Fix operator_spaces.py::kron_bases initial value (quantumlib#4716)

Fix dimensionality of initial array in kron_bases. This was causing failures when working with zero-qubit gates: empty pauli strings, Identity(0), and the new GlobalPhaseGate. cc @viathor to review
PhaseCraft · Nov 30, 2021 · 196318f · 196318f
1 parent 1021db7
commit 196318f
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Showing 2 changed files with 2 additions and 4 deletions.
diff --git a/cirq/linalg/operator_spaces.py b/cirq/linalg/operator_spaces.py
@@ -31,7 +31,7 @@
 
 def kron_bases(*bases: Dict[str, np.ndarray], repeat: int = 1) -> Dict[str, np.ndarray]:
     """Creates tensor product of bases."""
-    product_basis = {'': np.ones(1)}
+    product_basis = {'': np.array([[1]])}
     for basis in bases * repeat:
         product_basis = {
             name1 + name2: np.kron(matrix1, matrix2)

diff --git a/cirq/linalg/operator_spaces_test.py b/cirq/linalg/operator_spaces_test.py
@@ -120,9 +120,7 @@ def test_kron_bases_consistency(basis1, basis2):
         assert np.all(basis1[name] == basis2[name])
 
 
-@pytest.mark.parametrize(
-    'basis,repeat', itertools.product((PAULI_BASIS, STANDARD_BASIS), range(1, 5))
-)
+@pytest.mark.parametrize('basis,repeat', itertools.product((PAULI_BASIS, STANDARD_BASIS), range(5)))
 def test_kron_bases_repeat_sanity_checks(basis, repeat):
     product_basis = cirq.kron_bases(basis, repeat=repeat)
     assert len(product_basis) == 4 ** repeat