Updates to lie_closure

doc/releases/changelog-dev.md

@@ -145,6 +145,9 @@
 * `qml.AmplitudeEmbedding` has better support for features using low precision integer data types.
 [(#5969)](https://github.com/PennyLaneAI/pennylane/pull/5969)
 
+* `qml.lie_closure` works with sums of Paulis.
+ [(#6023)](https://github.com/PennyLaneAI/pennylane/pull/6023)
+
 
 <h3>Contributors ✍️</h3>
 
@@ -160,6 +163,7 @@ Pietropaolo Frisoni,
 Emiliano Godinez,
 Renke Huang,
 Soran Jahangiri,
+Korbinian Kottmann,
 Christina Lee,
 Austin Huang,
 Christina Lee,

pennylane/pauli/dla/lie_closure.py

@@ -14,6 +14,7 @@
 """A function to compute the Lie closure of a set of operators"""
 # pylint: disable=too-many-arguments
 import itertools
+import warnings
 from copy import copy
 from functools import reduce
 from typing import Iterable, Union
@@ -134,22 +135,29 @@ def lie_closure(
  while (new_length > old_length) and (epoch < max_iterations):
  if verbose:
  print(f"epoch {epoch+1} of lie_closure, DLA size is {new_length}")
+
  for ps1, ps2 in itertools.combinations(vspace.basis, 2):
  com = ps1.commutator(ps2)
+ com.simplify()
+
  if len(com) == 0: # skip because operators commute
  continue
 
  # result is always purely imaginary
  # remove common factor 2 with Pauli commutators
  for pw, val in com.items():
  com[pw] = val.imag / 2
+
  vspace.add(com, tol=tol)
 
  # Updated number of linearly independent PauliSentences from previous and current step
  old_length = new_length
  new_length = len(vspace)
  epoch += 1
 
+ if epoch == max_iterations:
+ warnings.warn(f"reached the maximum number of iterations {max_iterations}", UserWarning)
+
  if verbose > 0:
  print(f"After {epoch} epochs, reached a DLA size of {new_length}")
 

tests/pauli/dla/test_lie_closure.py

@@ -344,7 +344,9 @@ def test_max_iterations(self, capsys):
  PauliSentence({PauliWord({i: "X", (i + 1) % n: "Z"}): 1.0}) for i in range(n - 1)
  ]
 
- res = qml.pauli.lie_closure(generators, verbose=True, max_iterations=1)
+ with pytest.warns(UserWarning, match="reached the maximum number of iterations"):
+ res = qml.pauli.lie_closure(generators, verbose=True, max_iterations=1)
+
  captured = capsys.readouterr()
  assert (
  captured.out
@@ -501,6 +503,17 @@ def test_lie_closure_heisenberg_generators_even(self):
  res = qml.pauli.lie_closure(generators)
  assert len(res) == 4 * ((2 ** (n - 2)) ** 2 - 1)
 
+ @pytest.mark.parametrize("n, res", [(3, 4), (4, 12)])
+ def test_lie_closure_heisenberg(self, n, res):
+ """Test the resulting DLA from Heisenberg model with summed generators"""
+ genXX = [X(i) @ X(i + 1) for i in range(n - 1)]
+ genYY = [Y(i) @ Y(i + 1) for i in range(n - 1)]
+ genZZ = [Z(i) @ Z(i + 1) for i in range(n - 1)]
+
+ generators = [qml.sum(XX + YY + ZZ) for XX, YY, ZZ in zip(genXX, genYY, genZZ)]
+ g = qml.lie_closure(generators)
+ assert len(g) == res
+
  def test_universal_gate_set(self):
  """Test universal gate set"""
  n = 3