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Performance improvement of SparsePauliOp.to_matrix #9620

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merged 9 commits into from
Mar 23, 2023
Merged

Performance improvement of SparsePauliOp.to_matrix #9620

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9b4adc0
optimize SparsePauliOp.to_matrix
t-imamichi Feb 17, 2023
e0f541f
optimize dense
t-imamichi Feb 18, 2023
fb1428c
proper vectorize
t-imamichi Feb 22, 2023
7cfb91c
Merge branch 'main' into opt-tomatrix2
t-imamichi Feb 22, 2023
d962498
update copyright year
t-imamichi Feb 22, 2023
cea780e
small opt
t-imamichi Feb 22, 2023
972b406
update a comment
t-imamichi Feb 22, 2023
8094903
Merge branch 'main' into opt-tomatrix2
t-imamichi Feb 23, 2023
6915b1e
Merge branch 'main' into opt-tomatrix2
t-imamichi Mar 21, 2023
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13 changes: 8 additions & 5 deletions qiskit/quantum_info/operators/symplectic/base_pauli.py
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Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2020
# (C) Copyright IBM 2017, 2023
#
# 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
Expand All @@ -26,6 +26,10 @@
from qiskit.quantum_info.operators.mixins import AdjointMixin, MultiplyMixin


# utility for _to_matrix
_PARITY = np.array([-1 if bin(i).count("1") % 2 else 1 for i in range(256)], dtype=complex)


class BasePauli(BaseOperator, AdjointMixin, MultiplyMixin):
r"""Symplectic representation of a list of N-qubit Paulis.

Expand Down Expand Up @@ -394,7 +398,7 @@ def _from_array(z, x, phase=0):

@staticmethod
def _to_matrix(z, x, phase=0, group_phase=False, sparse=False):
"""Return the matrix matrix from symplectic representation.
"""Return the matrix from symplectic representation.

The Pauli is defined as :math:`P = (-i)^{phase + z.x} * Z^z.x^x`
where ``array = [x, z]``.
Expand Down Expand Up @@ -432,7 +436,7 @@ def _to_matrix(z, x, phase=0, group_phase=False, sparse=False):
coeff = (-1j) ** phase
else:
coeff = 1
data = np.array([coeff * (-1) ** (bin(i).count("1") % 2) for i in z_indices & indptr])
data = coeff * _PARITY[(z_indices & indptr).astype(np.uint8)]
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Why is uint8 enough? Is it possible to have a larger number?

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The original code computes the parity of each value of z_indices & indptr.
My code first applies astype(np.uint8) to take mod 256 (parity does not change) and then applies a lookup table _PARITY[256] to compute the parity. Does it make sense?

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Thanks for your explanation. I misunderstood the definition of parity.

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@t-imamichi t-imamichi Feb 22, 2023 •
edited
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Sorry. I noticed that I misunderstood the party of the code. Your concern is correct. This code does not work with many qubits. I will rethink the optimization.

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fixed at fb1428c

if sparse:
# Return sparse matrix
from scipy.sparse import csr_matrix
Expand All @@ -441,8 +445,7 @@ def _to_matrix(z, x, phase=0, group_phase=False, sparse=False):

# Build dense matrix using csr format
mat = np.zeros((dim, dim), dtype=complex)
for i in range(dim):
mat[i][indices[indptr[i] : indptr[i + 1]]] = data[indptr[i] : indptr[i + 1]]
mat[range(dim), indices[:dim]] = data[:dim]
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return mat

@staticmethod
Expand Down