpace([op.pauli_rep for op in ops])
        >>> vspace.is_independent(X(0).pauli_rep)
        False
        >>> vspace.is_independent(Y(0).pauli_rep)
        True

        """
        if tol is None:
            tol = self.tol

        _, _, _, _, is_independent = self._check_independence(
            self._M, pauli_sentence, self._pw_to_idx, self._rank, self._num_pw, tol
        )
        return is_independent

    @staticmethod
    def _check_independence(
        M, pauli_sentence, pw_to_idx, rank, num_pw, tol
    ):  # pylint: disable=too-many-positional-arguments
        r"""
        Checks if :class:`~PauliSentence` ``pauli_sentence`` is linearly independent and provides the updated class attributes in case the vector is added.

        This is done in the following way: ``M`` (see description in class) is extended by ``pauli_sentence``.
        If the added operator has a PauliWord (key) that is new to ``pw_to_idx``, then we have to add a new row
        and already know that it has to be linearly independent.
        If it contains the same PauliWords, we need to compute the new rank and compare it with the old rank.
        If the rank is the same, the operator is linearly dependent and not added. Else, the rank is incrased by 1
        and the extended M becomes our new M.

        Args:
            M (ndarray): coefficient matrix for current LIS
            pauli_sentence (`~.PauliSentence`): Pauli sentence for which to add a column if independent
            pw_to_idx (dict): map from :class:`~pennylane.pauli.PauliWord` to row index in ``M``
            rank (int): current rank of ``M``, equal to its number of columns
            num_pw (int): current number of :class:`~pennylane.pauli.PauliWord`\ s, equal to the number of rows in ``M``
            tol (float): Numerical tolerance for linear independence check.

        Returns:
            ndarray: updated coefficient matrix for the LIS
            dict: updated map from :class:`~pennylane.pauli.PauliWord` to row index in ``M``. Includes new :class:`~pennylane.pauli.PauliWord` keys
                from the input ``pauli_sentence`` if it was linearly independent
            int: updated rank/number of columns of ``M``
            int: updated number of :class:`~pennylane.pauli.PauliWord`\ s/number of rows of ``M``
            bool: whether ``pauli_sentence`` was linearly independent and whether its column was added to ``M``
        """
        new_pws = [pw for pw in pauli_sentence.keys() if pw not in pw_to_idx]
        new_num_pw = num_pw + len(new_pws)

        if new_num_pw < rank + 1:
            # Can't span rank+1 independent vectors in fewer than rank+1 dimensions
            # The input PauliSentence must have been linearly dependent
            return M, pw_to_idx, rank, num_pw, False

        M = np.pad(M, ((0, new_num_pw - num_pw), (0, 1)))

        # If there are new PauliWords (i.e. new basis vectors), the candidate vector must be linearly independent
        if new_num_pw > num_pw:
            new_pw_to_idx = copy(pw_to_idx)
            for i, pw in enumerate(new_pws, start=num_pw):
                new_pw_to_idx[pw] = i
            # Add new PauliSentence entries to matrix
            for pw, value in pauli_sentence.items():
                M[new_pw_to_idx[pw], rank] = value

            M[:, rank] /= np.linalg.norm(M[:, rank])

            return M, new_pw_to_idx, rank + 1, new_num_pw, True

        # Add new PauliSentence entries to matrix
        for pw, value in pauli_sentence.items():
            M[pw_to_idx[pw], rank] = value

        M[:, rank] /= np.linalg.norm(M[:, rank])

        # Check if new vector is linearly dependent on the current basis
        s = scipy.linalg.svdvals(M)
        new_rank = np.count_nonzero(s > tol)

        if rank + 1 == new_rank:
            return M, pw_to_idx, new_rank, new_num_pw, True

        return M[:num_pw, :rank], pw_to_idx, rank, num_pw, False

    def __repr__(self):
        return str(self.basis)

    def __eq__(self, other):
        """
        Two PauliVSpaces are equivalent when they span the same dimensional space.
        This is checked here by having matching PauliWord keys in the sparse DOK representation and having the same rank.
        """
        if not self._num_pw == other._num_pw:
            return False
        if not set(self._pw_to_idx.keys()) == set(other._pw_to_idx.keys()):
            return False

        rank1 = np.linalg.matrix_rank(self._M)
        rank2 = np.linalg.matrix_rank(other._M)

        # To accommodate the case where the _pw_to_idx have
        # different permutations, re-arrange ``other`` with the order of ``self``
        other_M = np.zeros((other._num_pw, other._rank), dtype=float)
        for i, ps in enumerate(other.basis):
            for pw, value in ps.items():
                other_M[self._pw_to_idx[pw], i] = value
        rank3 = np.linalg.matrix_rank(np.concatenate([self._M, other_M], axis=1))

        return rank1 == rank2 and rank2 == rank3

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