fixed bug in symmer.utils._get_sparse_matrix_large_pauliwordop functi…

…on. Empty matrix now built properly (previously getting sparse matrix of Identity term, which causes inf recursion. Now fixed and unit tested

symmer/utils.py

@@ -297,7 +297,7 @@ def _get_sparse_matrix_large_pauliwordop(P_op: PauliwordOp) -> csr_matrix:
  """
  """
  nq = P_op.n_qubits
- mat = PauliwordOp.empty(nq).to_sparse_matrix
+ mat = csr_matrix(([], ([],[])), shape=(2**nq,2**nq))
  for op in P_op:
  left_tensor = np.hstack((op.X_block[:, :nq // 2],
  op.Z_block[:, :nq // 2]))

tests/test_operators/test_base.py

@@ -711,6 +711,29 @@ def test_to_sparse_matrix_2(
  P = PauliwordOp.from_dictionary(P_dict)
  assert np.all(P.to_sparse_matrix.toarray() == P_array)
 
+def test_to_sparse_matrix_large_operator():
+ """ Tests multiplication and the Qiskit conversion
+ """
+ H = PauliwordOp.from_dictionary({'ZIIIIIIIZIXXXIII': (-1.333664871035997-0.6347579982999967j),
+ 'IIIIIYIIXYZZIXXI': (0.6121055433989232+2.0175827791182313j),
+ 'IIIXIZZIIZIIXIZI': (-0.5187971729475656+1.2184045529704965j),
+ 'ZIIXYYZZIYYXXIZY': (0.6788676757886678+1.867085666718753j),
+ 'IZXIYIXYXIIIZZIX': (-1.0665060328185856-0.5702647494844407j),
+ 'ZIXXIIIZIIIIZIXX': (0.17268863171166954-0.07117422292367692j),
+ 'IIYXIIYIIIXIIZXI': (0.03704770372393225-0.21589376964746243j),
+ 'IYIZXXIXZXXZIIII': (0.29998428856285453-0.9742733999161437j),
+ 'YXIXIIZXZIIIIIYX': (0.3421035543407282-0.20273712913326358j),
+ 'XXXYIIIIXIIXIXIZ': (1.1502457768722+1.3148268876228302j)})
+ mat_sparse = H.to_sparse_matrix
+
+ basis = H.copy()
+ basis.coeff_vec = np.ones_like(basis.coeff_vec)
+ out = PauliwordOp.from_matrix(mat_sparse,
+ strategy='full_basis',
+ operator_basis=basis,
+ disable_loading_bar=True)
+ assert H == out, 'to_sparse_matrix of large Pauli operator is failing'
+
 
 def test_QuantumState_overlap():
  for n_q in range(2,5):